linux-kernel - [PATCH] mutex: set owner only once on acquisition

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Message-ID: <20090115083951.GA13930@cmpxchg.org>
Date:	Thu, 15 Jan 2009 09:41:01 +0100
From:	Johannes Weiner <hannes@...xchg.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Gregory Haskins <ghaskins@...ell.com>,
	Matthew Wilcox <matthew@....cx>,
	Andi Kleen <andi@...stfloor.org>,
	Chris Mason <chris.mason@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-btrfs <linux-btrfs@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Nick Piggin <npiggin@...e.de>,
	Peter Morreale <pmorreale@...ell.com>,
	Sven Dietrich <SDietrich@...ell.com>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>
Subject: [PATCH] mutex: set owner only once on acquisition

mutex_lock() sets the lock owner, no need to set it upfront in
__mutex_lock_common().

Inside __mutex_lock_common() we can cope with the case where the
successful acquirer got preempted by us before setting the owner
field: there is an explicit check in the spinning code and the
sleeping part doesn't rely on it.

The debug code does owner checks only on unlock where the field is
garuanteed to be set.

Signed-off-by: Johannes Weiner <hannes@...xchg.org>
---
 kernel/mutex.c |    2 --
 1 file changed, 2 deletions(-)

Just a small observation.  Peter said it wouldn't matter much as the
write is to a hot cache line.  But otoh, why keep it if it's not
necessary. :)

--- a/kernel/mutex.c
+++ b/kernel/mutex.c
@@ -195,7 +195,6 @@ __mutex_lock_common(struct mutex *lock, 
 
 		if (atomic_cmpxchg(&lock->count, 1, 0) == 1) {
 			lock_acquired(&lock->dep_map, ip);
-			mutex_set_owner(lock);
 			preempt_enable();
 			return 0;
 		}
@@ -263,7 +262,6 @@ done:
 	lock_acquired(&lock->dep_map, ip);
 	/* got the lock - rejoice! */
 	mutex_remove_waiter(lock, &waiter, current_thread_info());
-	mutex_set_owner(lock);
 
 	/* set it to 0 if there are no waiters left: */
 	if (likely(list_empty(&lock->wait_list)))
--
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