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

Heiko Carstens
2007-03-02T14:24:43Z

On Fri, Mar 02, 2007 at 02:04:33PM +0100, Ingo Molnar wrote:
>
> * Heiko Carstens <heiko.carstens@de.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);
}