linux-kernel - Re: [PATCH v2 5/6] blk-mq: fix freeze queue race

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Message-ID: <CACVXFVO8R-6ztByYkyZaDFnasszg-x60VsyeF-Ut+GR8bgPRiA@mail.gmail.com>
Date:	Fri, 10 Jul 2015 16:08:07 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Akinobu Mita <akinobu.mita@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH v2 5/6] blk-mq: fix freeze queue race

On Fri, Jul 10, 2015 at 5:37 AM, Tejun Heo <tj@...nel.org> wrote:
> On Thu, Jul 09, 2015 at 02:55:10PM +0800, Ming Lei wrote:
>> This patch looks fine since at least changing DEAD state of percpu ref
>> state should have been synchronized by caller.
>>
>> Also looks __percpu_ref_switch_to_percpu() need to check if the refcount
>> becomes dead after current switching, and seems something like following
>> is needed:
>
> Hmmm... but the caller is responsible for synchronization for these
> operations.  If the state changes while waiting, it indicates a
> synchronization bug and testing it one more time is kinda weird.  What
> if it changes right after that test?

OK.

Also looks it is helpful to detect the bug early by adding
WARN_ON(ref->percpu_count_ptr & __PERCPU_REF_DEAD)
after wait_event().

--
Ming Lei
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