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printk: ringbuffer: Skip non-finalized records in panic

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Normally a reader will stop once reaching a non-finalized
record. However, when a panic happens, writers from other CPUs
(or an interrupted context on the panic CPU) may have been
writing a record and were unable to finalize it. The panic CPU
will reserve/commit/finalize its panic records, but these will
be located after the non-finalized records. This results in
panic() not flushing the panic messages.

Extend _prb_read_valid() to skip over non-finalized records if
on the panic CPU.

Fixes: 896fbe20b4 ("printk: use the lockless ringbuffer")
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20240207134103.1357162-11-john.ogness@linutronix.de
Signed-off-by: Petr Mladek <pmladek@suse.com>
This commit is contained in:
John Ogness 2024-02-07 14:46:59 +01:06 committed by Petr Mladek
parent ac7d7844c6
commit b1c4c67a5e
1 changed files with 26 additions and 2 deletions
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28
kernel/printk/printk_ringbuffer.c
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@ -2099,6 +2099,10 @@ try_again:
*
* On failure @seq is updated to a record that is not yet available to the
* reader, but it will be the next record available to the reader.
*
* Note: When the current CPU is in panic, this function will skip over any
* non-existent/non-finalized records in order to allow the panic CPU
* to print any and all records that have been finalized.
*/
static bool _prb_read_valid(struct printk_ringbuffer *rb, u64 *seq,
struct printk_record *r, unsigned int *line_count)
@ -2121,8 +2125,28 @@ static bool _prb_read_valid(struct printk_ringbuffer *rb, u64 *seq,
(*seq)++;
} else {
/* Non-existent/non-finalized record. Must stop. */
return false;
/*
* Non-existent/non-finalized record. Must stop.
*
* For panic situations it cannot be expected that
* non-finalized records will become finalized. But
* there may be other finalized records beyond that
* need to be printed for a panic situation. If this
* is the panic CPU, skip this
* non-existent/non-finalized record unless it is
* at or beyond the head, in which case it is not
* possible to continue.
*
* Note that new messages printed on panic CPU are
* finalized when we are here. The only exception
* might be the last message without trailing newline.
* But it would have the sequence number returned
* by "prb_next_reserve_seq() - 1".
*/
if (this_cpu_in_panic() && ((*seq + 1) < prb_next_reserve_seq(rb)))
(*seq)++;
else
return false;
}
}