linux-kernel - [PATCH 1/2] locking/rwbase: Optimize rwbase_read

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Date:   Sun, 19 Sep 2021 22:20:30 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     tglx@...utronix.de
Cc:     peterz@...radead.org, mingo@...nel.org, rostedt@...dmis.org,
        longman@...hat.com, bigeasy@...utronix.de, boqun.feng@...il.com,
        dave@...olabs.net, linux-kernel@...r.kernel.org,
        Davidlohr Bueso <dbueso@...e.de>
Subject: [PATCH 1/2] 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@...e.de>
---
 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 88191f6e252c..a9034784a5a0 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;
@@ -183,7 +182,7 @@ 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);
-- 
2.26.2