linux-kernel - [tip:locking/core] locking/rwsem: Micro-optimize rwsem_try_read_lock

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Message-ID: <tip-a338ecb07a338c9a8b0ca0010e862ebe598b1551@git.kernel.org>
Date:   Tue, 16 Apr 2019 03:03:17 -0700
From:   tip-bot for Waiman Long <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     paulmck@...ux.vnet.ibm.com, hpa@...or.com, a.p.zijlstra@...llo.nl,
        bp@...en8.de, dave@...olabs.net, mingo@...nel.org,
        torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
        longman@...hat.com, peterz@...radead.org, dbueso@...e.de,
        tglx@...utronix.de, will.deacon@....com,
        linux-kernel@...r.kernel.org, tim.c.chen@...ux.intel.com,
        arnd@...db.de
Subject: [tip:locking/core] locking/rwsem: Micro-optimize
 rwsem_try_read_lock_unqueued()

Commit-ID:  a338ecb07a338c9a8b0ca0010e862ebe598b1551
Gitweb:     https://git.kernel.org/tip/a338ecb07a338c9a8b0ca0010e862ebe598b1551
Author:     Waiman Long <longman@...hat.com>
AuthorDate: Thu, 4 Apr 2019 13:43:13 -0400
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Wed, 10 Apr 2019 10:56:01 +0200

locking/rwsem: Micro-optimize rwsem_try_read_lock_unqueued()

The atomic_long_cmpxchg_acquire() in rwsem_try_read_lock_unqueued() is
replaced by atomic_long_try_cmpxchg_acquire() to simpify the code and
generate slightly better assembly code.

There is no functional change.

Signed-off-by: Waiman Long <longman@...hat.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Acked-by: Will Deacon <will.deacon@....com>
Acked-by: Davidlohr Bueso <dbueso@...e.de>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Arnd Bergmann <arnd@...db.de>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Davidlohr Bueso <dave@...olabs.net>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Tim Chen <tim.c.chen@...ux.intel.com>
Link: http://lkml.kernel.org/r/20190404174320.22416-5-longman@redhat.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/locking/rwsem-xadd.c | 15 +++++----------
 1 file changed, 5 insertions(+), 10 deletions(-)

diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index c213869e1aa7..f6198e1a58f6 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -259,21 +259,16 @@ static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem)
  */
 static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
 {
-	long old, count = atomic_long_read(&sem->count);
+	long count = atomic_long_read(&sem->count);
 
-	while (true) {
-		if (!(count == 0 || count == RWSEM_WAITING_BIAS))
-			return false;
-
-		old = atomic_long_cmpxchg_acquire(&sem->count, count,
-				      count + RWSEM_ACTIVE_WRITE_BIAS);
-		if (old == count) {
+	while (!count || count == RWSEM_WAITING_BIAS) {
+		if (atomic_long_try_cmpxchg_acquire(&sem->count, &count,
+					count + RWSEM_ACTIVE_WRITE_BIAS)) {
 			rwsem_set_owner(sem);
 			return true;
 		}
-
-		count = old;
 	}
+	return false;
 }
 
 static inline bool owner_on_cpu(struct task_struct *owner)