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SubjectRe: [PATCH 2/2] mm: call complete_vfork_done() after clearing child_tid and flushing rss-counters
Andrew Morton wrote:
> On Thu, 12 Apr 2012 12:09:53 +0400
> Konstantin Khlebnikov<khlebnikov@openvz.org> wrote:
>
>> Child should wake ups parent from vfork() only after finishing all operations with
>> shared mm. There is no sense to use CLONE_CHILD_CLEARTID together with CLONE_VFORK,
>> but it looks more accurate now.
>>
>> ...
>>
>> --- a/kernel/fork.c
>> +++ b/kernel/fork.c
>> @@ -728,9 +728,6 @@ void mm_release(struct task_struct *tsk, struct mm_struct *mm)
>> /* Get rid of any cached register state */
>> deactivate_mm(tsk, mm);
>>
>> - if (tsk->vfork_done)
>> - complete_vfork_done(tsk);
>> -
>> /*
>> * If we're exiting normally, clear a user-space tid field if
>> * requested. We leave this alone when dying by signal, to leave
>> @@ -759,6 +756,13 @@ void mm_release(struct task_struct *tsk, struct mm_struct *mm)
>> */
>> if (mm)
>> sync_mm_rss(mm);
>> +
>> + /*
>> + * All done, finally we can wake up parent and return this mm to him.
>> + * Also kthread_stop() uses this completion for synchronization.
>> + */
>> + if (tsk->vfork_done)
>> + complete_vfork_done(tsk);
>> }
>
> That does look a bit racy.
>
> But are we really sure that the patch really does fix something?
> Because it does increase vfork() latency a tiny bit.
>
> I'm going to call this a patch against the fork subsystem, not the mm
> subsystem.
>
> I believe that this patch is unrelated to "mm: set task exit code
> before complete_vfork_done()", yes?

Yes, unrelated.


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