linux-kernel - Re: double call identical release when there is a race hitting

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Date:   Tue, 6 Jun 2017 17:56:01 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     zhong jiang <zhongjiang@...wei.com>
Cc:     Michal Hocko <mhocko@...nel.org>, Vlastimil Babka <vbabka@...e.cz>,
        David Rientjes <rientjes@...gle.com>,
        Hugh Dickins <hughd@...gle.com>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Xishi Qiu <qiuxishi@...wei.com>
Subject: Re: double call identical release when there is a race hitting

I can't answer authoritatively, but

On 06/06, zhong jiang wrote:
>
> Hi
>
> when I review the code, I find the following scenario will lead to a race ,
> but I am not sure whether the real issue will hit or not.
>
> cpu1                                                                      cpu2
> exit_mmap                                               mmu_notifier_unregister
>    __mmu_notifier_release                                 srcu_read_lock
>             srcu_read_lock
>             mm->ops->release(mn, mm)                 mm->ops->release(mn,mm)
>            srcu_read_unlock                                         srcu_read_unlock
>
>
> obviously,  the specified mm will call identical release function when
> the related condition satisfy.  is it right?

I think you are right, this is possible, perhaps the comments should mention
this explicitly.

See the changelog in d34883d4e35c0a994e91dd847a82b4c9e0c31d83 "mm: mmu_notifier:
re-fix freed page still mapped in secondary MMU":

	"multiple ->release() callouts", we needn't care it too much ...

Oleg.