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    SubjectRe: [PATCH 1/4] introduce complete_vfork_done()
    On 02/16, Andrew Morton wrote:
    >
    > On Thu, 16 Feb 2012 18:26:47 +0100
    > Oleg Nesterov <oleg@redhat.com> wrote:
    >
    > > ...
    > > +void complete_vfork_done(struct task_struct *tsk)
    > > +{
    > > + struct completion *vfork_done = tsk->vfork_done;
    > > +
    > > + tsk->vfork_done = NULL;
    > > + complete(vfork_done);
    > > +}
    > > +
    > > /* Please note the differences between mmput and mm_release.
    > > * mmput is called whenever we stop holding onto a mm_struct,
    > > * error success whatever.
    > > @@ -682,8 +690,6 @@ struct mm_struct *mm_access(struct task_struct *task, unsigned int mode)
    > > */
    > > void mm_release(struct task_struct *tsk, struct mm_struct *mm)
    > > {
    > > - struct completion *vfork_done = tsk->vfork_done;
    > > -
    > > /* Get rid of any futexes when releasing the mm */
    > > #ifdef CONFIG_FUTEX
    > > if (unlikely(tsk->robust_list)) {
    > > @@ -703,11 +709,8 @@ void mm_release(struct task_struct *tsk, struct mm_struct *mm)
    > > /* Get rid of any cached register state */
    > > deactivate_mm(tsk, mm);
    > >
    > > - /* notify parent sleeping on vfork() */
    > > - if (vfork_done) {
    > > - tsk->vfork_done = NULL;
    > > - complete(vfork_done);
    > > - }
    > > + if (tsk->vfork_done)
    > > + complete_vfork_done(tsk);
    >
    > This all looks somewhat smelly.

    First of all, let me repeat that this patch changes nothing, justs
    move this code into the new helper.


    > - Why do we zero tsk->vfork_done in this manner? It *looks* like
    > it's done to prevent the kernel from running complete() twice against
    > a single task

    Yes,

    > in a race situation.

    No. More precisely, not before/after this patch.

    "if (vfork_done) complete_vfork_done()" is called twice very often.
    A vforked child does exec and notifies its parent. It should clear
    ->vfork_done, otherwise it will do complete_vfork_done() again on
    exit when ->vfork_done points to nowhere.

    The caller can never race with another user of ->vfork_done. It
    is the parent sleeping in do_fork(CLONE_VFORK). (I am ignoring
    the kernel threads created by kthread_create).

    > We'd need external locking to firm that up
    > and I'm not seeing it.

    After the next patch, parent/child can race with each other, that
    is why the next patch moves complete() under task_lock(). I'll write
    another email in reply to 2/4.

    > - Moving the test for non-null tsk->vfork_done into
    > complete_vfork_done() would simplify things a bit?

    Yes, perhaps this makes sense. After 3/4 mm_release() becomes the
    only caller and this microoptimization buys nothing, this helper
    will be static.

    I like the explicit test a bit more, just because it looks more
    clear to me. But this is subjective, I can redo.

    > - The complete_vfork_done() interface isn't wonderful. What prevents
    > tsk from getting freed? Presumably the caller must have pinned it in
    > some fashion? Or must hold some lock? Or it's always run against
    > `current',

    Yes, it is always current,

    > in which case it would be clearer to not pass the
    > task_struct arg at all?

    Well, may be... But mm_release() already has the 'tsk' argument which
    is always current. It would be strange to not use it.

    Oleg.



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