linux-kernel - [PATCH] locking/mutex: Refactor mutex_spin_on

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Date:	Mon, 09 Mar 2015 13:14:54 -0700
From:	Jason Low <jason.low2@...com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Davidlohr Bueso <dave@...olabs.net>
Cc:	LKML <linux-kernel@...r.kernel.org>, Jason Low <jason.low2@...com>
Subject: [PATCH] locking/mutex: Refactor mutex_spin_on_owner()

This patch applies on top of tip.

-------------------------------------------------------------------
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). This improves the
readability of the code.

Signed-off-by: Jason Low <jason.low2@...com>
---
 kernel/locking/mutex.c |   17 +++++------------
 1 files changed, 5 insertions(+), 12 deletions(-)

diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index 16b2d3c..1c3b7c5 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -224,16 +224,8 @@ 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;
-
 	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,
@@ -242,16 +234,17 @@ bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner)
 		 */
 		barrier();
 
+		/* Stop spinning when need_resched or owner is not running. */
 		if (!owner->on_cpu || need_resched()) {
-			ret = false;
-			break;
+			rcu_read_unlock();
+			return false;
 		}
 
 		cpu_relax_lowlatency();
 	}
 	rcu_read_unlock();
 
-	return ret;
+	return true;
 }
 
 /*
-- 
1.7.2.5



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