[tip: locking/core] locking/rwbase: Optimize rwbase_read_trylock

["tip-bot2 for Davidlohr Bueso" <tip-bot2@linutronix.de>]

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     c78416d122243c92992a1d1063f17ddd0bc80e6c
Gitweb:        https://git.kernel.org/tip/c78416d122243c92992a1d1063f17ddd0bc80e6c
Author:        Davidlohr Bueso <dave@stgolabs.net>
AuthorDate:    Sun, 19 Sep 2021 22:20:30 -07:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Thu, 07 Oct 2021 13:51:07 +02:00

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: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Waiman Long <longman@redhat.com>
Link: https://lkml.kernel.org/r/20210920052031.54220-2-dave@stgolabs.net
---
 kernel/locking/rwbase_rt.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/kernel/locking/rwbase_rt.c b/kernel/locking/rwbase_rt.c
index 15c8110..6fd3162 100644
--- a/kernel/locking/rwbase_rt.c
+++ b/kernel/locking/rwbase_rt.c
@@ -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);