[PATCH 6.8 164/715] printk: ringbuffer: Skip non-finalized records in panic

Sasha Levin
2024-03-24T22:37:40Z

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.de
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;
}
}

--
2.43.0