linux-kernel - Re: [patch 07/44] posix-cpu-timers: Simplify sighand locking in run_posix_cpu

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Message-ID: <alpine.DEB.2.21.1908211911580.1926@nanos.tec.linutronix.de>
Date:   Wed, 21 Aug 2019 19:13:41 +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 07/44] posix-cpu-timers: Simplify sighand locking in
 run_posix_cpu_timers()

On Wed, 21 Aug 2019, Frederic Weisbecker wrote:
> On Wed, Aug 21, 2019 at 03:25:01PM +0200, Thomas Gleixner wrote:
> > On Wed, 21 Aug 2019, Frederic Weisbecker wrote:
> > 
> > > On Mon, Aug 19, 2019 at 04:31:48PM +0200, Thomas Gleixner wrote:
> > > > run_posix_cpu_timers() is called from the timer interrupt. The posix timer
> > > > expiry always affects the current task which got interrupted.
> > > > 
> > > > sighand locking is only racy when done on a foreign task, which must use
> > > > lock_task_sighand(). But in case of run_posix_cpu_timers() that's
> > > > pointless.
> > > > 
> > > > sighand of a task can only be dropped or changed by the task itself. Drop
> > > > happens in do_exit()
> > > 
> > > Well, that's only in case of autoreap. Otherwise this is dropped by the reaper.
> > 
> > Right, but in the reaper case the task cannot be on the CPU running and
> > being interrupted by the tick. I might be missing something subtle though.
> 
> That looks possible. After exit_notify() and until the final schedule(), the exiting task
> can execute concurrently with the reaper calling release_task().

Duh, indeed.

We only can rely on that when we are in task context and not post state = ZOMBIE.

So for now this needs to stay unfortunately, but once we moved the expiry
to task work this becomes interesting.

/me goes fix.