linux-kernel - [tip:locking/core] locking/mutex: Further simplify mutex_spin_on

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Date:	Thu, 9 Apr 2015 02:00:38 -0700
From:	tip-bot for Jason Low <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	mingo@...nel.org, linux-kernel@...r.kernel.org,
	tim.c.chen@...ux.intel.com, tglx@...utronix.de,
	akpm@...ux-foundation.org, aswin@...com, peterz@...radead.org,
	torvalds@...ux-foundation.org, paulmck@...ux.vnet.ibm.com,
	dave@...olabs.net, hpa@...or.com, jason.low2@...com
Subject: [tip:locking/core] locking/mutex:
  Further simplify mutex_spin_on_owner()

Commit-ID:  01ac33c1f907b366dcc50551316b372f1519cca9
Gitweb:     http://git.kernel.org/tip/01ac33c1f907b366dcc50551316b372f1519cca9
Author:     Jason Low <jason.low2@...com>
AuthorDate: Wed, 8 Apr 2015 12:39:19 -0700
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Thu, 9 Apr 2015 08:10:23 +0200

locking/mutex: Further simplify mutex_spin_on_owner()

Similar to what Linus suggested for rwsem_spin_on_owner(), in
mutex_spin_on_owner() instead of having while (true) and
breaking out of the spin loop on lock->owner != owner, we can
have the loop directly check for while (lock->owner == owner) to
improve the readability of the code.

It also shrinks the code a bit:

   text    data     bss     dec     hex filename
   3721       0       0    3721     e89 mutex.o.before
   3705       0       0    3705     e79 mutex.o.after

Signed-off-by: Jason Low <jason.low2@...com>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Aswin Chandramouleeswaran <aswin@...com>
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/1428521960-5268-2-git-send-email-jason.low2@hp.com
[ Added code generation info. ]
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/locking/mutex.c | 14 ++++----------
 1 file changed, 4 insertions(+), 10 deletions(-)

diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index 16b2d3c..4cccea6 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -224,20 +224,14 @@ ww_mutex_set_context_slowpath(struct ww_mutex *lock,
 static noinline
 bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner)
 {
-	bool ret;
+	bool ret = true;
 
 	rcu_read_lock();
-	while (true) {
-		/* Return success when the lock owner changed */
-		if (lock->owner != owner) {
-			ret = true;
-			break;
-		}
-
+	while (lock->owner == owner) {
 		/*
 		 * Ensure we emit the owner->on_cpu, dereference _after_
-		 * checking lock->owner still matches owner, if that fails,
-		 * owner might point to free()d memory, if it still matches,
+		 * checking lock->owner still matches owner. If that fails,
+		 * owner might point to freed memory. If it still matches,
 		 * the rcu_read_lock() ensures the memory stays valid.
 		 */
 		barrier();
--
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