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


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