[tip: locking/core] seqlock: Implement raw_seqcount_begin() in terms of raw_read_seqcount()

["tip-bot2 for Ahmed S. Darwish" <tip-bot2@linutronix.de>]

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 <a.darwish@linutronix.de>
AuthorDate:    Mon, 20 Jul 2020 17:55:12 +02:00
Committer:     Peter Zijlstra <peterz@infradead.org>
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 <a.darwish@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200720155530.1173732-7-a.darwish@linutronix.de
---
 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;
 }
 
 /**