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

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 85fb3ac..e885702 100644 (file)
--- 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;
 }
 
 /**