[tip: locking/core] seqlock: seqcount_t latch: End read sections with read_seqcount_retry()

tip-bot2 for Ahmed S. Darwish
2020-07-29T14:34:11Z

The following commit has been merged into the locking/core branch of tip:

Commit-ID: d3b35b87f436c1b226a8061bee9c8875ba6658bd
Gitweb: https://git.kernel.org/tip/d3b35b87f436c1b226a8061bee9c8875ba6658bd
Author: Ahmed S. Darwish <a.darwish@linutronix.de>
AuthorDate: Mon, 20 Jul 2020 17:55:09 +02:00
Committer: Peter Zijlstra <peterz@infradead.org>
CommitterDate: Wed, 29 Jul 2020 16:14:23 +02:00

seqlock: seqcount_t latch: End read sections with read_seqcount_retry()

The seqcount_t latch reader example at the raw_write_seqcount_latch()
kernel-doc comment ends the latch read section with a manual smp memory
barrier and sequence counter comparison.

This is technically correct, but it is suboptimal: read_seqcount_retry()
already contains the same logic of an smp memory barrier and sequence
counter comparison.

End the latch read critical section example with read_seqcount_retry().

Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200720155530.1173732-4-a.darwish@linutronix.de
---
include/linux/seqlock.h | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 6c4f68e..d724b5e 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -363,8 +363,8 @@ static inline int raw_read_seqcount_latch(seqcount_t *s)
* idx = seq & 0x01;
* entry = data_query(latch->data[idx], ...);
*
- * smp_rmb();
- * } while (seq != latch->seq);
+ * // read_seqcount_retry() includes needed smp_rmb()
+ * } while (read_seqcount_retry(&latch->seq, seq));
*
* return entry;
* }