This is a note to let you know that I've just added the patch titled
printk: ringbuffer: Skip non-finalized records in panic
to the 6.8-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
printk-ringbuffer-skip-non-finalized-records-in-pani.patch
and it can be found in the queue-6.8 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit 4192bebb0d30ce796c111ccd41c31e72c964ebbd
Author: John Ogness <john.ogness@xxxxxxxxxxxxx>
Date: Wed Feb 7 14:46:59 2024 +0106
printk: ringbuffer: Skip non-finalized records in panic
[ 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@xxxxxxxxxxxxx>
Reviewed-by: Petr Mladek <pmladek@xxxxxxxx>
Link: https://lore.kernel.org/r/20240207134103.1357162-11-john.ogness@xxxxxxxxxxxxx
Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
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;
}
}
