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

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Date:	Tue, 11 Mar 2014 16:59:15 +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 Tue, Mar 11, 2014 at 11:34:21PM +0800, Ming Lei wrote:
> >> 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.
> 
> I mean the below block can be put inside the previous lock:
> 
>          if (nr_free == 1)
>              cpumask_set_cpu()
> 
> As I mentioned, 'nr_free == 1' doesn't happen frequently, so
> it won't be big deal, will it?

No. The lock will be taken *each* time in your suggestion, which is bad.

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