linux-kernel - Re: [patch 04/44] posix-cpu-timers: Fixup stale comment

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Message-ID: <alpine.DEB.2.21.1908211525150.2223@nanos.tec.linutronix.de>
Date:   Wed, 21 Aug 2019 15:31:39 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Frederic Weisbecker <frederic@...nel.org>
cc:     LKML <linux-kernel@...r.kernel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        John Stultz <john.stultz@...aro.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Anna-Maria Behnsen <anna-maria@...utronix.de>
Subject: Re: [patch 04/44] posix-cpu-timers: Fixup stale comment

On Wed, 21 Aug 2019, Frederic Weisbecker wrote:
> On Tue, Aug 20, 2019 at 11:43:26PM +0200, Thomas Gleixner wrote:
> > On Tue, 20 Aug 2019, Frederic Weisbecker wrote:
> > No it can't do that throughout posix_cpu_timer_del() because exit_itimers()
> > can only look at current->signal->posix_timers which does not contain the
> > posix timers owned by a different task/process.
> > 
> > We could of course invoke posix_cpu_timers_exit() from exit_itimers() but
> > does that buy anything?
> >  
> > > It would make things more simple to delete the timer off the target from
> > > the same caller and place and we could remove posix_cpu_timers_exit*().
> > 
> > We can't. The foreign owned cpu timers are not in cur->signal->posix_timers
> > so how should we invoke posix_cpu_timer_del() on them. Only the owner task
> > can. The only thing the exiting task can do is to remove the foreign timer
> > from it's expiry list which has nothing to do with cur->signal->posix_timers.
> 
> That's exactly what I'm proposing. I think you're misunderstanding me.
> 
> I want the owner to handle all the list deletion work from the target.
> 
> Ok let's imagine a timer $ITIMER, owned by task $OWNER and whose target is task $TARGET.
> 
> So it's enqueued on $OWNER->signal->posix_timers and $TARGET->cputime_expires.
> 
> Two scenarios can happen:
> 
> 1) $TARGET exits first and is released. So it calls posix_cpu_timers_exit()
>    which deletes $ITIMER from $TARGET->cputime_expires.
> 
>    Later on, $OWNER exits and calls exit_itimers() -> timer_delete_hook($ITIMER)
>    -> posix_cpu_timer_del($ITIMER). It finds $TARGET as the target of $ITIMER but no
>    more sighand. So it returns.
> 
> 2) $OWNER exits first and calls exit_itimer() -> timer_delete_hook($ITIMER)
>    -> posix_cpu_timer_del($ITIMER). It finds $TARGET as the target of $ITIMER and it
>    finds a sighand to lock. So it deletes $ITIMER from $TARGET->cputime_expires
>    (see list_del(&timer->it.cpu.entry)).
> 
> 
> So I propose to change the behaviour of case 1) so that $TARGET doesn't call
> posix_cpu_timers_exit(). We instead wait for $OWNER to exit and call
> exit_itimers()  -> timer_delete_hook($ITIMER) -> posix_cpu_timer_del($ITIMER).
> It is going to find $TARGET as the target of $ITIMER but no more sighand. Then
> finally it removes $ITIMER from $TARGET->cputime_expires.
> We basically do the same thing as in 2) but without locking sighand since it's NULL
> on $TARGET at this time.

But what do we win with that? Horrors like this:

task A		task B	   	task C

     		arm_timer(A)	arm_timer(A)

do_exit()

		del_timer(A)	del_timer(A)
		no sighand	no_sighand
		 list_del()       list_del()

Guess how well concurrent list deletion works.

We must remove armed timers from the task/signal _before_ dropping sighand,
really.

Thanks,

	tglx