Commit b1c4c67a authored by John Ogness's avatar John Ogness Committed by Petr Mladek
Browse files

printk: ringbuffer: Skip non-finalized records in panic 



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: 896fbe20 ("printk: use the lockless ringbuffer")
Signed-off-by: default avatarJohn Ogness <john.ogness@linutronix.de>
Reviewed-by: default avatarPetr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20240207134103.1357162-11-john.ogness@linutronix.de


Signed-off-by: default avatarPetr Mladek <pmladek@suse.com>
parent ac7d7844

kernel/printk/printk_ringbuffer.c

+26 −2
Original line number Diff line number Diff line
@@ -2099,6 +2099,10 @@ 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)
@@ -2121,7 +2125,27 @@ static bool _prb_read_valid(struct printk_ringbuffer *rb, u64 *seq, 
			(*seq)++;



		} else {

			/* Non-existent/non-finalized record. Must stop. */

			/*

			 * 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;

		}

	}