linux-kernel - [PATCH-queue/locking/core] locking/mutex: Unify yield_to_waiter & waiter

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Date:   Thu, 18 Aug 2016 21:15:28 -0400
From:   Waiman Long <Waiman.Long@....com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ding Tianhong <dingtianhong@...wei.com>,
        Jason Low <jason.low2@....com>,
        Davidlohr Bueso <dave@...olabs.net>,
        "Paul E. McKenney" <paulmck@...ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <Will.Deacon@....com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Imre Deak <imre.deak@...el.com>,
        Waiman Long <Waiman.Long@....com>
Subject: [PATCH-queue/locking/core] locking/mutex: Unify yield_to_waiter & waiter_spinning

Both waiter_spinning and yield_to_waiter are used for somewhat similar
purpose. The waiter_spinning flag is used in CONFIG_MUTEX_SPIN_ON_OWNER
to make optimistic spinner yield to spinning waiter; whereas the
yield_to_waiter is used in !CONFIG_MUTEX_SPIN_ON_OWNER to make new
incoming mutex locker to yield to the sleeping waiter.

This patch unifies these 2 flags into a single yield_to_waiter
flag that is used in both CONFIG_MUTEX_SPIN_ON_OWNER and
!CONFIG_MUTEX_SPIN_ON_OWNER codes.

Signed-off-by: Waiman Long <Waiman.Long@....com>
---
 include/linux/mutex.h  |    8 ++++++--
 kernel/locking/mutex.c |   12 ++++++------
 2 files changed, 12 insertions(+), 8 deletions(-)

diff --git a/include/linux/mutex.h b/include/linux/mutex.h
index 988c020..2b3dcdb 100644
--- a/include/linux/mutex.h
+++ b/include/linux/mutex.h
@@ -57,8 +57,12 @@ struct mutex {
 #endif
 #ifdef CONFIG_MUTEX_SPIN_ON_OWNER
 	struct optimistic_spin_queue osq; /* Spinner MCS lock */
-	int waiter_spinning;
-#elif defined(CONFIG_SMP)
+#endif
+#ifdef CONFIG_SMP
+	/*
+	 * Used by both CONFIG_MUTEX_SPIN_ON_OWNER (depends on CONFIG_SMP) &
+	 * !CONFIG_MUTEX_SPIN_ON_OWNER codes.
+	 */
 	int yield_to_waiter;
 #endif
 #ifdef CONFIG_DEBUG_MUTEXES
diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index c4261fa..7218835 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -55,8 +55,8 @@ __mutex_init(struct mutex *lock, const char *name, struct lock_class_key *key)
 	mutex_clear_owner(lock);
 #ifdef CONFIG_MUTEX_SPIN_ON_OWNER
 	osq_lock_init(&lock->osq);
-	lock->waiter_spinning = false;
-#elif defined(CONFIG_SMP)
+#endif
+#ifdef CONFIG_SMP
 	lock->yield_to_waiter = false;
 #endif
 
@@ -351,7 +351,7 @@ static bool mutex_optimistic_spin(struct mutex *lock,
 		 * Turn on the waiter spinning flag to discourage the spinner
 		 * from getting the lock.
 		 */
-		lock->waiter_spinning = true;
+		lock->yield_to_waiter = true;
 	}
 
 	for (;;) {
@@ -374,11 +374,11 @@ static bool mutex_optimistic_spin(struct mutex *lock,
 		}
 
 		/*
-		 * For regular opt-spinner, it waits until the waiter_spinning
+		 * For regular opt-spinner, it waits until the yield_to_waiter
 		 * flag isn't set. This will ensure forward progress for
 		 * the waiter spinner.
 		 */
-		if (!waiter && READ_ONCE(lock->waiter_spinning)) {
+		if (!waiter && READ_ONCE(lock->yield_to_waiter)) {
 			if (need_resched())
 				break;
 			goto relax;
@@ -430,7 +430,7 @@ relax:
 	if (!waiter)
 		osq_unlock(&lock->osq);
 	else
-		lock->waiter_spinning = false;
+		lock->yield_to_waiter = false;
 done:
 	/*
 	 * If we fell out of the spin path because of need_resched(),
-- 
1.7.1