The following commit has been merged into the locking/core branch of tip: Commit-ID: 932e46365226324d2cf26d8bdec8b51ceb296948 Gitweb: https://git.kernel.org/tip/932e46365226324d2cf26d8bdec8b51ceb296948 Author: Ahmed S. Darwish <[email protected]> AuthorDate: Mon, 20 Jul 2020 17:55:12 +02:00 Committer: Peter Zijlstra <[email protected]> CommitterDate: Wed, 29 Jul 2020 16:14:24 +02:00
seqlock: Implement raw_seqcount_begin() in terms of raw_read_seqcount() raw_seqcount_begin() has the same code as raw_read_seqcount(), with the exception of masking the sequence counter's LSB before returning it to the caller. Note, raw_seqcount_begin() masks the counter's LSB before returning it to the caller so that read_seqcount_retry() can fail if the counter is odd -- without the overhead of an extra branching instruction. Signed-off-by: Ahmed S. Darwish <[email protected]> Signed-off-by: Peter Zijlstra (Intel) <[email protected]> Link: https://lkml.kernel.org/r/[email protected] --- include/linux/seqlock.h | 9 +++++---- 1 file changed, 5 insertions(+), 4 deletions(-) diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h index 85fb3ac..e885702 100644 --- a/include/linux/seqlock.h +++ b/include/linux/seqlock.h @@ -199,10 +199,11 @@ static inline unsigned raw_read_seqcount(const seqcount_t *s) */ static inline unsigned raw_seqcount_begin(const seqcount_t *s) { - unsigned ret = READ_ONCE(s->sequence); - smp_rmb(); - kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX); - return ret & ~1; + /* + * If the counter is odd, let read_seqcount_retry() fail + * by decrementing the counter. + */ + return raw_read_seqcount(s) & ~1; } /**
