В Чт, 05/02/2015 в 14:38 +0100, Oleg Nesterov пишет:
> On 02/05, Kirill Tkhai wrote:
> >
> > The write operation may be reordered with the setting of group_exit_task.
> > If so, this fires in exit_notify().
>
> How?
>
> OK, yes, "sig->notify_count = -1" can be reordered with the last unlock,
> but we do not care?
>
> group_exit_task + notify_count is only checked under the same lock, and
> "notify_count = -1" can't happen until de_thread() sees it is zero.
>
> Could you explain why this is bad in more details?
Can't exit_notify() see tsk->signal->notify_count == -1 before
tsk->signal->group_exit_task?
As I see in Documentation/memory-barriers.txt:
RELEASE operation implication:
Memory operations issued after the RELEASE may be completed
before the
RELEASE operation has completed.
>
> > --- a/fs/exec.c
> > +++ b/fs/exec.c
> > @@ -920,10 +920,16 @@ static int de_thread(struct task_struct *tsk)
> > if (!thread_group_leader(tsk)) {
> > struct task_struct *leader = tsk->group_leader;
> >
> > - sig->notify_count = -1; /* for exit_notify() */
> > for (;;) {
> > threadgroup_change_begin(tsk);
> > write_lock_irq(&tasklist_lock);
> > + /*
> > + * We could set it once outside the for() cycle, but
> > + * this requires to use SMP barriers there and in
> > + * exit_notify(), because the write operation may
> > + * be reordered with the setting of group_exit_task.
> > + */
> > + sig->notify_count = -1; /* for exit_notify() */
> > if (likely(leader->exit_state))
> > break;
> > __set_current_state(TASK_KILLABLE);
>
> Perhaps something like this makes sense anyway to make the code more
> clear, but in this case I'd suggest to set ->notify_count after we
> check ->exit_state. And without the (afaics!) misleading comment...
>
> Or I missed something?
>
> Oleg.
>
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
