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

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Message-ID: <20190821155125.GB22020@lenoir>
Date:   Wed, 21 Aug 2019 17:51:25 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
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, Aug 21, 2019 at 03:31:39PM +0200, Thomas Gleixner wrote:
> On Wed, 21 Aug 2019, Frederic Weisbecker wrote:
> > 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.

Ah right, there can be concurrent owners, nevermind.