linux-kernel - Re: Refactoring mutex spin on owner code

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Date:	Thu, 29 Jan 2015 17:52:48 -0800
From:	Jason Low <jason.low2@...com>
To:	Davidlohr Bueso <dave@...olabs.net>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Michel Lespinasse <walken@...gle.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	linux-kernel@...r.kernel.org, jason.low2@...com
Subject: Re: Refactoring mutex spin on owner code

On Thu, 2015-01-29 at 15:15 -0800, Davidlohr Bueso wrote:
> On Thu, 2015-01-29 at 12:18 -0800, Jason Low wrote:
> >  	/*
> > -	 * We break out the loop above on need_resched() and when the
> > -	 * owner changed, which is a sign for heavy contention. Return
> > -	 * success only when lock->owner is NULL.
> > +	 * We break out the loop above on either need_resched(), when
> > +	 * the owner is not running, or when the lock owner changed.
> > +	 * Return success only when the lock owner changed.
> >  	 */
> > -	return lock->owner == NULL;
> > +	return lock->owner != owner;
> >  }
> 
> Ideally we would refactor all this, along with getting rid of
> owner_running() at some point. It no longer makes sense to split up
> mutex_spin_on_owner() and we're doing duplicate owner checks. It would
> also be simpler than having to guess why we broke out of the loop, for
> example.

Sure, that makes sense. What do you think of this additional change for
refactoring the mutex version?

---
diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index 8711505..b6a8633 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -204,44 +204,45 @@ ww_mutex_set_context_fastpath(struct ww_mutex *lock,
  * Mutex spinning code migrated from kernel/sched/core.c
  */
 
-static inline bool owner_running(struct mutex *lock, struct task_struct *owner)
-{
-	if (lock->owner != owner)
-		return false;
-
-	/*
-	 * 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, the rcu_read_lock()
-	 * ensures the memory stays valid.
-	 */
-	barrier();
-
-	return owner->on_cpu;
-}
-
 /*
  * Look out! "owner" is an entirely speculative pointer
  * access and not reliable.
  */
 static noinline
-int mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner)
+bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner)
 {
+	bool ret;
+
 	rcu_read_lock();
-	while (owner_running(lock, owner)) {
-		if (need_resched())
+	while (true) {
+		/* Return success when the lock owner changed */
+		if (lock->owner != owner) {
+			ret = true;
 			break;
+		}
+
+		/*
+		 * 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,
+		 * the rcu_read_lock() ensures the memory stays valid.
+		 */
+		barrier();
+
+		/*
+		 * Stop spinning if we need to reschedule or if owner is
+		 * not running.
+		 */
+		if (!owner->on_cpu || need_resched()) {
+			ret = false;
+			break;
+		}
 
 		cpu_relax_lowlatency();
 	}
 	rcu_read_unlock();
 
-	/*
-	 * We break out the loop above on either need_resched(), when
-	 * the owner is not running, or when the lock owner changed.
-	 * Return success only when the lock owner changed.
-	 */
-	return lock->owner != owner;
+	return ret;
 }
 
 /*


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