linux-kernel - Re: [patch] timer/hrtimer: take per cpu locks in sane order

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Date:	Fri, 2 Mar 2007 15:23:08 +0100
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	john stultz <johnstul@...ibm.com>,
	Roman Zippel <zippel@...ux-m68k.org>,
	Christian Borntraeger <cborntra@...ibm.com>,
	linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [patch] timer/hrtimer: take per cpu locks in sane order

On Fri, Mar 02, 2007 at 02:04:33PM +0100, Ingo Molnar wrote:
> 
> * Heiko Carstens <heiko.carstens@...ibm.com> wrote:
> 
> > -	spin_lock(&new_base->lock);
> > -	spin_lock(&old_base->lock);
> > +	/*
> > +	 * If we take a lock from a different cpu, make sure we have always
> > +	 * the same locking order. That is the lock that belongs to the cpu
> > +	 * with the lowest number is taken first.
> > +	 */
> > +	lock1 = smp_processor_id() < cpu ? &new_base->lock : &old_base->lock;
> > +	lock2 = smp_processor_id() < cpu ? &old_base->lock : &new_base->lock;
> > +	spin_lock(lock1);
> > +	spin_lock(lock2);
> 
> looks good to me. Wouldnt this be cleaner via double_lock_timer() - 
> similar to how double_rq_lock() works in kernel/sched.c - instead of 
> open-coding it?

Something like the stuff below? Exploits the knowledge that the two
tvec_base_t's are in a per_cpu array. Otherwise I would end up passing
a lot of redundant stuff. But still I think that isn't a good solution
but rather a hack...?
I'd go for the patch above.

---
Index: linux-2.6/kernel/timer.c
===================================================================
--- linux-2.6.orig/kernel/timer.c
+++ linux-2.6/kernel/timer.c
@@ -1640,6 +1640,28 @@ static void migrate_timer_list(tvec_base
 	}
 }
 
+static void __devinit double_tvec_lock(tvec_base_t *base1, tvec_base_t *base2)
+{
+	if (base1 < base2) {
+		spin_lock(&base1->lock);
+		spin_lock(&base2->lock);
+	} else {
+		spin_lock(&base2->lock);
+		spin_lock(&base1->lock);
+	}
+}
+
+static void __devinit double_tvec_unlock(tvec_base_t *base1, tvec_base_t *base2)
+{
+	if (base1 < base2) {
+		spin_unlock(&base1->lock);
+		spin_unlock(&base2->lock);
+	} else {
+		spin_unlock(&base2->lock);
+		spin_unlock(&base1->lock);
+	}
+}
+
 static void __devinit migrate_timers(int cpu)
 {
 	tvec_base_t *old_base;
@@ -1651,8 +1673,7 @@ static void __devinit migrate_timers(int
 	new_base = get_cpu_var(tvec_bases);
 
 	local_irq_disable();
-	spin_lock(&new_base->lock);
-	spin_lock(&old_base->lock);
+	double_tvec_lock(new_base, old_base);
 
 	BUG_ON(old_base->running_timer);
 
@@ -1665,8 +1686,7 @@ static void __devinit migrate_timers(int
 		migrate_timer_list(new_base, old_base->tv5.vec + i);
 	}
 
-	spin_unlock(&old_base->lock);
-	spin_unlock(&new_base->lock);
+	double_tvec_unlock(new_base, old_base);
 	local_irq_enable();
 	put_cpu_var(tvec_bases);
 }
-
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