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seqlock: Implement raw_seqcount_begin() in terms of raw_read_seqcount()

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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
This commit is contained in:
Ahmed S. Darwish 2020-07-20 17:55:12 +02:00 • committed by Peter Zijlstra
parent 89b88845e0
commit 932e463652
1 changed files with 5 additions and 4 deletions
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9
include/linux/seqlock.h
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@ -199,10 +199,11 @@ static inline unsigned raw_read_seqcount(const seqcount_t *s)
*/ */
static inline unsigned raw_seqcount_begin(const seqcount_t *s) static inline unsigned raw_seqcount_begin(const seqcount_t *s)
{ {
unsigned ret = READ_ONCE(s->sequence); /*
smp_rmb(); * If the counter is odd, let read_seqcount_retry() fail
kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX); * by decrementing the counter.
return ret & ~1; */
return raw_read_seqcount(s) & ~1;
} }
/** /**