From 932e46365226324d2cf26d8bdec8b51ceb296948 Mon Sep 17 00:00:00 2001
From: "Ahmed S. Darwish" <a.darwish@linutronix.de>
Date: Mon, 20 Jul 2020 17:55:12 +0200
Subject: [PATCH] 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 85fb3ac93ffb..e885702d8b82 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;
}
/**