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From:   Waiman Long <longman@redhat.com>
To:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Will Deacon <will.deacon@arm.com>,
        Thomas Gleixner <tglx@linutronix.de>
Cc:     linux-kernel@vger.kernel.org, x86@kernel.org,
        Arnd Bergmann <arnd@arndb.de>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Waiman Long <longman@redhat.com>
Subject: [PATCH-tip v2 04/10] locking/rwsem: Micro-optimize rwsem_try_read_lock_unqueued()
Date:   Fri, 15 Feb 2019 15:50:04 -0500
Message-Id: <1550263810-31947-5-git-send-email-longman@redhat.com>
In-Reply-To: <1550263810-31947-1-git-send-email-longman@redhat.com>
References: <1550263810-31947-1-git-send-email-longman@redhat.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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@redhat.com>
---
 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 c213869..f6198e1 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)
-- 
1.8.3.1