From: "Ahmed S. Darwish" <a.darwish@linutronix.de>
Date: Mon, 20 Jul 2020 17:55:12 +0200
Subject: seqlock: Implement raw_seqcount_begin() in terms of
 raw_read_seqcount()
Patch-mainline: v5.9-rc1
Git-commit: 932e46365226324d2cf26d8bdec8b51ceb296948
References: bsc#1176564 bsc#1162702

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
[dwagner: update context due to missing kcsan]
Acked-by: Daniel Wagner <dwagner@suse.de>
---
 include/linux/seqlock.h |    8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -181,9 +181,11 @@ static inline unsigned raw_read_seqcount
  */
 static inline unsigned raw_seqcount_begin(const seqcount_t *s)
 {
-	unsigned ret = READ_ONCE(s->sequence);
-	smp_rmb();
-	return ret & ~1;
+	/*
+	 * If the counter is odd, let read_seqcount_retry() fail
+	 * by decrementing the counter.
+	 */
+	return raw_read_seqcount(s) & ~1;
 }
 
 /**