linux-kernel - Re: [patch V2 3/5] posix-cpu-timers: Provide mechanisms to defer timer handling to task

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Date:   Thu, 23 Jul 2020 03:03:15 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Oleg Nesterov <oleg@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        John Stultz <john.stultz@...aro.org>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [patch V2 3/5] posix-cpu-timers: Provide mechanisms to defer
 timer handling to task_work

On Fri, Jul 17, 2020 at 12:50:34AM +0200, Peter Zijlstra wrote:
> On Thu, Jul 16, 2020 at 10:19:26PM +0200, Thomas Gleixner wrote:
> > +static void __run_posix_cpu_timers(struct task_struct *tsk)
> > +{
> > +	struct posix_cputimers *pct = &tsk->posix_cputimers;
> > +
> > +	if (!test_and_set_bit(CPUTIMERS_WORK_SCHEDULED, &pct->flags))
> > +		task_work_add(tsk, &pct->task_work, true);
> > +}
> > +
> > +static inline void posix_cpu_timers_enable_work(struct task_struct *tsk)
> > +{
> > +	clear_bit(CPUTIMERS_WORK_SCHEDULED, &tsk->posix_cputimers.flags);
> 	/*
> 	 * Ensure we observe everything before a failing test_and_set()
> 	 * in __run_posix_cpu_timers().
> 	 */
> 	smp_mb__after_atomic();
> > +}
> 
> Such that when another timer interrupt happens while we run this, we're
> guaranteed to either see it, or get re-queued and thus re-run the
> function.

But each thread in the process enqueues its own task work and flips its
own flags. So if task A runs the task work and task B runs __run_posix_cpu_timers(),
they wouldn't be ordering against the same flags.