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

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Date:	Thu, 2 Oct 2014 22:29:20 -0700
From:	tip-bot for Jason Low <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, hpa@...or.com, mingo@...nel.org,
	torvalds@...ux-foundation.org, peterz@...radead.org,
	tim.c.chen@...ux.intel.com, peter@...leysoftware.com,
	jason.low2@...com, dbueso@...e.de, chegu_vinod@...com,
	tglx@...utronix.de, aswin@...com
Subject: [tip:locking/core] locking/rwsem:
  Avoid double checking before try acquiring write lock

Commit-ID:  debfab74e453f079cd8b12b0604387a8c510ef3a
Gitweb:     http://git.kernel.org/tip/debfab74e453f079cd8b12b0604387a8c510ef3a
Author:     Jason Low <jason.low2@...com>
AuthorDate: Tue, 16 Sep 2014 17:16:57 -0700
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Fri, 3 Oct 2014 06:09:29 +0200

locking/rwsem: Avoid double checking before try acquiring write lock

Commit 9b0fc9c09f1b ("rwsem: skip initial trylock in rwsem_down_write_failed")
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 overhead.

This patch modifies it so that we only do a check for if
count == RWSEM_WAITING_BIAS.

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

Signed-off-by: Jason Low <jason.low2@...com>
Acked-by: Davidlohr Bueso <dbueso@...e.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Aswin Chandramouleeswaran <aswin@...com>
Cc: Chegu Vinod <chegu_vinod@...com>
Cc: Peter Hurley <peter@...leysoftware.com>
Cc: Tim Chen <tim.c.chen@...ux.intel.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Link: http://lkml.kernel.org/r/1410913017.2447.22.camel@j-VirtualBox
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/locking/rwsem-xadd.c | 20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 12166ec..7628c3f 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -250,16 +250,18 @@ EXPORT_SYMBOL(rwsem_down_read_failed);
 
 static inline bool rwsem_try_write_lock(long count, 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 count first in order
+	 * to reduce unnecessary expensive cmpxchg() operations.
+	 */
+	if (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;
 }
 
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