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

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Message-ID: <alpine.DEB.2.21.1908201946320.2223@nanos.tec.linutronix.de>
Date:   Tue, 20 Aug 2019 19:57:37 +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 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.

Thanks,

	tglx