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Message-Id: <20231106210730.115192-7-john.ogness@linutronix.de>
Date: Mon, 6 Nov 2023 22:13:27 +0106
From: John Ogness <john.ogness@...utronix.de>
To: Petr Mladek <pmladek@...e.com>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>,
Steven Rostedt <rostedt@...dmis.org>,
Thomas Gleixner <tglx@...utronix.de>,
linux-kernel@...r.kernel.org
Subject: [PATCH printk v2 6/9] printk: Wait for all reserved records with pr_flush()
Currently pr_flush() will only wait for records that were
available to readers at the time of the call. But there may be
more records (non-finalized) with following finalized records.
pr_flush() should wait for these to print as well. Particularly
because any trailing finalized records may be the messages that
the calling context wants to ensure are printed.
Fixes: 3b604ca81202 ("printk: add pr_flush()")
Signed-off-by: John Ogness <john.ogness@...utronix.de>
---
kernel/printk/printk.c | 2 +-
kernel/printk/printk_ringbuffer.c | 96 +++++++++++++++++++++++++++++++
kernel/printk/printk_ringbuffer.h | 1 +
3 files changed, 98 insertions(+), 1 deletion(-)
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 82dc2c7949b7..f3a7f5a6f6f8 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3755,7 +3755,7 @@ static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progre
might_sleep();
- seq = prb_next_seq(prb);
+ seq = prb_next_reserve_seq(prb);
/* Flush the consoles so that records up to @seq are printed. */
console_lock();
diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c
index 7c121fd68330..dc83569d3a3a 100644
--- a/kernel/printk/printk_ringbuffer.c
+++ b/kernel/printk/printk_ringbuffer.c
@@ -2012,6 +2012,102 @@ static u64 prb_first_seq(struct printk_ringbuffer *rb)
return seq;
}
+/**
+ * prb_next_reserve_seq() - Get the sequence number after the most recently
+ * reserved record.
+ *
+ * @rb: The ringbuffer to get the sequence number from.
+ *
+ * This is the public function available to readers to see what sequence
+ * number will be assigned to the next reserved record.
+ *
+ * Note that depending on the situation, this value can be equal to or
+ * higher than the sequence number returned by prb_next_seq().
+ *
+ * Context: Any context.
+ * Return: The sequence number that will be assigned to the next record
+ * reserved.
+ */
+u64 prb_next_reserve_seq(struct printk_ringbuffer *rb)
+{
+ struct prb_desc_ring *desc_ring = &rb->desc_ring;
+ unsigned long last_finalized_id;
+ atomic_long_t *state_var;
+ u64 last_finalized_seq;
+ unsigned long head_id;
+ struct prb_desc desc;
+ unsigned long diff;
+ struct prb_desc *d;
+ int err;
+
+ /*
+ * It may not be possible to read a sequence number for @head_id.
+ * So the ID of @last_finailzed_seq is used to calculate what the
+ * sequence number of @head_id will be.
+ */
+
+try_again:
+ last_finalized_seq = desc_last_finalized_seq(desc_ring);
+
+ /*
+ * @head_id is loaded after @last_finalized_seq to ensure that it is
+ * at or beyond @last_finalized_seq.
+ *
+ * Memory barrier involvement:
+ *
+ * If desc_last_finalized_seq:A reads from
+ * desc_update_last_finalized:A, then
+ * prb_next_reserve_seq:A reads from desc_reserve:D.
+ *
+ * Relies on:
+ *
+ * RELEASE from desc_reserve:D to desc_update_last_finalized:A
+ * matching
+ * ACQUIRE from desc_last_finalized_seq:A to prb_next_reserve_seq:A
+ *
+ * Note: desc_reserve:D and desc_update_last_finalized:A can be
+ * different CPUs. However, the desc_update_last_finalized:A CPU
+ * (which performs the release) must have previously seen
+ * desc_read:C, which implies desc_reserve:D can be seen.
+ */
+ head_id = atomic_long_read(&desc_ring->head_id); /* LMM(prb_next_reserve_seq:A) */
+
+ d = to_desc(desc_ring, last_finalized_seq);
+ state_var = &d->state_var;
+
+ /* Extract the ID, used to specify the descriptor to read. */
+ last_finalized_id = DESC_ID(atomic_long_read(state_var));
+
+ /* Ensure @last_finalized_id is correct. */
+ err = desc_read_finalized_seq(desc_ring, last_finalized_id, last_finalized_seq, &desc);
+
+ if (err == -EINVAL) {
+ if (last_finalized_seq == 0) {
+ /*
+ * @last_finalized_seq still contains its initial
+ * value. Probably no record has been finalized yet.
+ * This means the ringbuffer is not yet full and the
+ * @head_id value can be used directly (subtracting
+ * off its initial value).
+ *
+ * Because of hack#2 of the bootstrapping phase, the
+ * @head_id initial value must be handled separately.
+ */
+ if (head_id == DESC0_ID(desc_ring->count_bits))
+ return 0;
+
+ last_finalized_id = DESC0_ID(desc_ring->count_bits) + 1;
+ } else {
+ /* Record must have been overwritten. Try again. */
+ goto try_again;
+ }
+ }
+
+ diff = head_id - last_finalized_id;
+
+ return (last_finalized_seq + diff);
+}
+
/*
* Non-blocking read of a record.
*
diff --git a/kernel/printk/printk_ringbuffer.h b/kernel/printk/printk_ringbuffer.h
index 5c09061433f3..b48b44ecbe6d 100644
--- a/kernel/printk/printk_ringbuffer.h
+++ b/kernel/printk/printk_ringbuffer.h
@@ -394,5 +394,6 @@ bool prb_read_valid_info(struct printk_ringbuffer *rb, u64 seq,
u64 prb_first_valid_seq(struct printk_ringbuffer *rb);
u64 prb_next_seq(struct printk_ringbuffer *rb);
+u64 prb_next_reserve_seq(struct printk_ringbuffer *rb);
#endif /* _KERNEL_PRINTK_RINGBUFFER_H */
--
2.39.2
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