On Fri, Jun 01, 2012 at 12:42:06AM +0200, Jan Kara wrote:
> On Fri 01-06-12 00:26:05, Peter Zijlstra wrote:
> > On Fri, 2012-06-01 at 00:11 +0200, Jan Kara wrote:
> > > bool fprop_new_period(struct fprop_global *p, int periods)
> > > {
> > > - u64 events = percpu_counter_sum(&p->events);
> > > + u64 events;
> > > + unsigned long flags;
> > >
> > > + local_irq_save(flags);
> > > + events = percpu_counter_sum(&p->events);
> > > + local_irq_restore(flags);
> > > /*
> > > * Don't do anything if there are no events.
> > > */
> > > @@ -73,7 +77,9 @@ bool fprop_new_period(struct fprop_global *p, int periods)
> > > if (periods < 64)
> > > events -= events >> periods;
> > > /* Use addition to avoid losing events happening between sum and set */
> > > + local_irq_save(flags);
> > > percpu_counter_add(&p->events, -events);
> > > + local_irq_restore(flags);
> > > p->period += periods;
> > > write_seqcount_end(&p->sequence);
> >
> > Uhm, why bother enabling it in between? Just wrap the whole function in
> > a single IRQ disable.
> I wanted to have interrupts disabled for as short as possible but if you
> think it doesn't matter, I'll take your advice. The result is attached.
Thank you! I applied this incremental fix next to the commit
"lib: Proportions with flexible period".
Thanks,
Fengguang
> From: Jan Kara <jack@xxxxxxx>
> Subject: lib: Fix possible deadlock in flexible proportion code
>
> When percpu counter function in fprop_new_period() is interrupted by an
> interrupt while holding counter lock, it can cause deadlock when the
> interrupt wants to take the lock as well. Fix the problem by disabling
> interrupts when calling percpu counter functions.
>
> Signed-off-by: Jan Kara <jack@xxxxxxx>
>
> diff -u b/lib/flex_proportions.c b/lib/flex_proportions.c
> --- b/lib/flex_proportions.c
> +++ b/lib/flex_proportions.c
> @@ -62,13 +62,18 @@
> */
> bool fprop_new_period(struct fprop_global *p, int periods)
> {
> - u64 events = percpu_counter_sum(&p->events);
> + u64 events;
> + unsigned long flags;
>
> + local_irq_save(flags);
> + events = percpu_counter_sum(&p->events);
> /*
> * Don't do anything if there are no events.
> */
> - if (events <= 1)
> + if (events <= 1) {
> + local_irq_restore(flags);
> return false;
> + }
> write_seqcount_begin(&p->sequence);
> if (periods < 64)
> events -= events >> periods;
> @@ -76,6 +81,7 @@
> percpu_counter_add(&p->events, -events);
> p->period += periods;
> write_seqcount_end(&p->sequence);
> + local_irq_restore(flags);
>
> return true;
> }
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