From: John Ogness john.ogness@linutronix.de
[ Upstream commit 584528d621459d1a5c31da7a591218ad3bb96d6c ]
With the lockless ringbuffer, it is allowed that multiple CPUs/contexts write simultaneously into the buffer. This creates an ambiguity as some writers will finalize sooner.
The documentation for the prb_read functions is not clear as it refers to "not yet written" and "no data available". Clarify the return values and language to be in terms of the reader: records available for reading.
Signed-off-by: John Ogness john.ogness@linutronix.de Reviewed-by: Petr Mladek pmladek@suse.com Link: https://lore.kernel.org/r/20240207134103.1357162-9-john.ogness@linutronix.de Signed-off-by: Petr Mladek pmladek@suse.com Stable-dep-of: b1c4c67a5e90 ("printk: ringbuffer: Skip non-finalized records in panic") Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/printk/printk_ringbuffer.c | 16 +++++++++------- 1 file changed, 9 insertions(+), 7 deletions(-)
diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c index d152b6bd35c9a..97ec25d227976 100644 --- a/kernel/printk/printk_ringbuffer.c +++ b/kernel/printk/printk_ringbuffer.c @@ -2080,11 +2080,13 @@ u64 prb_next_reserve_seq(struct printk_ringbuffer *rb) }
/* - * Non-blocking read of a record. Updates @seq to the last finalized record - * (which may have no data available). + * Non-blocking read of a record. * - * See the description of prb_read_valid() and prb_read_valid_info() - * for details. + * On success @seq is updated to the record that was read and (if provided) + * @r and @line_count will contain the read/calculated data. + * + * 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. */ static bool _prb_read_valid(struct printk_ringbuffer *rb, u64 *seq, struct printk_record *r, unsigned int *line_count) @@ -2103,7 +2105,7 @@ static bool _prb_read_valid(struct printk_ringbuffer *rb, u64 *seq, *seq = tail_seq;
} else if (err == -ENOENT) { - /* Record exists, but no data available. Skip. */ + /* Record exists, but the data was lost. Skip. */ (*seq)++;
} else { @@ -2136,7 +2138,7 @@ static bool _prb_read_valid(struct printk_ringbuffer *rb, u64 *seq, * On success, the reader must check r->info.seq to see which record was * actually read. This allows the reader to detect dropped records. * - * Failure means @seq refers to a not yet written record. + * Failure means @seq refers to a record not yet available to the reader. */ bool prb_read_valid(struct printk_ringbuffer *rb, u64 seq, struct printk_record *r) @@ -2166,7 +2168,7 @@ bool prb_read_valid(struct printk_ringbuffer *rb, u64 seq, * On success, the reader must check info->seq to see which record meta data * was actually read. This allows the reader to detect dropped records. * - * Failure means @seq refers to a not yet written record. + * Failure means @seq refers to a record not yet available to the reader. */ bool prb_read_valid_info(struct printk_ringbuffer *rb, u64 seq, struct printk_info *info, unsigned int *line_count)