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Date: Thu, 27 Aug 2020 12:04:58 +0206
From: John Ogness <john.ogness@...utronix.de>
To: Petr Mladek <pmladek@...e.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
Steven Rostedt <rostedt@...dmis.org>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Thomas Gleixner <tglx@...utronix.de>,
Peter Zijlstra <peterz@...radead.org>,
Andrea Parri <parri.andrea@...il.com>,
Paul McKenney <paulmck@...nel.org>, kexec@...ts.infradead.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 5/7][next] printk: ringbuffer: add finalization/extension support
On 2020-08-26, Petr Mladek <pmladek@...e.com> wrote:
>> This series makes a very naive assumption that the previous
>> descriptor is either in the reserved or committed queried states. The
>> fact is, it can be in any of the 4 queried states. Adding support for
>> finalization of all the states then gets quite complex, since any
>> state transition (cmpxchg) may have to deal with an unexpected FINAL
>> flag.
>
> It has to be done in two steps to avoid race:
>
> prb_commit()
>
> + set PRB_COMMIT_MASK
> + check if it is still the last descriptor in the array
> + set PRB_FINAL_MASK when it is not the last descriptor
>
> It should work because prb_reserve() finalizes the previous
> descriptor after the new one is reserved. As a result:
>
> + prb_reserve() should either see PRB_COMMIT_MASK in the previous
> descriptor and be able to finalize it.
>
> + or prb_commit() will see that the head moved and it is not
> longer the last reserved one.
I do not like the idea of relying on descriptors to finalize
themselves. I worry that there might be some hole there. Failing to
finalize basically disables printk, so that is pretty serious.
Below is a patch against this series that adds support for finalizing
all 4 queried states. It passes all my tests. Note that the code handles
2 corner cases:
1. When seq is 0, there is no previous descriptor to finalize. This
exception is important because we don't want to finalize the -1
placeholder. Otherwise, upon the first wrap, a descriptor will be
prematurely finalized.
2. When a previous descriptor is being reserved for the first time, it
might have a state_var value of 0 because the writer is still in
prb_reserve() and has not set the initial value yet. I added
considerable comments on this special case.
I am comfortable with adding this new code, although it clearly adds
complexity.
John Ogness
diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c
index 90d48973ac9e..1ed1e9eb930f 100644
--- a/kernel/printk/printk_ringbuffer.c
+++ b/kernel/printk/printk_ringbuffer.c
@@ -860,9 +860,11 @@ 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;
+ unsigned long state_val;
struct prb_desc *desc;
unsigned long head_id;
unsigned long id;
+ bool is_final;
head_id = atomic_long_read(&desc_ring->head_id); /* LMM(desc_reserve:A) */
@@ -953,10 +955,17 @@ static bool desc_reserve(struct printk_ringbuffer *rb, unsigned long *id_out)
* 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 &&
- get_desc_state(id_prev_wrap, prev_state_val, NULL) != desc_reusable) {
- WARN_ON_ONCE(1);
- return false;
+ if (get_desc_state(id_prev_wrap, prev_state_val, &is_final) != desc_reusable) {
+ /*
+ * If this descriptor has never been used, @prev_state_val
+ * will be 0. However, even though it may have never been
+ * used, it may have been finalized. So that flag must be
+ * ignored.
+ */
+ if ((prev_state_val & ~DESC_FINAL_MASK)) {
+ WARN_ON_ONCE(1);
+ return false;
+ }
}
/*
@@ -967,10 +976,25 @@ static bool desc_reserve(struct printk_ringbuffer *rb, unsigned long *id_out)
* 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;
+ if (is_final)
+ state_val = id | 0 | DESC_FINAL_MASK;
+ else
+ state_val = id | 0;
+ if (atomic_long_cmpxchg(&desc->state_var, prev_state_val,
+ state_val) != prev_state_val) { /* LMM(desc_reserve:F) */
+ /*
+ * This reusable descriptor must have been finalized already.
+ * Retry with a reusable+final expected value.
+ */
+ prev_state_val |= DESC_FINAL_MASK;
+ state_val |= DESC_FINAL_MASK;
+
+ if (!atomic_long_try_cmpxchg(&desc->state_var, &prev_state_val,
+ state_val)) { /* LMM(desc_reserve:FIXME) */
+
+ WARN_ON_ONCE(1);
+ return false;
+ }
}
/* Now data in @desc can be modified: LMM(desc_reserve:G) */
@@ -1364,9 +1388,37 @@ static void desc_finalize(struct prb_desc_ring *desc_ring, unsigned long id)
while (!atomic_long_try_cmpxchg_relaxed(&d->state_var, &prev_state_val,
prev_state_val | DESC_FINAL_MASK)) {
- if (get_desc_state(id, prev_state_val, &is_final) != desc_reserved)
+ switch (get_desc_state(id, prev_state_val, &is_final)) {
+ 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 (get_desc_state(DESC_ID_PREV_WRAP(desc_ring, id),
+ prev_state_val, &is_final) != desc_reusable) {
+ /*
+ * If this descriptor has never been used, @prev_state_val
+ * will be 0. However, even though it may have never been
+ * used, it may have been finalized. So that flag must be
+ * ignored.
+ */
+ if ((prev_state_val & ~DESC_FINAL_MASK)) {
+ WARN_ON_ONCE(1);
+ return;
+ }
+ }
+ fallthrough;
+ case desc_reserved:
+ case desc_reusable:
+ /* finalizable, try again */
break;
+ case desc_committed:
+ /* already finalized */
+ return;
+ }
+ /* already finalized? */
if (is_final)
break;
}
@@ -1431,14 +1483,6 @@ bool prb_reserve(struct prb_reserved_entry *e, struct printk_ringbuffer *rb,
goto fail;
}
- /*
- * New data is about to be reserved. Once that happens, previous
- * descriptors are no longer able to be extended. Finalize the
- * previous descriptor now so that it can be made available to
- * readers (when committed).
- */
- desc_finalize(desc_ring, DESC_ID(id - 1));
-
d = to_desc(desc_ring, id);
/*
@@ -1464,6 +1508,16 @@ bool prb_reserve(struct prb_reserved_entry *e, struct printk_ringbuffer *rb,
else
d->info.seq += DESCS_COUNT(desc_ring);
+ /*
+ * New data is about to be reserved. Once that happens, previous
+ * descriptors are no longer able to be extended. Finalize the
+ * previous descriptor now so that it can be made available to
+ * readers (when committed). (For the first descriptor, there is
+ * no previous record to finalize.)
+ */
+ if (d->info.seq > 0)
+ desc_finalize(desc_ring, DESC_ID(id - 1));
+
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. */
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