From: John Ogness john.ogness@linutronix.de
[ Upstream commit b1c4c67a5e90db8fbdb5b5504fe16e17b564cca8 ]
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: 896fbe20b4e2 ("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.d... Signed-off-by: Petr Mladek pmladek@suse.com Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/printk/printk_ringbuffer.c | 28 ++++++++++++++++++++++++++-- 1 file changed, 26 insertions(+), 2 deletions(-)
diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c index 97ec25d227976..3d98232902cfd 100644 --- a/kernel/printk/printk_ringbuffer.c +++ b/kernel/printk/printk_ringbuffer.c @@ -2087,6 +2087,10 @@ u64 prb_next_reserve_seq(struct printk_ringbuffer *rb) * * 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) @@ -2109,8 +2113,28 @@ 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; } }