Commit c78416d1 authored by Davidlohr Bueso's avatar Davidlohr Bueso Committed by Peter Zijlstra
Browse files

locking/rwbase: Optimize rwbase_read_trylock 



Instead of a full barrier around the Rmw insn, micro-optimize
for weakly ordered archs such that we only provide the required
ACQUIRE semantics when taking the read lock.

Signed-off-by: default avatarDavidlohr Bueso <dbueso@suse.de>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: default avatarWaiman Long <longman@redhat.com>
Link: https://lkml.kernel.org/r/20210920052031.54220-2-dave@stgolabs.net
parent 3f48565b

kernel/locking/rwbase_rt.c

+2 −3
Original line number Diff line number Diff line
@@ -59,8 +59,7 @@ static __always_inline int rwbase_read_trylock(struct rwbase_rt *rwb) 
	 * set.

	 */

	for (r = atomic_read(&rwb->readers); r < 0;) {

		/* Fully-ordered if cmpxchg() succeeds, provides ACQUIRE */

		if (likely(atomic_try_cmpxchg(&rwb->readers, &r, r + 1)))

		if (likely(atomic_try_cmpxchg_acquire(&rwb->readers, &r, r + 1)))

			return 1;

	}

	return 0;
@@ -187,7 +186,7 @@ static inline void __rwbase_write_unlock(struct rwbase_rt *rwb, int bias, 


	/*

	 * _release() is needed in case that reader is in fast path, pairing

	 * with atomic_try_cmpxchg() in rwbase_read_trylock(), provides RELEASE

	 * with atomic_try_cmpxchg_acquire() in rwbase_read_trylock().

	 */

	(void)atomic_add_return_release(READER_BIAS - bias, &rwb->readers);

	raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);