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SubjectRe: [PATCH 2/2] mm: call complete_vfork_done() after clearing child_tid and flushing rss-counters
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?


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