В Чт, 05/02/2015 в 17:15 +0300, Kirill Tkhai пишет:
> 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.
Thread group leader (I) Thread (II)
exit_notify() de_thread()
sig->group_exit_task =
tsk;
sig->notify_count =
zap_other_threads(tsk); // == 1
if
(!thread_group_leader(tsk))
sig->notify_count--; // == 0
spin_unlock_irq(lock);
sig->notify_count = -1;
if (tsk->signal->notify_count < 0) (== -1)
wake_up_process(tsk->signal->group_exit_task); (garbage in
group_exit_task)
> >
> > > --- 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.
> >
>
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