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Message-ID: <20200629153226.GW9247@paulmck-ThinkPad-P72>
Date: Mon, 29 Jun 2020 08:32:26 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: John Ogness <john.ogness@...utronix.de>
Cc: Petr Mladek <pmladek@...e.com>,
Peter Zijlstra <peterz@...radead.org>,
Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
Steven Rostedt <rostedt@...dmis.org>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Andrea Parri <parri.andrea@...il.com>,
Thomas Gleixner <tglx@...utronix.de>,
kexec@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/3] printk: add lockless ringbuffer
On Thu, Jun 18, 2020 at 04:55:18PM +0206, John Ogness wrote:
> Introduce a multi-reader multi-writer lockless ringbuffer for storing
> the kernel log messages. Readers and writers may use their API from
> any context (including scheduler and NMI). This ringbuffer will make
> it possible to decouple printk() callers from any context, locking,
> or console constraints. It also makes it possible for readers to have
> full access to the ringbuffer contents at any time and context (for
> example from any panic situation).
>
> The printk_ringbuffer is made up of 3 internal ringbuffers:
>
> desc_ring:
> A ring of descriptors. A descriptor contains all record meta data
> (sequence number, timestamp, loglevel, etc.) as well as internal state
> information about the record and logical positions specifying where in
> the other ringbuffers the text and dictionary strings are located.
>
> text_data_ring:
> A ring of data blocks. A data block consists of an unsigned long
> integer (ID) that maps to a desc_ring index followed by the text
> string of the record.
>
> dict_data_ring:
> A ring of data blocks. A data block consists of an unsigned long
> integer (ID) that maps to a desc_ring index followed by the dictionary
> string of the record.
>
> The internal state information of a descriptor is the key element to
> allow readers and writers to locklessly synchronize access to the data.
>
> Co-developed-by: Petr Mladek <pmladek@...e.com>
> Signed-off-by: John Ogness <john.ogness@...utronix.de>
The orderings match the comments, although a number could (later!)
be weakened to the easier-to-read smp_load_acquire() and/or
smp_store_release(). So, from a memory-ordering perspective:
Reviewed-by: Paul E. McKenney <paulmck@...nel.org>
> ---
> kernel/printk/Makefile | 1 +
> kernel/printk/printk_ringbuffer.c | 1674 +++++++++++++++++++++++++++++
> kernel/printk/printk_ringbuffer.h | 352 ++++++
> 3 files changed, 2027 insertions(+)
> create mode 100644 kernel/printk/printk_ringbuffer.c
> create mode 100644 kernel/printk/printk_ringbuffer.h
>
> diff --git a/kernel/printk/Makefile b/kernel/printk/Makefile
> index 4d052fc6bcde..eee3dc9b60a9 100644
> --- a/kernel/printk/Makefile
> +++ b/kernel/printk/Makefile
> @@ -2,3 +2,4 @@
> obj-y = printk.o
> obj-$(CONFIG_PRINTK) += printk_safe.o
> obj-$(CONFIG_A11Y_BRAILLE_CONSOLE) += braille.o
> +obj-$(CONFIG_PRINTK) += printk_ringbuffer.o
> diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c
> new file mode 100644
> index 000000000000..75d056436cc5
> --- /dev/null
> +++ b/kernel/printk/printk_ringbuffer.c
> @@ -0,0 +1,1674 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/kernel.h>
> +#include <linux/irqflags.h>
> +#include <linux/string.h>
> +#include <linux/errno.h>
> +#include <linux/bug.h>
> +#include "printk_ringbuffer.h"
> +
> +/**
> + * DOC: printk_ringbuffer overview
> + *
> + * Data Structure
> + * --------------
> + * The printk_ringbuffer is made up of 3 internal ringbuffers:
> + *
> + * desc_ring
> + * A ring of descriptors. A descriptor contains all record meta data
> + * (sequence number, timestamp, loglevel, etc.) as well as internal state
> + * information about the record and logical positions specifying where in
> + * the other ringbuffers the text and dictionary strings are located.
> + *
> + * text_data_ring
> + * A ring of data blocks. A data block consists of an unsigned long
> + * integer (ID) that maps to a desc_ring index followed by the text
> + * string of the record.
> + *
> + * dict_data_ring
> + * A ring of data blocks. A data block consists of an unsigned long
> + * integer (ID) that maps to a desc_ring index followed by the dictionary
> + * string of the record.
> + *
> + * The internal state information of a descriptor is the key element to allow
> + * readers and writers to locklessly synchronize access to the data.
> + *
> + * Implementation
> + * --------------
> + *
> + * Descriptor Ring
> + * ~~~~~~~~~~~~~~~
> + * The descriptor ring is an array of descriptors. A descriptor contains all
> + * the meta data of a printk record as well as blk_lpos structs pointing to
> + * associated text and dictionary data blocks (see "Data Rings" below). Each
> + * descriptor is assigned an ID that maps directly to index values of the
> + * descriptor array and has a state. The ID and the state are bitwise combined
> + * into a single descriptor field named @state_var, allowing ID and state to
> + * be synchronously and atomically updated.
> + *
> + * Descriptors have three states:
> + *
> + * reserved
> + * A writer is modifying the record.
> + *
> + * committed
> + * The record and all its data are complete and available for reading.
> + *
> + * reusable
> + * The record exists, but its text and/or dictionary data may no longer
> + * be available.
> + *
> + * Querying the @state_var of a record requires providing the ID of the
> + * descriptor to query. This can yield a possible fourth (pseudo) state:
> + *
> + * miss
> + * The descriptor being queried has an unexpected ID.
> + *
> + * The descriptor ring has a @tail_id that contains the ID of the oldest
> + * descriptor and @head_id that contains the ID of the newest descriptor.
> + *
> + * When a new descriptor should be created (and the ring is full), the tail
> + * descriptor is invalidated by first transitioning to the reusable state and
> + * then invalidating all tail data blocks up to and including the data blocks
> + * associated with the tail descriptor (for text and dictionary rings). Then
> + * @tail_id is advanced, followed by advancing @head_id. And finally the
> + * @state_var of the new descriptor is initialized to the new ID and reserved
> + * state.
> + *
> + * The @tail_id can only be advanced if the the new @tail_id would be in the
> + * committed or reusable queried state. This makes it possible that a valid
> + * sequence number of the tail is always available.
> + *
> + * Data Rings
> + * ~~~~~~~~~~
> + * The two data rings (text and dictionary) function identically. They exist
> + * separately so that their buffer sizes can be individually set and they do
> + * not affect one another.
> + *
> + * Data rings are byte arrays composed of data blocks. Data blocks are
> + * referenced by blk_lpos structs that point to the logical position of the
> + * beginning of a data block and the beginning of the next adjacent data
> + * block. Logical positions are mapped directly to index values of the byte
> + * array ringbuffer.
> + *
> + * Each data block consists of an ID followed by the raw data. The ID is the
> + * identifier of a descriptor that is associated with the data block. A data
> + * block is considered valid if all of the following conditions are met:
> + *
> + * 1) The descriptor associated with the data block is in the committed
> + * or reusable queried state.
> + *
> + * 2) The blk_lpos struct within the descriptor associated with the data
> + * block references back to the same data block.
> + *
> + * 3) The data block is within the head/tail logical position range.
> + *
> + * If the raw data of a data block would extend beyond the end of the byte
> + * array, only the ID of the data block is stored at the logical position
> + * and the full data block (ID and raw data) is stored at the beginning of
> + * the byte array. The referencing blk_lpos will point to the ID before the
> + * wrap and the next data block will be at the logical position adjacent the
> + * full data block after the wrap.
> + *
> + * Data rings have a @tail_lpos that points to the beginning of the oldest
> + * data block and a @head_lpos that points to the logical position of the
> + * next (not yet existing) data block.
> + *
> + * When a new data block should be created (and the ring is full), tail data
> + * blocks will first be invalidated by putting their associated descriptors
> + * into the reusable state and then pushing the @tail_lpos forward beyond
> + * them. Then the @head_lpos is pushed forward and is associated with a new
> + * descriptor. If a data block is not valid, the @tail_lpos cannot be
> + * advanced beyond it.
> + *
> + * Usage
> + * -----
> + * Here are some simple examples demonstrating writers and readers. For the
> + * examples a global ringbuffer (test_rb) is available (which is not the
> + * actual ringbuffer used by printk)::
> + *
> + * DECLARE_PRINTKRB(test_rb, 15, 5, 3);
> + *
> + * This ringbuffer allows up to 32768 records (2 ^ 15) and has a size of
> + * 1 MiB (2 ^ (15 + 5)) for text data and 256 KiB (2 ^ (15 + 3)) for
> + * dictionary data.
> + *
> + * Sample writer code::
> + *
> + * const char *dictstr = "dictionary text";
> + * const char *textstr = "message text";
> + * struct prb_reserved_entry e;
> + * struct printk_record r;
> + *
> + * // specify how much to allocate
> + * prb_rec_init_wr(&r, strlen(textstr) + 1, strlen(dictstr) + 1);
> + *
> + * if (prb_reserve(&e, &test_rb, &r)) {
> + * snprintf(r.text_buf, r.text_buf_size, "%s", textstr);
> + *
> + * // dictionary allocation may have failed
> + * if (r.dict_buf)
> + * snprintf(r.dict_buf, r.dict_buf_size, "%s", dictstr);
> + *
> + * r.info->ts_nsec = local_clock();
> + *
> + * prb_commit(&e);
> + * }
> + *
> + * Sample reader code::
> + *
> + * struct printk_info info;
> + * struct printk_record r;
> + * char text_buf[32];
> + * char dict_buf[32];
> + * u64 seq;
> + *
> + * prb_rec_init_rd(&r, &info, &text_buf[0], sizeof(text_buf),
> + * &dict_buf[0], sizeof(dict_buf));
> + *
> + * prb_for_each_record(0, &test_rb, &seq, &r) {
> + * if (info.seq != seq)
> + * pr_warn("lost %llu records\n", info.seq - seq);
> + *
> + * if (info.text_len > r.text_buf_size) {
> + * pr_warn("record %llu text truncated\n", info.seq);
> + * text_buf[sizeof(text_buf) - 1] = 0;
> + * }
> + *
> + * if (info.dict_len > r.dict_buf_size) {
> + * pr_warn("record %llu dict truncated\n", info.seq);
> + * dict_buf[sizeof(dict_buf) - 1] = 0;
> + * }
> + *
> + * pr_info("%llu: %llu: %s;%s\n", info.seq, info.ts_nsec,
> + * &text_buf[0], info.dict_len ? &dict_buf[0] : "");
> + * }
> + *
> + * Note that additional less convenient reader functions are available to
> + * allow complex record access.
> + *
> + * ABA Issues
> + * ~~~~~~~~~~
> + * To help avoid ABA issues, descriptors are referenced by IDs (array index
> + * values with tagged states) and data blocks are referenced by logical
> + * positions (array index values with tagged states). However, on 32-bit
> + * systems the number of tagged states is relatively small such that an ABA
> + * incident is (at least theoretically) possible. For example, if 4 million
> + * maximally sized (1KiB) printk messages were to occur in NMI context on a
> + * 32-bit system, the interrupted context would not be able to recognize that
> + * the 32-bit integer completely wrapped and thus represents a different
> + * data block than the one the interrupted context expects.
> + *
> + * To help combat this possibility, additional state checking is performed
> + * (such as using cmpxchg() even though set() would suffice). These extra
> + * checks are commented as such and will hopefully catch any ABA issue that
> + * a 32-bit system might experience.
> + *
> + * Memory Barriers
> + * ~~~~~~~~~~~~~~~
> + * Multiple memory barriers are used. To simplify proving correctness and
> + * generating litmus tests, lines of code related to memory barriers
> + * (loads, stores, and the associated memory barriers) are labeled::
> + *
> + * LMM(function:letter)
> + *
> + * Comments reference the labels using only the "function:letter" part.
> + *
> + * The memory barrier pairs and their ordering are:
> + *
> + * desc_reserve:D / desc_reserve:B
> + * push descriptor tail (id), then push descriptor head (id)
> + *
> + * desc_reserve:D / data_push_tail:B
> + * push data tail (lpos), then set new descriptor reserved (state)
> + *
> + * desc_reserve:D / desc_push_tail:C
> + * push descriptor tail (id), then set new descriptor reserved (state)
> + *
> + * desc_reserve:D / prb_first_seq:C
> + * push descriptor tail (id), then set new descriptor reserved (state)
> + *
> + * desc_reserve:F / desc_read:D
> + * set new descriptor id and reserved (state), then allow writer changes
> + *
> + * data_alloc:A / desc_read:D
> + * set old descriptor reusable (state), then modify new data block area
> + *
> + * data_alloc:A / data_push_tail:B
> + * push data tail (lpos), then modify new data block area
> + *
> + * prb_commit:B / desc_read:B
> + * store writer changes, then set new descriptor committed (state)
> + *
> + * data_push_tail:D / data_push_tail:A
> + * set descriptor reusable (state), then push data tail (lpos)
> + *
> + * desc_push_tail:B / desc_reserve:D
> + * set descriptor reusable (state), then push descriptor tail (id)
> + */
> +
> +#define DATA_SIZE(data_ring) _DATA_SIZE((data_ring)->size_bits)
> +#define DATA_SIZE_MASK(data_ring) (DATA_SIZE(data_ring) - 1)
> +
> +#define DESCS_COUNT(desc_ring) _DESCS_COUNT((desc_ring)->count_bits)
> +#define DESCS_COUNT_MASK(desc_ring) (DESCS_COUNT(desc_ring) - 1)
> +
> +/* Determine the data array index from a logical position. */
> +#define DATA_INDEX(data_ring, lpos) ((lpos) & DATA_SIZE_MASK(data_ring))
> +
> +/* Determine the desc array index from an ID or sequence number. */
> +#define DESC_INDEX(desc_ring, n) ((n) & DESCS_COUNT_MASK(desc_ring))
> +
> +/* Determine how many times the data array has wrapped. */
> +#define DATA_WRAPS(data_ring, lpos) ((lpos) >> (data_ring)->size_bits)
> +
> +/* Get the logical position at index 0 of the current wrap. */
> +#define DATA_THIS_WRAP_START_LPOS(data_ring, lpos) \
> +((lpos) & ~DATA_SIZE_MASK(data_ring))
> +
> +/* Get the ID for the same index of the previous wrap as the given ID. */
> +#define DESC_ID_PREV_WRAP(desc_ring, id) \
> +DESC_ID((id) - DESCS_COUNT(desc_ring))
> +
> +/* A data block: maps to the raw data within the data ring. */
> +struct prb_data_block {
> + unsigned long id;
> + char data[0];
> +};
> +
> +static struct prb_desc *to_desc(struct prb_desc_ring *desc_ring, u64 n)
> +{
> + return &desc_ring->descs[DESC_INDEX(desc_ring, n)];
> +}
> +
> +static struct prb_data_block *to_block(struct prb_data_ring *data_ring,
> + unsigned long begin_lpos)
> +{
> + return (void *)&data_ring->data[DATA_INDEX(data_ring, begin_lpos)];
> +}
> +
> +/*
> + * Increase the data size to account for data block meta data plus any
> + * padding so that the adjacent data block is aligned on the ID size.
> + */
> +static unsigned int to_blk_size(unsigned int size)
> +{
> + struct prb_data_block *db = NULL;
> +
> + size += sizeof(*db);
> + size = ALIGN(size, sizeof(db->id));
> + return size;
> +}
> +
> +/*
> + * Sanity checker for reserve size. The ringbuffer code assumes that a data
> + * block does not exceed the maximum possible size that could fit within the
> + * ringbuffer. This function provides that basic size check so that the
> + * assumption is safe.
> + *
> + * Writers are also not allowed to write 0-sized (data-less) records. Such
> + * records are used only internally by the ringbuffer.
> + */
> +static bool data_check_size(struct prb_data_ring *data_ring, unsigned int size)
> +{
> + struct prb_data_block *db = NULL;
> +
> + /*
> + * Writers are not allowed to write data-less records. Such records
> + * are used only internally by the ringbuffer to denote records where
> + * their data failed to allocate or have been lost.
> + */
> + if (size == 0)
> + return false;
> +
> + /*
> + * Ensure the alignment padded size could possibly fit in the data
> + * array. The largest possible data block must still leave room for
> + * at least the ID of the next block.
> + */
> + size = to_blk_size(size);
> + if (size > DATA_SIZE(data_ring) - sizeof(db->id))
> + return false;
> +
> + return true;
> +}
> +
> +/* The possible responses of a descriptor state-query. */
> +enum desc_state {
> + desc_miss, /* ID mismatch */
> + desc_reserved, /* reserved, in use by writer */
> + desc_committed, /* committed, writer is done */
> + desc_reusable, /* free, not yet used by any writer */
> +};
> +
> +/* Query the state of a descriptor. */
> +static enum desc_state get_desc_state(unsigned long id,
> + unsigned long state_val)
> +{
> + if (id != DESC_ID(state_val))
> + return desc_miss;
> +
> + if (state_val & DESC_REUSE_MASK)
> + return desc_reusable;
> +
> + if (state_val & DESC_COMMITTED_MASK)
> + return desc_committed;
> +
> + return desc_reserved;
> +}
> +
> +/*
> + * Get a copy of a specified descriptor and its queried state. A descriptor
> + * that is not in the committed or reusable state must be considered garbage
> + * by the reader.
> + */
> +static enum desc_state desc_read(struct prb_desc_ring *desc_ring,
> + unsigned long id, struct prb_desc *desc_out)
> +{
> + struct prb_desc *desc = to_desc(desc_ring, id);
> + atomic_long_t *state_var = &desc->state_var;
> + enum desc_state d_state;
> + unsigned long state_val;
> +
> + /* Check the descriptor state. */
> + state_val = atomic_long_read(state_var); /* LMM(desc_read:A) */
> + d_state = get_desc_state(id, state_val);
> + if (d_state != desc_committed && d_state != desc_reusable)
> + return d_state;
> +
> + /*
> + * Guarantee the state is loaded before copying the descriptor
> + * content. This avoids copying obsolete descriptor content that might
> + * not apply to the descriptor state. This pairs with prb_commit:B.
> + *
> + * Memory barrier involvement:
> + *
> + * If desc_read:A reads from prb_commit:B, then desc_read:C reads
> + * from prb_commit:A.
> + *
> + * Relies on:
> + *
> + * WMB from prb_commit:A to prb_commit:B
> + * matching
> + * RMB from desc_read:A to desc_read:C
> + */
> + smp_rmb(); /* LMM(desc_read:B) */
> +
> + /*
> + * Copy the descriptor data. The data is not valid until the
> + * state has been re-checked.
> + */
> + memcpy(desc_out, desc, sizeof(*desc_out)); /* LMM(desc_read:C) */
> +
> + /*
> + * 1. Guarantee the descriptor content is loaded before re-checking
> + * the state. This avoids reading an obsolete descriptor state
> + * that may not apply to the copied content. This pairs with
> + * desc_reserve:F.
> + *
> + * Memory barrier involvement:
> + *
> + * If desc_read:C reads from desc_reserve:G, then desc_read:E
> + * reads from desc_reserve:F.
> + *
> + * Relies on:
> + *
> + * WMB from desc_reserve:F to desc_reserve:G
> + * matching
> + * RMB from desc_read:C to desc_read:E
> + *
> + * 2. Guarantee the record data is loaded before re-checking the
> + * state. This avoids reading an obsolete descriptor state that may
> + * not apply to the copied data. This pairs with data_alloc:A.
> + *
> + * Memory barrier involvement:
> + *
> + * If copy_data:A reads from data_alloc:B, then desc_read:E
> + * reads from desc_make_reusable:A.
> + *
> + * Relies on:
> + *
> + * MB from desc_make_reusable:A to data_alloc:B
> + * matching
> + * RMB from desc_read:C to desc_read:E
> + *
> + * Note: desc_make_reusable:A and data_alloc:B can be different
> + * CPUs. However, the data_alloc:B CPU (which performs the
> + * full memory barrier) must have previously seen
> + * desc_make_reusable:A.
> + */
> + smp_rmb(); /* LMM(desc_read:D) */
> +
> + /* Re-check the descriptor state. */
> + state_val = atomic_long_read(state_var); /* LMM(desc_read:E) */
> + return get_desc_state(id, state_val);
> +}
> +
> +/*
> + * Take a specified descriptor out of the committed state by attempting
> + * the transition from committed to reusable. Either this context or some
> + * other context will have been successful.
> + */
> +static void desc_make_reusable(struct prb_desc_ring *desc_ring,
> + unsigned long id)
> +{
> + unsigned long val_committed = id | DESC_COMMITTED_MASK;
> + unsigned long val_reusable = val_committed | DESC_REUSE_MASK;
> + struct prb_desc *desc = to_desc(desc_ring, id);
> + atomic_long_t *state_var = &desc->state_var;
> +
> + atomic_long_cmpxchg_relaxed(state_var, val_committed,
> + val_reusable); /* LMM(desc_make_reusable:A) */
> +}
> +
> +/*
> + * Given a data ring (text or dict), put the associated descriptor of each
> + * data block from @lpos_begin until @lpos_end into the reusable state.
> + *
> + * If there is any problem making the associated descriptor reusable, either
> + * the descriptor has not yet been committed or another writer context has
> + * already pushed the tail lpos past the problematic data block. Regardless,
> + * on error the caller can re-load the tail lpos to determine the situation.
> + */
> +static bool data_make_reusable(struct printk_ringbuffer *rb,
> + struct prb_data_ring *data_ring,
> + unsigned long lpos_begin,
> + unsigned long lpos_end,
> + unsigned long *lpos_out)
> +{
> + struct prb_desc_ring *desc_ring = &rb->desc_ring;
> + struct prb_data_blk_lpos *blk_lpos;
> + struct prb_data_block *blk;
> + enum desc_state d_state;
> + struct prb_desc desc;
> + unsigned long id;
> +
> + /*
> + * Using the provided @data_ring, point @blk_lpos to the correct
> + * blk_lpos within the local copy of the descriptor.
> + */
> + if (data_ring == &rb->text_data_ring)
> + blk_lpos = &desc.text_blk_lpos;
> + else
> + blk_lpos = &desc.dict_blk_lpos;
> +
> + /* Loop until @lpos_begin has advanced to or beyond @lpos_end. */
> + while ((lpos_end - lpos_begin) - 1 < DATA_SIZE(data_ring)) {
> + blk = to_block(data_ring, lpos_begin);
> +
> + /*
> + * Load the block ID from the data block. This is a data race
> + * against a writer that may have newly reserved this data
> + * area. If the loaded value matches a valid descriptor ID,
> + * the blk_lpos of that descriptor will be checked to make
> + * sure it points back to this data block. If the check fails,
> + * the data area has been recycled by another writer.
> + */
> + id = blk->id; /* LMM(data_make_reusable:A) */
> +
> + d_state = desc_read(desc_ring, id, &desc); /* LMM(data_make_reusable:B) */
> +
> + switch (d_state) {
> + case desc_miss:
> + return false;
> + case desc_reserved:
> + return false;
> + case desc_committed:
> + /*
> + * This data block is invalid if the descriptor
> + * does not point back to it.
> + */
> + if (blk_lpos->begin != lpos_begin)
> + return false;
> + desc_make_reusable(desc_ring, id);
> + break;
> + case desc_reusable:
> + /*
> + * This data block is invalid if the descriptor
> + * does not point back to it.
> + */
> + if (blk_lpos->begin != lpos_begin)
> + return false;
> + break;
> + }
> +
> + /* Advance @lpos_begin to the next data block. */
> + lpos_begin = blk_lpos->next;
> + }
> +
> + *lpos_out = lpos_begin;
> + return true;
> +}
> +
> +/*
> + * Advance the data ring tail to at least @lpos. This function puts
> + * descriptors into the reusable state if the tail is pushed beyond
> + * their associated data block.
> + */
> +static bool data_push_tail(struct printk_ringbuffer *rb,
> + struct prb_data_ring *data_ring,
> + unsigned long lpos)
> +{
> + unsigned long tail_lpos_new;
> + unsigned long tail_lpos;
> + unsigned long next_lpos;
> +
> + /* If @lpos is not valid, there is nothing to do. */
> + if (lpos == INVALID_LPOS)
> + return true;
> +
> + /*
> + * Any descriptor states that have transitioned to reusable due to the
> + * data tail being pushed to this loaded value will be visible to this
> + * CPU. This pairs with data_push_tail:D.
> + *
> + * Memory barrier involvement:
> + *
> + * If data_push_tail:A reads from data_push_tail:D, then this CPU can
> + * see desc_make_reusable:A.
> + *
> + * Relies on:
> + *
> + * MB from desc_make_reusable:A to data_push_tail:D
> + * matches
> + * READFROM from data_push_tail:D to data_push_tail:A
> + * thus
> + * READFROM from desc_make_reusable:A to this CPU
> + */
> + tail_lpos = atomic_long_read(&data_ring->tail_lpos); /* LMM(data_push_tail:A) */
> +
> + /*
> + * Loop until the tail lpos is at or beyond @lpos. This condition
> + * may already be satisfied, resulting in no full memory barrier
> + * from data_push_tail:D being performed. However, since this CPU
> + * sees the new tail lpos, any descriptor states that transitioned to
> + * the reusable state must already be visible.
> + */
> + while ((lpos - tail_lpos) - 1 < DATA_SIZE(data_ring)) {
> + /*
> + * Make all descriptors reusable that are associated with
> + * data blocks before @lpos.
> + */
> + if (!data_make_reusable(rb, data_ring, tail_lpos, lpos,
> + &next_lpos)) {
> + /*
> + * 1. Guarantee the block ID loaded in
> + * data_make_reusable() is performed before
> + * reloading the tail lpos. The failed
> + * data_make_reusable() may be due to a newly
> + * recycled data area causing the tail lpos to
> + * have been previously pushed. This pairs with
> + * data_alloc:A.
> + *
> + * Memory barrier involvement:
> + *
> + * If data_make_reusable:A reads from data_alloc:B,
> + * then data_push_tail:C reads from
> + * data_push_tail:D.
> + *
> + * Relies on:
> + *
> + * MB from data_push_tail:D to data_alloc:B
> + * matching
> + * RMB from data_make_reusable:A to
> + * data_push_tail:C
> + *
> + * Note: data_push_tail:D and data_alloc:B can be
> + * different CPUs. However, the data_alloc:B
> + * CPU (which performs the full memory
> + * barrier) must have previously seen
> + * data_push_tail:D.
> + *
> + * 2. Guarantee the descriptor state loaded in
> + * data_make_reusable() is performed before
> + * reloading the tail lpos. The failed
> + * data_make_reusable() may be due to a newly
> + * recycled descriptor causing the tail lpos to
> + * have been previously pushed. This pairs with
> + * desc_reserve:D.
> + *
> + * Memory barrier involvement:
> + *
> + * If data_make_reusable:B reads from
> + * desc_reserve:F, then data_push_tail:C reads
> + * from data_push_tail:D.
> + *
> + * Relies on:
> + *
> + * MB from data_push_tail:D to desc_reserve:F
> + * matching
> + * RMB from data_make_reusable:B to
> + * data_push_tail:C
> + *
> + * Note: data_push_tail:D and desc_reserve:F can
> + * be different CPUs. However, the
> + * desc_reserve:F CPU (which performs the
> + * full memory barrier) must have previously
> + * seen data_push_tail:D.
> + */
> + smp_rmb(); /* LMM(data_push_tail:B) */
> +
> + tail_lpos_new = atomic_long_read(&data_ring->tail_lpos
> + ); /* LMM(data_push_tail:C) */
> + if (tail_lpos_new == tail_lpos)
> + return false;
> +
> + /* Another CPU pushed the tail. Try again. */
> + tail_lpos = tail_lpos_new;
> + continue;
> + }
> +
> + /*
> + * Guarantee any descriptor states that have transitioned to
> + * reusable are stored before pushing the tail lpos. A full
> + * memory barrier is needed since other CPUs may have made
> + * the descriptor states reusable. This pairs with
> + * data_push_tail:A.
> + */
> + if (atomic_long_try_cmpxchg(&data_ring->tail_lpos, &tail_lpos,
> + next_lpos)) { /* LMM(data_push_tail:D) */
> + break;
> + }
> + }
> +
> + return true;
> +}
> +
> +/*
> + * Advance the desc ring tail. This function advances the tail by one
> + * descriptor, thus invalidating the oldest descriptor. Before advancing
> + * the tail, the tail descriptor is made reusable and all data blocks up to
> + * and including the descriptor's data block are invalidated (i.e. the data
> + * ring tail is pushed past the data block of the descriptor being made
> + * reusable).
> + */
> +static bool desc_push_tail(struct printk_ringbuffer *rb,
> + unsigned long tail_id)
> +{
> + struct prb_desc_ring *desc_ring = &rb->desc_ring;
> + enum desc_state d_state;
> + struct prb_desc desc;
> +
> + d_state = desc_read(desc_ring, tail_id, &desc);
> +
> + switch (d_state) {
> + case desc_miss:
> + /*
> + * If the ID is exactly 1 wrap behind the expected, it is
> + * in the process of being reserved by another writer and
> + * must be considered reserved.
> + */
> + if (DESC_ID(atomic_long_read(&desc.state_var)) ==
> + DESC_ID_PREV_WRAP(desc_ring, tail_id)) {
> + return false;
> + }
> +
> + /*
> + * The ID has changed. Another writer must have pushed the
> + * tail and recycled the descriptor already. Success is
> + * returned because the caller is only interested in the
> + * specified tail being pushed, which it was.
> + */
> + return true;
> + case desc_reserved:
> + return false;
> + case desc_committed:
> + desc_make_reusable(desc_ring, tail_id);
> + break;
> + case desc_reusable:
> + break;
> + }
> +
> + /*
> + * Data blocks must be invalidated before their associated
> + * descriptor can be made available for recycling. Invalidating
> + * them later is not possible because there is no way to trust
> + * data blocks once their associated descriptor is gone.
> + */
> +
> + if (!data_push_tail(rb, &rb->text_data_ring, desc.text_blk_lpos.next))
> + return false;
> + if (!data_push_tail(rb, &rb->dict_data_ring, desc.dict_blk_lpos.next))
> + return false;
> +
> + /*
> + * Check the next descriptor after @tail_id before pushing the tail
> + * to it because the tail must always be in a committed or reusable
> + * state. The implementation of prb_first_seq() relies on this.
> + *
> + * A successful read implies that the next descriptor is less than or
> + * equal to @head_id so there is no risk of pushing the tail past the
> + * head.
> + */
> + d_state = desc_read(desc_ring, DESC_ID(tail_id + 1), &desc); /* LMM(desc_push_tail:A) */
> +
> + if (d_state == desc_committed || d_state == desc_reusable) {
> + /*
> + * Guarantee any descriptor states that have transitioned to
> + * reusable are stored before pushing the tail ID. This allows
> + * verifying the recycled descriptor state. A full memory
> + * barrier is needed since other CPUs may have made the
> + * descriptor states reusable. This pairs with desc_reserve:D.
> + */
> + atomic_long_cmpxchg(&desc_ring->tail_id, tail_id,
> + DESC_ID(tail_id + 1)); /* LMM(desc_push_tail:B) */
> + } else {
> + /*
> + * Guarantee the last state load from desc_read() is before
> + * reloading @tail_id in order to see a new tail ID in the
> + * case that the descriptor has been recycled. This pairs
> + * with desc_reserve:D.
> + *
> + * Memory barrier involvement:
> + *
> + * If desc_push_tail:A reads from desc_reserve:F, then
> + * desc_push_tail:D reads from desc_push_tail:B.
> + *
> + * Relies on:
> + *
> + * MB from desc_push_tail:B to desc_reserve:F
> + * matching
> + * RMB from desc_push_tail:A to desc_push_tail:D
> + *
> + * Note: desc_push_tail:B and desc_reserve:F can be different
> + * CPUs. However, the desc_reserve:F CPU (which performs
> + * the full memory barrier) must have previously seen
> + * desc_push_tail:B.
> + */
> + smp_rmb(); /* LMM(desc_push_tail:C) */
> +
> + /*
> + * Re-check the tail ID. The descriptor following @tail_id is
> + * not in an allowed tail state. But if the tail has since
> + * been moved by another CPU, then it does not matter.
> + */
> + if (atomic_long_read(&desc_ring->tail_id) == tail_id) /* LMM(desc_push_tail:D) */
> + return false;
> + }
> +
> + return true;
> +}
> +
> +/* Reserve a new descriptor, invalidating the oldest if necessary. */
> +static bool desc_reserve(struct printk_ringbuffer *rb, unsigned long *id_out)
> +{
> + struct prb_desc_ring *desc_ring = &rb->desc_ring;
> + unsigned long prev_state_val;
> + unsigned long id_prev_wrap;
> + struct prb_desc *desc;
> + unsigned long head_id;
> + unsigned long id;
> +
> + head_id = atomic_long_read(&desc_ring->head_id); /* LMM(desc_reserve:A) */
> +
> + do {
> + desc = to_desc(desc_ring, head_id);
> +
> + id = DESC_ID(head_id + 1);
> + id_prev_wrap = DESC_ID_PREV_WRAP(desc_ring, id);
> +
> + /*
> + * Guarantee the head ID is read before reading the tail ID.
> + * Since the tail ID is updated before the head ID, this
> + * guarantees that @id_prev_wrap is never ahead of the tail
> + * ID. This pairs with desc_reserve:D.
> + *
> + * Memory barrier involvement:
> + *
> + * If desc_reserve:A reads from desc_reserve:D, then
> + * desc_reserve:C reads from desc_push_tail:B.
> + *
> + * Relies on:
> + *
> + * MB from desc_push_tail:B to desc_reserve:D
> + * matching
> + * RMB from desc_reserve:A to desc_reserve:C
> + *
> + * Note: desc_push_tail:B and desc_reserve:D can be different
> + * CPUs. However, the desc_reserve:D CPU (which performs
> + * the full memory barrier) must have previously seen
> + * desc_push_tail:B.
> + */
> + smp_rmb(); /* LMM(desc_reserve:B) */
> +
> + if (id_prev_wrap == atomic_long_read(&desc_ring->tail_id
> + )) { /* LMM(desc_reserve:C) */
> + /*
> + * Make space for the new descriptor by
> + * advancing the tail.
> + */
> + if (!desc_push_tail(rb, id_prev_wrap))
> + return false;
> + }
> +
> + /*
> + * 1. Guarantee the tail ID is read before validating the
> + * recycled descriptor state. A read memory barrier is
> + * sufficient for this. This pairs with desc_push_tail:B.
> + *
> + * Memory barrier involvement:
> + *
> + * If desc_reserve:C reads from desc_push_tail:B, then
> + * desc_reserve:E reads from desc_make_reusable:A.
> + *
> + * Relies on:
> + *
> + * MB from desc_make_reusable:A to desc_push_tail:B
> + * matching
> + * RMB from desc_reserve:C to desc_reserve:E
> + *
> + * Note: desc_make_reusable:A and desc_push_tail:B can be
> + * different CPUs. However, the desc_push_tail:B CPU
> + * (which performs the full memory barrier) must have
> + * previously seen desc_make_reusable:A.
> + *
> + * 2. Guarantee the tail ID is stored before storing the head
> + * ID. This pairs with desc_reserve:B.
> + *
> + * 3. Guarantee any data ring tail changes are stored before
> + * recycling the descriptor. Data ring tail changes can
> + * happen via desc_push_tail()->data_push_tail(). A full
> + * memory barrier is needed since another CPU may have
> + * pushed the data ring tails. This pairs with
> + * data_push_tail:B.
> + *
> + * 4. Guarantee a new tail ID is stored before recycling the
> + * descriptor. A full memory barrier is needed since
> + * another CPU may have pushed the tail ID. This pairs
> + * with desc_push_tail:C and this also pairs with
> + * prb_first_seq:C.
> + */
> + } while (!atomic_long_try_cmpxchg(&desc_ring->head_id, &head_id,
> + id)); /* LMM(desc_reserve:D) */
> +
> + desc = to_desc(desc_ring, id);
> +
> + /*
> + * If the descriptor has been recycled, verify the old state val.
> + * See "ABA Issues" about why this verification is performed.
> + */
> + prev_state_val = atomic_long_read(&desc->state_var); /* LMM(desc_reserve:E) */
> + if (prev_state_val &&
> + prev_state_val != (id_prev_wrap | DESC_COMMITTED_MASK | DESC_REUSE_MASK)) {
> + WARN_ON_ONCE(1);
> + return false;
> + }
> +
> + /*
> + * Assign the descriptor a new ID and set its state to reserved.
> + * See "ABA Issues" about why cmpxchg() instead of set() is used.
> + *
> + * Guarantee the new descriptor ID and state is stored before making
> + * any other changes. A write memory barrier is sufficient for this.
> + * This pairs with desc_read:D.
> + */
> + if (!atomic_long_try_cmpxchg(&desc->state_var, &prev_state_val,
> + id | 0)) { /* LMM(desc_reserve:F) */
> + WARN_ON_ONCE(1);
> + return false;
> + }
> +
> + /* Now data in @desc can be modified: LMM(desc_reserve:G) */
> +
> + *id_out = id;
> + return true;
> +}
> +
> +/* Determine the end of a data block. */
> +static unsigned long get_next_lpos(struct prb_data_ring *data_ring,
> + unsigned long lpos, unsigned int size)
> +{
> + unsigned long begin_lpos;
> + unsigned long next_lpos;
> +
> + begin_lpos = lpos;
> + next_lpos = lpos + size;
> +
> + /* First check if the data block does not wrap. */
> + if (DATA_WRAPS(data_ring, begin_lpos) == DATA_WRAPS(data_ring, next_lpos))
> + return next_lpos;
> +
> + /* Wrapping data blocks store their data at the beginning. */
> + return (DATA_THIS_WRAP_START_LPOS(data_ring, next_lpos) + size);
> +}
> +
> +/*
> + * Allocate a new data block, invalidating the oldest data block(s)
> + * if necessary. This function also associates the data block with
> + * a specified descriptor.
> + */
> +static char *data_alloc(struct printk_ringbuffer *rb,
> + struct prb_data_ring *data_ring, unsigned int size,
> + struct prb_data_blk_lpos *blk_lpos, unsigned long id)
> +{
> + struct prb_data_block *blk;
> + unsigned long begin_lpos;
> + unsigned long next_lpos;
> +
> + if (!data_ring->data || size == 0) {
> + /* Specify a data-less block. */
> + blk_lpos->begin = INVALID_LPOS;
> + blk_lpos->next = INVALID_LPOS;
> + return NULL;
> + }
> +
> + size = to_blk_size(size);
> +
> + begin_lpos = atomic_long_read(&data_ring->head_lpos);
> +
> + do {
> + next_lpos = get_next_lpos(data_ring, begin_lpos, size);
> +
> + if (!data_push_tail(rb, data_ring, next_lpos - DATA_SIZE(data_ring))) {
> + /* Failed to allocate, specify a data-less block. */
> + blk_lpos->begin = INVALID_LPOS;
> + blk_lpos->next = INVALID_LPOS;
> + return NULL;
> + }
> +
> + /*
> + * 1. Guarantee any descriptor states that have transitioned
> + * to reusable are stored before modifying the newly
> + * allocated data area. A full memory barrier is needed
> + * since other CPUs may have made the descriptor states
> + * reusable. See data_push_tail:A about why the reusable
> + * states are visible. This pairs with desc_read:D.
> + *
> + * 2. Guarantee any updated tail lpos is stored before
> + * modifying the newly allocated data area. Another CPU may
> + * be in data_make_reusable() and is reading a block ID
> + * from this area. data_make_reusable() can handle reading
> + * a garbage block ID value, but then it must be able to
> + * load a new tail lpos. A full memory barrier is needed
> + * since other CPUs may have updated the tail lpos. This
> + * pairs with data_push_tail:B.
> + */
> + } while (!atomic_long_try_cmpxchg(&data_ring->head_lpos, &begin_lpos,
> + next_lpos)); /* LMM(data_alloc:A) */
> +
> + blk = to_block(data_ring, begin_lpos);
> + blk->id = id; /* LMM(data_alloc:B) */
> +
> + if (DATA_WRAPS(data_ring, begin_lpos) != DATA_WRAPS(data_ring, next_lpos)) {
> + /* Wrapping data blocks store their data at the beginning. */
> + blk = to_block(data_ring, 0);
> +
> + /*
> + * Store the ID on the wrapped block for consistency.
> + * The printk_ringbuffer does not actually use it.
> + */
> + blk->id = id;
> + }
> +
> + blk_lpos->begin = begin_lpos;
> + blk_lpos->next = next_lpos;
> +
> + return &blk->data[0];
> +}
> +
> +/* Return the number of bytes used by a data block. */
> +static unsigned int space_used(struct prb_data_ring *data_ring,
> + struct prb_data_blk_lpos *blk_lpos)
> +{
> + if (DATA_WRAPS(data_ring, blk_lpos->begin) == DATA_WRAPS(data_ring, blk_lpos->next)) {
> + /* Data block does not wrap. */
> + return (DATA_INDEX(data_ring, blk_lpos->next) -
> + DATA_INDEX(data_ring, blk_lpos->begin));
> + }
> +
> + /*
> + * For wrapping data blocks, the trailing (wasted) space is
> + * also counted.
> + */
> + return (DATA_INDEX(data_ring, blk_lpos->next) +
> + DATA_SIZE(data_ring) - DATA_INDEX(data_ring, blk_lpos->begin));
> +}
> +
> +/**
> + * prb_reserve() - Reserve space in the ringbuffer.
> + *
> + * @e: The entry structure to setup.
> + * @rb: The ringbuffer to reserve data in.
> + * @r: The record structure to allocate buffers for.
> + *
> + * This is the public function available to writers to reserve data.
> + *
> + * The writer specifies the text and dict sizes to reserve by setting the
> + * @text_buf_size and @dict_buf_size fields of @r, respectively. Dictionaries
> + * are optional, so @dict_buf_size is allowed to be 0. To ensure proper
> + * initialization of @r, prb_rec_init_wr() should be used.
> + *
> + * Context: Any context. Disables local interrupts on success.
> + * Return: true if at least text data could be allocated, otherwise false.
> + *
> + * On success, the fields @info, @text_buf, @dict_buf of @r will be set by
> + * this function and should be filled in by the writer before committing. Also
> + * on success, prb_record_text_space() can be used on @e to query the actual
> + * space used for the text data block.
> + *
> + * If the function fails to reserve dictionary space (but all else succeeded),
> + * it will still report success. In that case @dict_buf is set to NULL and
> + * @dict_buf_size is set to 0. Writers must check this before writing to
> + * dictionary space.
> + *
> + * @info->text_len and @info->dict_len will already be set to @text_buf_size
> + * and @dict_buf_size, respectively. If dictionary space reservation fails,
> + * @info->dict_len is set to 0.
> + */
> +bool prb_reserve(struct prb_reserved_entry *e, struct printk_ringbuffer *rb,
> + struct printk_record *r)
> +{
> + struct prb_desc_ring *desc_ring = &rb->desc_ring;
> + struct prb_desc *d;
> + unsigned long id;
> +
> + if (!data_check_size(&rb->text_data_ring, r->text_buf_size))
> + goto fail;
> +
> + /* Records are allowed to not have dictionaries. */
> + if (r->dict_buf_size) {
> + if (!data_check_size(&rb->dict_data_ring, r->dict_buf_size))
> + goto fail;
> + }
> +
> + /*
> + * Descriptors in the reserved state act as blockers to all further
> + * reservations once the desc_ring has fully wrapped. Disable
> + * interrupts during the reserve/commit window in order to minimize
> + * the likelihood of this happening.
> + */
> + local_irq_save(e->irqflags);
> +
> + if (!desc_reserve(rb, &id)) {
> + /* Descriptor reservation failures are tracked. */
> + atomic_long_inc(&rb->fail);
> + local_irq_restore(e->irqflags);
> + goto fail;
> + }
> +
> + d = to_desc(desc_ring, id);
> +
> + /*
> + * Set the @e fields here so that prb_commit() can be used if
> + * text data allocation fails.
> + */
> + e->rb = rb;
> + e->id = id;
> +
> + /*
> + * Initialize the sequence number if it has "never been set".
> + * Otherwise just increment it by a full wrap.
> + *
> + * @seq is considered "never been set" if it has a value of 0,
> + * _except_ for @descs[0], which was specially setup by the ringbuffer
> + * initializer and therefore is always considered as set.
> + *
> + * See the "Bootstrap" comment block in printk_ringbuffer.h for
> + * details about how the initializer bootstraps the descriptors.
> + */
> + if (d->info.seq == 0 && DESC_INDEX(desc_ring, id) != 0)
> + d->info.seq = DESC_INDEX(desc_ring, id);
> + else
> + d->info.seq += DESCS_COUNT(desc_ring);
> +
> + r->text_buf = data_alloc(rb, &rb->text_data_ring, r->text_buf_size,
> + &d->text_blk_lpos, id);
> + /* If text data allocation fails, a data-less record is committed. */
> + if (r->text_buf_size && !r->text_buf) {
> + d->info.text_len = 0;
> + d->info.dict_len = 0;
> + prb_commit(e);
> + /* prb_commit() re-enabled interrupts. */
> + goto fail;
> + }
> +
> + r->dict_buf = data_alloc(rb, &rb->dict_data_ring, r->dict_buf_size,
> + &d->dict_blk_lpos, id);
> + /*
> + * If dict data allocation fails, the caller can still commit
> + * text. But dictionary information will not be available.
> + */
> + if (r->dict_buf_size && !r->dict_buf)
> + r->dict_buf_size = 0;
> +
> + r->info = &d->info;
> +
> + /* Set default values for the sizes. */
> + d->info.text_len = r->text_buf_size;
> + d->info.dict_len = r->dict_buf_size;
> +
> + /* Record full text space used by record. */
> + e->text_space = space_used(&rb->text_data_ring, &d->text_blk_lpos);
> +
> + return true;
> +fail:
> + /* Make it clear to the caller that the reserve failed. */
> + memset(r, 0, sizeof(*r));
> + return false;
> +}
> +
> +/**
> + * prb_commit() - Commit (previously reserved) data to the ringbuffer.
> + *
> + * @e: The entry containing the reserved data information.
> + *
> + * This is the public function available to writers to commit data.
> + *
> + * Context: Any context. Enables local interrupts.
> + */
> +void prb_commit(struct prb_reserved_entry *e)
> +{
> + struct prb_desc_ring *desc_ring = &e->rb->desc_ring;
> + struct prb_desc *d = to_desc(desc_ring, e->id);
> + unsigned long prev_state_val = e->id | 0;
> +
> + /* Now the writer has finished all writing: LMM(prb_commit:A) */
> +
> + /*
> + * Set the descriptor as committed. See "ABA Issues" about why
> + * cmpxchg() instead of set() is used.
> + *
> + * Guarantee all record data is stored before the descriptor state
> + * is stored as committed. A write memory barrier is sufficient for
> + * this. This pairs with desc_read:B.
> + */
> + if (!atomic_long_try_cmpxchg(&d->state_var, &prev_state_val,
> + e->id | DESC_COMMITTED_MASK)) { /* LMM(prb_commit:B) */
> + WARN_ON_ONCE(1);
> + }
> +
> + /* Restore interrupts, the reserve/commit window is finished. */
> + local_irq_restore(e->irqflags);
> +}
> +
> +/*
> + * Given @blk_lpos, return a pointer to the raw data from the data block
> + * and calculate the size of the data part. A NULL pointer is returned
> + * if @blk_lpos specifies values that could never be legal.
> + *
> + * This function (used by readers) performs strict validation on the lpos
> + * values to possibly detect bugs in the writer code. A WARN_ON_ONCE() is
> + * triggered if an internal error is detected.
> + */
> +static char *get_data(struct prb_data_ring *data_ring,
> + struct prb_data_blk_lpos *blk_lpos,
> + unsigned int *data_size)
> +{
> + struct prb_data_block *db;
> +
> + /* Data-less data block description. */
> + if (blk_lpos->begin == INVALID_LPOS &&
> + blk_lpos->next == INVALID_LPOS) {
> + return NULL;
> + }
> +
> + /* Regular data block: @begin less than @next and in same wrap. */
> + if (DATA_WRAPS(data_ring, blk_lpos->begin) == DATA_WRAPS(data_ring, blk_lpos->next) &&
> + blk_lpos->begin < blk_lpos->next) {
> + db = to_block(data_ring, blk_lpos->begin);
> + *data_size = blk_lpos->next - blk_lpos->begin;
> +
> + /* Wrapping data block: @begin is one wrap behind @next. */
> + } else if (DATA_WRAPS(data_ring, blk_lpos->begin + DATA_SIZE(data_ring)) ==
> + DATA_WRAPS(data_ring, blk_lpos->next)) {
> + db = to_block(data_ring, 0);
> + *data_size = DATA_INDEX(data_ring, blk_lpos->next);
> +
> + /* Illegal block description. */
> + } else {
> + WARN_ON_ONCE(1);
> + return NULL;
> + }
> +
> + /* A valid data block will always be aligned to the ID size. */
> + if (WARN_ON_ONCE(blk_lpos->begin != ALIGN(blk_lpos->begin, sizeof(db->id))) ||
> + WARN_ON_ONCE(blk_lpos->next != ALIGN(blk_lpos->next, sizeof(db->id)))) {
> + return NULL;
> + }
> +
> + /* A valid data block will always have at least an ID. */
> + if (WARN_ON_ONCE(*data_size < sizeof(db->id)))
> + return NULL;
> +
> + /* Subtract block ID space from size to reflect data size. */
> + *data_size -= sizeof(db->id);
> +
> + return &db->data[0];
> +}
> +
> +/**
> + * prb_count_lines() - Count the number of lines in provided text.
> + *
> + * @text: The text to count the lines of.
> + * @text_size: The size of the text to process.
> + *
> + * This is the public function available to readers to count the number of
> + * lines in a text string.
> + *
> + * Context: Any context.
> + * Return: The number of lines in the text.
> + *
> + * All text has at least 1 line (even if @text_size is 0). Each '\n'
> + * processed is counted as an additional line.
> + */
> +unsigned int prb_count_lines(char *text, unsigned int text_size)
> +{
> + unsigned int next_size = text_size;
> + unsigned int line_count = 1;
> + char *next = text;
> +
> + while (next_size) {
> + next = memchr(next, '\n', next_size);
> + if (!next)
> + break;
> + line_count++;
> + next++;
> + next_size = text_size - (next - text);
> + }
> +
> + return line_count;
> +}
> +
> +/*
> + * Given @blk_lpos, copy an expected @len of data into the provided buffer.
> + * If @line_count is provided, count the number of lines in the data.
> + *
> + * This function (used by readers) performs strict validation on the data
> + * size to possibly detect bugs in the writer code. A WARN_ON_ONCE() is
> + * triggered if an internal error is detected.
> + */
> +static bool copy_data(struct prb_data_ring *data_ring,
> + struct prb_data_blk_lpos *blk_lpos, u16 len, char *buf,
> + unsigned int buf_size, unsigned int *line_count)
> +{
> + unsigned int data_size;
> + char *data;
> +
> + /* Caller might not want any data. */
> + if ((!buf || !buf_size) && !line_count)
> + return true;
> +
> + data = get_data(data_ring, blk_lpos, &data_size);
> + if (!data)
> + return false;
> +
> + /*
> + * Actual cannot be less than expected. It can be more than expected
> + * because of the trailing alignment padding.
> + */
> + if (WARN_ON_ONCE(data_size < (unsigned int)len)) {
> + pr_warn_once("wrong data size (%u, expecting %hu) for data: %.*s\n",
> + data_size, len, data_size, data);
> + return false;
> + }
> +
> + /* Caller interested in the line count? */
> + if (line_count)
> + *line_count = prb_count_lines(data, data_size);
> +
> + /* Caller interested in the data content? */
> + if (!buf || !buf_size)
> + return true;
> +
> + data_size = min_t(u16, buf_size, len);
> +
> + if (!WARN_ON_ONCE(!data_size))
> + memcpy(&buf[0], data, data_size); /* LMM(copy_data:A) */
> + return true;
> +}
> +
> +/*
> + * This is an extended version of desc_read(). It gets a copy of a specified
> + * descriptor. However, it also verifies that the record is committed and has
> + * the sequence number @seq. On success, 0 is returned.
> + *
> + * Error return values:
> + * -EINVAL: A committed record with sequence number @seq does not exist.
> + * -ENOENT: A committed record with sequence number @seq exists, but its data
> + * is not available. This is a valid record, so readers should
> + * continue with the next record.
> + */
> +static int desc_read_committed_seq(struct prb_desc_ring *desc_ring,
> + unsigned long id, u64 seq,
> + struct prb_desc *desc_out)
> +{
> + struct prb_data_blk_lpos *blk_lpos = &desc_out->text_blk_lpos;
> + enum desc_state d_state;
> +
> + d_state = desc_read(desc_ring, id, desc_out);
> +
> + /*
> + * An unexpected @id (desc_miss) or @seq mismatch means the record
> + * does not exist. A descriptor in the reserved state means the
> + * record does not yet exist for the reader.
> + */
> + if (d_state == desc_miss ||
> + d_state == desc_reserved ||
> + desc_out->info.seq != seq) {
> + return -EINVAL;
> + }
> +
> + /*
> + * A descriptor in the reusable state may no longer have its data
> + * available; report it as a data-less record. Or the record may
> + * actually be a data-less record.
> + */
> + if (d_state == desc_reusable ||
> + (blk_lpos->begin == INVALID_LPOS && blk_lpos->next == INVALID_LPOS)) {
> + return -ENOENT;
> + }
> +
> + return 0;
> +}
> +
> +/*
> + * Copy the ringbuffer data from the record with @seq to the provided
> + * @r buffer. On success, 0 is returned.
> + *
> + * See desc_read_committed_seq() for error return values.
> + */
> +static int prb_read(struct printk_ringbuffer *rb, u64 seq,
> + struct printk_record *r, unsigned int *line_count)
> +{
> + struct prb_desc_ring *desc_ring = &rb->desc_ring;
> + struct prb_desc *rdesc = to_desc(desc_ring, seq);
> + atomic_long_t *state_var = &rdesc->state_var;
> + struct prb_desc desc;
> + unsigned long id;
> + int err;
> +
> + /* Extract the ID, used to specify the descriptor to read. */
> + id = DESC_ID(atomic_long_read(state_var));
> +
> + /* Get a local copy of the correct descriptor (if available). */
> + err = desc_read_committed_seq(desc_ring, id, seq, &desc);
> +
> + /*
> + * If @r is NULL, the caller is only interested in the availability
> + * of the record.
> + */
> + if (err || !r)
> + return err;
> +
> + /* If requested, copy meta data. */
> + if (r->info)
> + memcpy(r->info, &desc.info, sizeof(*(r->info)));
> +
> + /* Copy text data. If it fails, this is a data-less descriptor. */
> + if (!copy_data(&rb->text_data_ring, &desc.text_blk_lpos, desc.info.text_len,
> + r->text_buf, r->text_buf_size, line_count)) {
> + return -ENOENT;
> + }
> +
> + /*
> + * Copy dict data. Although this should not fail, dict data is not
> + * important. So if it fails, modify the copied meta data to report
> + * that there is no dict data, thus silently dropping the dict data.
> + */
> + if (!copy_data(&rb->dict_data_ring, &desc.dict_blk_lpos, desc.info.dict_len,
> + r->dict_buf, r->dict_buf_size, NULL)) {
> + if (r->info)
> + r->info->dict_len = 0;
> + }
> +
> + /* Ensure the record is still committed and has the same @seq. */
> + return desc_read_committed_seq(desc_ring, id, seq, &desc);
> +}
> +
> +/**
> + * prb_first_seq() - Get the sequence number of the tail descriptor.
> + *
> + * @rb: The ringbuffer to get the sequence number from.
> + *
> + * This is the public function available to readers to see what the
> + * first/oldest sequence number is.
> + *
> + * This provides readers a starting point to begin iterating the ringbuffer.
> + * Note that the returned sequence number might not belong to a valid record.
> + *
> + * Context: Any context.
> + * Return: The sequence number of the first/oldest record or, if the
> + * ringbuffer is empty, 0 is returned.
> + */
> +u64 prb_first_seq(struct printk_ringbuffer *rb)
> +{
> + struct prb_desc_ring *desc_ring = &rb->desc_ring;
> + enum desc_state d_state;
> + struct prb_desc desc;
> + unsigned long id;
> +
> + for (;;) {
> + id = atomic_long_read(&rb->desc_ring.tail_id); /* LMM(prb_first_seq:A) */
> +
> + d_state = desc_read(desc_ring, id, &desc); /* LMM(prb_first_seq:B) */
> +
> + /*
> + * This loop will not be infinite because the tail is
> + * _always_ in the committed or reusable state.
> + */
> + if (d_state == desc_committed || d_state == desc_reusable)
> + break;
> +
> + /*
> + * Guarantee the last state load from desc_read() is before
> + * reloading @tail_id in order to see a new tail in the case
> + * that the descriptor has been recycled. This pairs with
> + * desc_reserve:D.
> + *
> + * Memory barrier involvement:
> + *
> + * If prb_first_seq:B reads from desc_reserve:F, then
> + * prb_first_seq:A reads from desc_push_tail:B.
> + *
> + * Relies on:
> + *
> + * MB from desc_push_tail:B to desc_reserve:F
> + * matching
> + * RMB prb_first_seq:B to prb_first_seq:A
> + */
> + smp_rmb(); /* LMM(prb_first_seq:C) */
> + }
> +
> + return desc.info.seq;
> +}
> +
> +/*
> + * Non-blocking read of a record. Updates @seq to the last committed record
> + * (which may have no data).
> + *
> + * See the description of prb_read_valid() and prb_read_valid_info()
> + * for details.
> + */
> +static bool _prb_read_valid(struct printk_ringbuffer *rb, u64 *seq,
> + struct printk_record *r, unsigned int *line_count)
> +{
> + u64 tail_seq;
> + int err;
> +
> + while ((err = prb_read(rb, *seq, r, line_count))) {
> + tail_seq = prb_first_seq(rb);
> +
> + if (*seq < tail_seq) {
> + /*
> + * Behind the tail. Catch up and try again. This
> + * can happen for -ENOENT and -EINVAL cases.
> + */
> + *seq = tail_seq;
> +
> + } else if (err == -ENOENT) {
> + /* Record exists, but no data available. Skip. */
> + (*seq)++;
> +
> + } else {
> + /* Non-existent/non-committed record. Must stop. */
> + return false;
> + }
> + }
> +
> + return true;
> +}
> +
> +/**
> + * prb_read_valid() - Non-blocking read of a requested record or (if gone)
> + * the next available record.
> + *
> + * @rb: The ringbuffer to read from.
> + * @seq: The sequence number of the record to read.
> + * @r: A record data buffer to store the read record to.
> + *
> + * This is the public function available to readers to read a record.
> + *
> + * The reader provides the @info, @text_buf, @dict_buf buffers of @r to be
> + * filled in. Any of the buffer pointers can be set to NULL if the reader
> + * is not interested in that data. To ensure proper initialization of @r,
> + * prb_rec_init_rd() should be used.
> + *
> + * Context: Any context.
> + * Return: true if a record was read, otherwise false.
> + *
> + * On success, the reader must check r->info.seq to see which record was
> + * actually read. This allows the reader to detect dropped records.
> + *
> + * Failure means @seq refers to a not yet written record.
> + */
> +bool prb_read_valid(struct printk_ringbuffer *rb, u64 seq,
> + struct printk_record *r)
> +{
> + return _prb_read_valid(rb, &seq, r, NULL);
> +}
> +
> +/**
> + * prb_read_valid_info() - Non-blocking read of meta data for a requested
> + * record or (if gone) the next available record.
> + *
> + * @rb: The ringbuffer to read from.
> + * @seq: The sequence number of the record to read.
> + * @info: A buffer to store the read record meta data to.
> + * @line_count: A buffer to store the number of lines in the record text.
> + *
> + * This is the public function available to readers to read only the
> + * meta data of a record.
> + *
> + * The reader provides the @info, @line_count buffers to be filled in.
> + * Either of the buffer pointers can be set to NULL if the reader is not
> + * interested in that data.
> + *
> + * Context: Any context.
> + * Return: true if a record's meta data was read, otherwise false.
> + *
> + * On success, the reader must check info->seq to see which record meta data
> + * was actually read. This allows the reader to detect dropped records.
> + *
> + * Failure means @seq refers to a not yet written record.
> + */
> +bool prb_read_valid_info(struct printk_ringbuffer *rb, u64 seq,
> + struct printk_info *info, unsigned int *line_count)
> +{
> + struct printk_record r;
> +
> + prb_rec_init_rd(&r, info, NULL, 0, NULL, 0);
> +
> + return _prb_read_valid(rb, &seq, &r, line_count);
> +}
> +
> +/**
> + * prb_next_seq() - Get the sequence number after the last available record.
> + *
> + * @rb: The ringbuffer to get the sequence number from.
> + *
> + * This is the public function available to readers to see what the next
> + * newest sequence number available to readers will be.
> + *
> + * This provides readers a sequence number to jump to if all currently
> + * available records should be skipped.
> + *
> + * Context: Any context.
> + * Return: The sequence number of the next newest (not yet available) record
> + * for readers.
> + */
> +u64 prb_next_seq(struct printk_ringbuffer *rb)
> +{
> + u64 seq = 0;
> +
> + do {
> + /* Search forward from the oldest descriptor. */
> + if (!_prb_read_valid(rb, &seq, NULL, NULL))
> + return seq;
> + seq++;
> + } while (seq);
> +
> + return 0;
> +}
> +
> +/**
> + * prb_init() - Initialize a ringbuffer to use provided external buffers.
> + *
> + * @rb: The ringbuffer to initialize.
> + * @text_buf: The data buffer for text data.
> + * @textbits: The size of @text_buf as a power-of-2 value.
> + * @dict_buf: The data buffer for dictionary data.
> + * @dictbits: The size of @dict_buf as a power-of-2 value.
> + * @descs: The descriptor buffer for ringbuffer records.
> + * @descbits: The count of @descs items as a power-of-2 value.
> + *
> + * This is the public function available to writers to setup a ringbuffer
> + * during runtime using provided buffers.
> + *
> + * This must match the initialization of DECLARE_PRINTKRB().
> + *
> + * Context: Any context.
> + */
> +void prb_init(struct printk_ringbuffer *rb,
> + char *text_buf, unsigned int textbits,
> + char *dict_buf, unsigned int dictbits,
> + struct prb_desc *descs, unsigned int descbits)
> +{
> + memset(descs, 0, _DESCS_COUNT(descbits) * sizeof(descs[0]));
> +
> + rb->desc_ring.count_bits = descbits;
> + rb->desc_ring.descs = descs;
> + atomic_long_set(&rb->desc_ring.head_id, DESC0_ID(descbits));
> + atomic_long_set(&rb->desc_ring.tail_id, DESC0_ID(descbits));
> +
> + rb->text_data_ring.size_bits = textbits;
> + rb->text_data_ring.data = text_buf;
> + atomic_long_set(&rb->text_data_ring.head_lpos, BLK0_LPOS(textbits));
> + atomic_long_set(&rb->text_data_ring.tail_lpos, BLK0_LPOS(textbits));
> +
> + rb->dict_data_ring.size_bits = dictbits;
> + rb->dict_data_ring.data = dict_buf;
> + atomic_long_set(&rb->dict_data_ring.head_lpos, BLK0_LPOS(dictbits));
> + atomic_long_set(&rb->dict_data_ring.tail_lpos, BLK0_LPOS(dictbits));
> +
> + atomic_long_set(&rb->fail, 0);
> +
> + descs[0].info.seq = -(u64)_DESCS_COUNT(descbits);
> +
> + descs[_DESCS_COUNT(descbits) - 1].info.seq = 0;
> + atomic_long_set(&(descs[_DESCS_COUNT(descbits) - 1].state_var), DESC0_SV(descbits));
> + descs[_DESCS_COUNT(descbits) - 1].text_blk_lpos.begin = INVALID_LPOS;
> + descs[_DESCS_COUNT(descbits) - 1].text_blk_lpos.next = INVALID_LPOS;
> + descs[_DESCS_COUNT(descbits) - 1].dict_blk_lpos.begin = INVALID_LPOS;
> + descs[_DESCS_COUNT(descbits) - 1].dict_blk_lpos.next = INVALID_LPOS;
> +}
> +
> +/**
> + * prb_record_text_space() - Query the full actual used ringbuffer space for
> + * the text data of a reserved entry.
> + *
> + * @e: The successfully reserved entry to query.
> + *
> + * This is the public function available to writers to see how much actual
> + * space is used in the ringbuffer to store the text data of the specified
> + * entry.
> + *
> + * This function is only valid if @e has been successfully reserved using
> + * prb_reserve().
> + *
> + * Context: Any context.
> + * Return: The size in bytes used by the text data of the associated record.
> + */
> +unsigned int prb_record_text_space(struct prb_reserved_entry *e)
> +{
> + return e->text_space;
> +}
> diff --git a/kernel/printk/printk_ringbuffer.h b/kernel/printk/printk_ringbuffer.h
> new file mode 100644
> index 000000000000..df03039dca7e
> --- /dev/null
> +++ b/kernel/printk/printk_ringbuffer.h
> @@ -0,0 +1,352 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#ifndef _KERNEL_PRINTK_RINGBUFFER_H
> +#define _KERNEL_PRINTK_RINGBUFFER_H
> +
> +#include <linux/atomic.h>
> +
> +struct printk_info {
> + u64 seq; /* sequence number */
> + u64 ts_nsec; /* timestamp in nanoseconds */
> + u16 text_len; /* length of text message */
> + u16 dict_len; /* length of dictionary message */
> + u8 facility; /* syslog facility */
> + u8 flags:5; /* internal record flags */
> + u8 level:3; /* syslog level */
> + u32 caller_id; /* thread id or processor id */
> +};
> +
> +/*
> + * A structure providing the buffers, used by writers and readers.
> + *
> + * Writers:
> + * Using prb_rec_init_wr(), a writer sets @text_buf_size and @dict_buf_size
> + * before calling prb_reserve(). On success, prb_reserve() sets @info,
> + * @text_buf, @dict_buf to buffers reserved for that writer.
> + *
> + * Readers:
> + * Using prb_rec_init_rd(), a reader sets all fields before calling
> + * prb_read_valid(). Note that the reader provides the @info, @text_buf,
> + * @dict_buf buffers. On success, the struct pointed to by @info will be
> + * filled and the char arrays pointed to by @text_buf and @dict_buf will
> + * be filled with text and dict data.
> + */
> +struct printk_record {
> + struct printk_info *info;
> + char *text_buf;
> + char *dict_buf;
> + unsigned int text_buf_size;
> + unsigned int dict_buf_size;
> +};
> +
> +/* Specifies the position/span of a data block. */
> +struct prb_data_blk_lpos {
> + unsigned long begin;
> + unsigned long next;
> +};
> +
> +/* A descriptor: the complete meta-data for a record. */
> +struct prb_desc {
> + struct printk_info info;
> + atomic_long_t state_var;
> + struct prb_data_blk_lpos text_blk_lpos;
> + struct prb_data_blk_lpos dict_blk_lpos;
> +};
> +
> +/* A ringbuffer of "ID + data" elements. */
> +struct prb_data_ring {
> + unsigned int size_bits;
> + char *data;
> + atomic_long_t head_lpos;
> + atomic_long_t tail_lpos;
> +};
> +
> +/* A ringbuffer of "struct prb_desc" elements. */
> +struct prb_desc_ring {
> + unsigned int count_bits;
> + struct prb_desc *descs;
> + atomic_long_t head_id;
> + atomic_long_t tail_id;
> +};
> +
> +/* The high level structure representing the printk ringbuffer. */
> +struct printk_ringbuffer {
> + struct prb_desc_ring desc_ring;
> + struct prb_data_ring text_data_ring;
> + struct prb_data_ring dict_data_ring;
> + atomic_long_t fail;
> +};
> +
> +/* Used by writers as a reserve/commit handle. */
> +struct prb_reserved_entry {
> + struct printk_ringbuffer *rb;
> + unsigned long irqflags;
> + unsigned long id;
> + unsigned int text_space;
> +};
> +
> +#define _DATA_SIZE(sz_bits) (1UL << (sz_bits))
> +#define _DESCS_COUNT(ct_bits) (1U << (ct_bits))
> +#define DESC_SV_BITS (sizeof(unsigned long) * 8)
> +#define DESC_COMMITTED_MASK (1UL << (DESC_SV_BITS - 1))
> +#define DESC_REUSE_MASK (1UL << (DESC_SV_BITS - 2))
> +#define DESC_FLAGS_MASK (DESC_COMMITTED_MASK | DESC_REUSE_MASK)
> +#define DESC_ID_MASK (~DESC_FLAGS_MASK)
> +#define DESC_ID(sv) ((sv) & DESC_ID_MASK)
> +#define INVALID_LPOS 1
> +
> +#define INVALID_BLK_LPOS \
> +{ \
> + .begin = INVALID_LPOS, \
> + .next = INVALID_LPOS, \
> +}
> +
> +/*
> + * Descriptor Bootstrap
> + *
> + * The descriptor array is minimally initialized to allow immediate usage
> + * by readers and writers. The requirements that the descriptor array
> + * initialization must satisfy:
> + *
> + * Req1
> + * The tail must point to an existing (committed or reusable) descriptor.
> + * This is required by the implementation of prb_first_seq().
> + *
> + * Req2
> + * Readers must see that the ringbuffer is initially empty.
> + *
> + * Req3
> + * The first record reserved by a writer is assigned sequence number 0.
> + *
> + * To satisfy Req1, the tail initially points to a descriptor that is
> + * minimally initialized (having no data block, i.e. data-less with the
> + * data block's lpos @begin and @next values set to INVALID_LPOS).
> + *
> + * To satisfy Req2, the initial tail descriptor is initialized to the
> + * reusable state. Readers recognize reusable descriptors as existing
> + * records, but skip over them.
> + *
> + * To satisfy Req3, the last descriptor in the array is used as the initial
> + * head (and tail) descriptor. This allows the first record reserved by a
> + * writer (head + 1) to be the first descriptor in the array. (Only the first
> + * descriptor in the array could have a valid sequence number of 0.)
> + *
> + * The first time a descriptor is reserved, it is assigned a sequence number
> + * with the value of the array index. A "first time reserved" descriptor can
> + * be recognized because it has a sequence number of 0 but does not have an
> + * index of 0. (Only the first descriptor in the array could have a valid
> + * sequence number of 0.) After the first reservation, all future reservations
> + * (recycling) simply involve incrementing the sequence number by the array
> + * count.
> + *
> + * Hack #1
> + * Only the first descriptor in the array is allowed to have the sequence
> + * number 0. In this case it is not possible to recognize if it is being
> + * reserved the first time (set to index value) or has been reserved
> + * previously (increment by the array count). This is handled by _always_
> + * incrementing the sequence number by the array count when reserving the
> + * first descriptor in the array. In order to satisfy Req3, the sequence
> + * number of the first descriptor in the array is initialized to minus
> + * the array count. Then, upon the first reservation, it is incremented
> + * to 0, thus satisfying Req3.
> + *
> + * Hack #2
> + * prb_first_seq() can be called at any time by readers to retrieve the
> + * sequence number of the tail descriptor. However, due to Req2 and Req3,
> + * initially there are no records to report the sequence number of
> + * (sequence numbers are u64 and there is nothing less than 0). To handle
> + * this, the sequence number of the initial tail descriptor is initialized
> + * to 0. Technically this is incorrect, because there is no record with
> + * sequence number 0 (yet) and the tail descriptor is not the first
> + * descriptor in the array. But it allows prb_read_valid() to correctly
> + * report the existence of a record for _any_ given sequence number at all
> + * times. Bootstrapping is complete when the tail is pushed the first
> + * time, thus finally pointing to the first descriptor reserved by a
> + * writer, which has the assigned sequence number 0.
> + */
> +
> +/*
> + * Initiating Logical Value Overflows
> + *
> + * Both logical position (lpos) and ID values can be mapped to array indexes
> + * but may experience overflows during the lifetime of the system. To ensure
> + * that printk_ringbuffer can handle the overflows for these types, initial
> + * values are chosen that map to the correct initial array indexes, but will
> + * result in overflows soon.
> + *
> + * BLK0_LPOS
> + * The initial @head_lpos and @tail_lpos for data rings. It is at index
> + * 0 and the lpos value is such that it will overflow on the first wrap.
> + *
> + * DESC0_ID
> + * The initial @head_id and @tail_id for the desc ring. It is at the last
> + * index of the descriptor array (see Req3 above) and the ID value is such
> + * that it will overflow on the second wrap.
> + */
> +#define BLK0_LPOS(sz_bits) (-(_DATA_SIZE(sz_bits)))
> +#define DESC0_ID(ct_bits) DESC_ID(-(_DESCS_COUNT(ct_bits) + 1))
> +#define DESC0_SV(ct_bits) (DESC_COMMITTED_MASK | DESC_REUSE_MASK | DESC0_ID(ct_bits))
> +
> +/*
> + * Declare a ringbuffer with an external text data buffer. The same as
> + * DECLARE_PRINTKRB() but requires specifying an external buffer for the
> + * text data.
> + *
> + * Note: The specified external buffer must be of the size:
> + * 2 ^ (descbits + avgtextbits)
> + */
> +#define _DECLARE_PRINTKRB(name, descbits, avgtextbits, avgdictbits, text_buf) \
> +char _##name##_dict[1U << ((avgdictbits) + (descbits))] __aligned(__alignof__(unsigned long)); \
> +struct prb_desc _##name##_descs[_DESCS_COUNT(descbits)] = { \
> + /* this will be the first record reserved by a writer */ \
> + [0] = { \
> + .info = { \
> + /* will be incremented to 0 on the first reservation */ \
> + .seq = -(u64)_DESCS_COUNT(descbits), \
> + }, \
> + }, \
> + /* the initial head and tail */ \
> + [_DESCS_COUNT(descbits) - 1] = { \
> + .info = { \
> + /* reports the first seq value during the bootstrap phase */ \
> + .seq = 0, \
> + }, \
> + /* reusable */ \
> + .state_var = ATOMIC_INIT(DESC0_SV(descbits)), \
> + /* no associated data block */ \
> + .text_blk_lpos = INVALID_BLK_LPOS, \
> + .dict_blk_lpos = INVALID_BLK_LPOS, \
> + }, \
> +}; \
> +struct printk_ringbuffer name = { \
> + .desc_ring = { \
> + .count_bits = descbits, \
> + .descs = &_##name##_descs[0], \
> + .head_id = ATOMIC_INIT(DESC0_ID(descbits)), \
> + .tail_id = ATOMIC_INIT(DESC0_ID(descbits)), \
> + }, \
> + .text_data_ring = { \
> + .size_bits = (avgtextbits) + (descbits), \
> + .data = text_buf, \
> + .head_lpos = ATOMIC_LONG_INIT(BLK0_LPOS((avgtextbits) + (descbits))), \
> + .tail_lpos = ATOMIC_LONG_INIT(BLK0_LPOS((avgtextbits) + (descbits))), \
> + }, \
> + .dict_data_ring = { \
> + .size_bits = (avgtextbits) + (descbits), \
> + .data = &_##name##_dict[0], \
> + .head_lpos = ATOMIC_LONG_INIT(BLK0_LPOS((avgtextbits) + (descbits))), \
> + .tail_lpos = ATOMIC_LONG_INIT(BLK0_LPOS((avgtextbits) + (descbits))), \
> + }, \
> + .fail = ATOMIC_LONG_INIT(0), \
> +}
> +
> +/**
> + * DECLARE_PRINTKRB() - Declare a ringbuffer.
> + *
> + * @name: The name of the ringbuffer variable.
> + * @descbits: The number of descriptors as a power-of-2 value.
> + * @avgtextbits: The average text data size per record as a power-of-2 value.
> + * @avgdictbits: The average dictionary data size per record as a
> + * power-of-2 value.
> + *
> + * This is a macro for declaring a ringbuffer and all internal structures
> + * such that it is ready for immediate use. See _DECLARE_PRINTKRB() for a
> + * variant where the text data buffer can be specified externally.
> + */
> +#define DECLARE_PRINTKRB(name, descbits, avgtextbits, avgdictbits) \
> +char _##name##_text[1U << ((avgtextbits) + (descbits))] __aligned(__alignof__(unsigned long)); \
> +_DECLARE_PRINTKRB(name, descbits, avgtextbits, avgdictbits, &_##name##_text[0])
> +
> +/* Writer Interface */
> +
> +/**
> + * prb_rec_init_wd() - Initialize a buffer for writing records.
> + *
> + * @r: The record to initialize.
> + * @text_buf_size: The needed text buffer size.
> + * @dict_buf_size: The needed dictionary buffer size.
> + *
> + * Initialize all the fields that a writer is interested in. If
> + * @dict_buf_size is 0, a dictionary buffer will not be reserved.
> + * @text_buf_size must be greater than 0.
> + *
> + * Note that although @dict_buf_size may be initialized to non-zero,
> + * its value must be rechecked after a successful call to prb_reserve()
> + * to verify a dictionary buffer was actually reserved. Dictionary buffer
> + * reservation is allowed to fail.
> + */
> +static inline void prb_rec_init_wr(struct printk_record *r,
> + unsigned int text_buf_size,
> + unsigned int dict_buf_size)
> +{
> + r->info = NULL;
> + r->text_buf = NULL;
> + r->dict_buf = NULL;
> + r->text_buf_size = text_buf_size;
> + r->dict_buf_size = dict_buf_size;
> +}
> +
> +bool prb_reserve(struct prb_reserved_entry *e, struct printk_ringbuffer *rb,
> + struct printk_record *r);
> +void prb_commit(struct prb_reserved_entry *e);
> +
> +void prb_init(struct printk_ringbuffer *rb,
> + char *text_buf, unsigned int text_buf_size,
> + char *dict_buf, unsigned int dict_buf_size,
> + struct prb_desc *descs, unsigned int descs_count_bits);
> +unsigned int prb_record_text_space(struct prb_reserved_entry *e);
> +
> +/* Reader Interface */
> +
> +/**
> + * prb_rec_init_rd() - Initialize a buffer for reading records.
> + *
> + * @r: The record to initialize.
> + * @info: A buffer to store record meta-data.
> + * @text_buf: A buffer to store text data.
> + * @text_buf_size: The size of @text_buf.
> + * @dict_buf: A buffer to store dictionary data.
> + * @dict_buf_size: The size of @dict_buf.
> + *
> + * Initialize all the fields that a reader is interested in. All arguments
> + * (except @r) are optional. Only record data for arguments that are
> + * non-NULL or non-zero will be read.
> + */
> +static inline void prb_rec_init_rd(struct printk_record *r,
> + struct printk_info *info,
> + char *text_buf, unsigned int text_buf_size,
> + char *dict_buf, unsigned int dict_buf_size)
> +{
> + r->info = info;
> + r->text_buf = text_buf;
> + r->dict_buf = dict_buf;
> + r->text_buf_size = text_buf_size;
> + r->dict_buf_size = dict_buf_size;
> +}
> +
> +/**
> + * prb_for_each_record() - Iterate over a ringbuffer.
> + *
> + * @from: The sequence number to begin with.
> + * @rb: The ringbuffer to iterate over.
> + * @s: A u64 to store the sequence number on each iteration.
> + * @r: A printk_record to store the record on each iteration.
> + *
> + * This is a macro for conveniently iterating over a ringbuffer.
> + *
> + * Context: Any context.
> + */
> +#define prb_for_each_record(from, rb, s, r) \
> +for ((s) = from; prb_read_valid(rb, s, r); (s) = (r)->info->seq + 1)
> +
> +bool prb_read_valid(struct printk_ringbuffer *rb, u64 seq,
> + struct printk_record *r);
> +bool prb_read_valid_info(struct printk_ringbuffer *rb, u64 seq,
> + struct printk_info *info, unsigned int *line_count);
> +
> +u64 prb_first_seq(struct printk_ringbuffer *rb);
> +u64 prb_next_seq(struct printk_ringbuffer *rb);
> +
> +unsigned int prb_count_lines(char *text, unsigned int text_size);
> +
> +#endif /* _KERNEL_PRINTK_RINGBUFFER_H */
> --
> 2.20.1
>
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