Commit 932e4636 authored Jul 20, 2020 by Ahmed S. Darwish Committed by Peter Zijlstra Jul 29, 2020

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

parent 89b88845

include/linux/seqlock.h

+5 −4

Original line number	Diff line number	Diff line
		@@ -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;
		}

		/**