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

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Message-ID: <CACVXFVMB4YyyEDph5CsVPRLFe4ZncxVrarxV7-VT2wC=pbPTZQ@mail.gmail.com>
Date:	Tue, 11 Mar 2014 23:34:21 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Alexander Gordeev <agordeev@...hat.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 10:03 PM, Alexander Gordeev <agordeev@...hat.com> 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?

Thanks,
-- 
Ming Lei
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