Re: [PATCH] de_thread: Move notify_count write under lock

[Kirill Tkhai]

Ð ÐÑ, 05/02/2015 Ð 17:15 +0300, Kirill Tkhai ÐÐÑÐÑ:
> Ð ÐÑ, 05/02/2015 Ð 14:38 +0100, Oleg Nesterov ÐÐÑÐÑ:
> > 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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