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

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Message-ID: <alpine.DEB.2.21.1908202331080.2223@nanos.tec.linutronix.de>
Date:   Tue, 20 Aug 2019 23:43:26 +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 Tue, 20 Aug 2019, Frederic Weisbecker wrote:
> On Tue, Aug 20, 2019 at 07:57:37PM +0200, Thomas Gleixner wrote:
> > On Tue, 20 Aug 2019, Frederic Weisbecker wrote:
> > > On Mon, Aug 19, 2019 at 04:31:45PM +0200, Thomas Gleixner wrote:
> > > >  /*
> > > > - * Clean out CPU timers still ticking when a thread exited.  The task
> > > > - * pointer is cleared, and the expiry time is replaced with the residual
> > > > - * time for later timer_gettime calls to return.
> > > > + * Clean out CPU timers which are still armed when a thread exits. The
> > > > + * timers are only removed from the list. No other updates are done. The
> > > > + * corresponding posix timers are still accessible, but cannot be rearmed.
> > > > + *
> > > >   * This must be called with the siglock held.
> > > >   */
> > > >  static void cleanup_timers(struct list_head *head)
> > > 
> > > Indeed and I believe we could avoid that step. We remove the sighand at the same
> > > time so those can't be accessed anymore anyway.
> > > 
> > > exit_itimers() takes care of the last call release and could force remove from
> > > the list (although it might be taken care of in your series, haven't checked yet):
> > 
> > No. The posix timer is not necessarily owned by the exiting task or
> > process. It can be owned by a different entity which has permissions,
> > e.g. parent.
> > 
> > So those are not in the posix timer list of the exiting task, which gets
> > cleaned up in exit_itimers(). Those are in the list of the task which armed
> > the timer. The timer is merily queued in the 'active timers' list of the
> > exiting task and posix_cpu_timers_exit()/posix_cpu_timers_exit_group()
> > remove it before the task/signal structs go away.
> 
> Sure, I understand there's two distinct things here: the owner that queues
> timers in owner->sig->posix_timers (cleaned in exit_itimers()) and the target that queues
> in target->[signal->]cputime_expires (cleaned in posix_cpu_timers_exit[_group]().
>
> So I'm wondering why we bother with posix_cpu_timers_exit[_group]() at
> all when exit_itimers() could handle the list deletion from
> target->[signal]->cputime_expires throughout posix_cpu_timer_del() as it
> already does on targets that still have their sighands.

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.

cur->signal->posix_timers only contains posix timers which are owned by
current not those which are owned by a different task and armed on the
exiting one.

exit_itimers() handles cur->signal->posix_timers, i.e. timers owned by
current.

posix_cpu_timers_exit() handles timers enqueued on current, which are
foreign owned timers because exit_itimers() removed those which were owned
by current already.

posix_cpu_timers_exit_group() handles timers enqueued on current->signal,
which are foreign owned timers because exit_itimers() removed those which
were owned by current already.

> Or is there something I'm awkwardly missing as usual? :-) 

I think so :)

Thanks,

	tglx