From: John Ogness Date: Fri, 13 Oct 2023 10:23:11 +0000 Subject: [PATCH 10/50] printk: ringbuffer: Skip non-finalized records in panic Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/6.7/older/patches-6.7-rt6.tar.xz 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 Signed-off-by: Sebastian Andrzej Siewior --- kernel/printk/printk_ringbuffer.c | 28 ++++++++++++++++++++++++++-- 1 file changed, 26 insertions(+), 2 deletions(-) --- a/kernel/printk/printk_ringbuffer.c +++ b/kernel/printk/printk_ringbuffer.c @@ -2107,6 +2107,10 @@ u64 prb_next_reserve_seq(struct printk_r * * 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) @@ -2129,8 +2133,28 @@ static bool _prb_read_valid(struct print (*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; } }