linux-kernel - Re: [PATCH RESEND 1/3] percpu_ida: Fix data race on cpus_have

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Date:	Tue, 11 Mar 2014 15:03:23 +0100
From:	Alexander Gordeev <agordeev@...hat.com>
To:	Ming Lei <tom.leiming@...il.com>
Cc:	Kent Overstreet <kmo@...erainc.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Jens Axboe <axboe@...nel.dk>,
	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
Subject: Re: [PATCH RESEND 1/3] percpu_ida: Fix data race on cpus_have_tags
 cpumask

On Sun, Mar 02, 2014 at 10:42:05PM +0800, Ming Lei wrote:
> >> @@ -237,8 +242,11 @@ void percpu_ida_free(struct percpu_ida *pool, unsigned tag)
> >>       spin_unlock(&tags->lock);
> >>
> >>       if (nr_free == 1) {
> >> -             cpumask_set_cpu(smp_processor_id(),
> >> -                             &pool->cpus_have_tags);
> >> +             cpumask_set_cpu(smp_processor_id(), &pool->cpus_have_tags);
> >> +             /*
> >> +              * Pairs with smp_rmb() in steal_tags()
> >> +              */
> >> +             smp_wmb();
> >>               wake_up(&pool->wait);
> >
> > I think I'm nacking this - there's a lot of code in the kernel that relies on
> > the fact that prepare_to_wait)/wake_up() do the appropriate fences, we really
> > shouldn't be adding to the barriers those do.
> 
> In theory, it still might cause percpu_ida_alloc(TASK_RUNNING) failed,
> looks it isn't a big deal for the case.
> 
> But I am wondering why cpumask_set_cpu() isn't called with
> holding lock inside percpu_ida_free()? Looks 'nr_free == 1'
> shouldn't have happened frequently.

Because bouncing on the lock is more expensive than occasionally putting
a thread into sleep.

> 
> 
> Thanks,
> -- 
> Ming Lei

-- 
Regards,
Alexander Gordeev
agordeev@...hat.com
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