linux-kernel - Re: [patch 01/20] posix-timers: Prevent RT livelock in itimer

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Date:   Thu, 4 May 2023 19:06:50 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Anna-Maria Behnsen <anna-maria@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Sebastian Siewior <bigeasy@...utronix.de>,
        syzbot+5c54bd3eb218bb595aa9@...kaller.appspotmail.com,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [patch 01/20] posix-timers: Prevent RT livelock in
 itimer_delete()

Le Tue, Apr 25, 2023 at 08:48:56PM +0200, Thomas Gleixner a écrit :
> itimer_delete() has a retry loop when the timer is concurrently expired. On
> non-RT kernels this just spin-waits until the timer callback has
> completed. On RT kernels this is a potential livelock when the exiting task
> preempted the hrtimer soft interrupt.
> 
> This only affects hrtimer based timers as Posix CPU timers cannot be
> concurrently expired. For CONFIG_POSIX_CPU_TIMERS_TASK_WORK=y this is
> obviously impossible as the task cannot run task work and exit at the same
> time. The CONFIG_POSIX_CPU_TIMERS_TASK_WORK=n (only non-RT) is prevented
> because interrupts are disabled.

But the owner of the timer is not the same as the target of the timer, right?

Though I seem to remember that we forbid setting a timer to a target outside
the current process, in which case the owner and the target are the same at
this exit stage. But I can't remember what enforces that permission in pid_for_clock()...

Thanks.