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

Andrew Morton
2007-03-02T16:56:38Z

On Fri, 2 Mar 2007 15:23:08 +0100 Heiko Carstens <heiko.carstens@de.ibm.com> wrote:

> 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.

Yeah, it'd be nicer to pass in the CPU number(s), use that to make the
ordering decision. Perhaps (smp_processor_id() - cpu).

> ---
> 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);
> + }
> +}

And to undo the locks in the reverse order from that in which they were
taken.