double call identical release when there is a race hitting

double call identical release when there is a race hitting

[zhong jiang]

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?

Thanks
zhongjiang

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

[Oleg Nesterov]

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.

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

[zhong jiang]

On 2017/6/6 23:56, Oleg Nesterov wrote:
> 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.
>
>
> .
>
Thank you for clarification.
 yes,  I see that the author admit that this is a issue.   The patch describe that it is really rare.
 Anyway, this issue should be fixed in a separate patch.

but so far  the issue still exist unfortunately.

Regards
zhongjiang




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