linux-kernel - [PATCH v2] locking/rwsem: Avoid double checking before try acquiring write lock

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Date:	Tue, 16 Sep 2014 12:01:58 -0700
From:	Jason Low <jason.low2@...com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Davidlohr Bueso <dbueso@...e.de>
Cc:	linux-kernel@...r.kernel.org,
	Aswin Chandramouleeswaran <aswin@...com>,
	Chegu Vinod <chegu_vinod@...com>, Jason Low <jason.low2@...com>
Subject: [PATCH v2] locking/rwsem: Avoid double checking before try
 acquiring write lock

Commit 9b0fc9c09f1b checks for if there are known active lockers in
order to avoid write trylocking using expensive cmpxchg() when it
likely wouldn't get the lock.

However, a subsequent patch was added such that we directly check for
sem->count == RWSEM_WAITING_BIAS right before trying that cmpxchg().
Thus, commit 9b0fc9c09f1b now just adds extra overhead. This patch
deletes it.

Also, add a comment on why we do an "extra check" of sem->count before
the cmpxchg().

Signed-off-by: Jason Low <jason.low2@...com>
---
 kernel/locking/rwsem-xadd.c |   24 +++++++++++++-----------
 1 files changed, 13 insertions(+), 11 deletions(-)

diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index d6203fa..63d3ef2 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -247,18 +247,20 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
 	return sem;
 }
 
-static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem)
+static inline bool rwsem_try_write_lock(struct rw_semaphore *sem)
 {
-	if (!(count & RWSEM_ACTIVE_MASK)) {
-		/* try acquiring the write lock */
-		if (sem->count == RWSEM_WAITING_BIAS &&
-		    cmpxchg(&sem->count, RWSEM_WAITING_BIAS,
-			    RWSEM_ACTIVE_WRITE_BIAS) == RWSEM_WAITING_BIAS) {
-			if (!list_is_singular(&sem->wait_list))
-				rwsem_atomic_update(RWSEM_WAITING_BIAS, sem);
-			return true;
-		}
+	/*
+	 * Try acquiring the write lock. Check sem->count first
+	 * in order to reduce unnecessary expensive cmpxchg() operations.
+	 */
+	if (sem->count == RWSEM_WAITING_BIAS &&
+	    cmpxchg(&sem->count, RWSEM_WAITING_BIAS,
+		    RWSEM_ACTIVE_WRITE_BIAS) == RWSEM_WAITING_BIAS) {
+		if (!list_is_singular(&sem->wait_list))
+			rwsem_atomic_update(RWSEM_WAITING_BIAS, sem);
+		return true;
 	}
+
 	return false;
 }
 
@@ -446,7 +448,7 @@ struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem)
 	/* wait until we successfully acquire the lock */
 	set_current_state(TASK_UNINTERRUPTIBLE);
 	while (true) {
-		if (rwsem_try_write_lock(count, sem))
+		if (rwsem_try_write_lock(sem))
 			break;
 		raw_spin_unlock_irq(&sem->wait_lock);
 
-- 
1.7.1



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