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

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Date:   Fri, 17 Jul 2020 00:50:34 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Oleg Nesterov <oleg@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        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 Thu, Jul 16, 2020 at 10:19:26PM +0200, Thomas Gleixner wrote:

> @@ -1096,6 +1099,12 @@ static void __run_posix_cpu_timers(struc
>  	check_process_timers(tsk, &firing);
>  
>  	/*
> +	 * Allow new work to be scheduled. The expiry cache
> +	 * is up to date.
> +	 */
> +	posix_cpu_timers_enable_work(tsk);
> +
> +	/*
>  	 * We must release these locks before taking any timer's lock.
>  	 * There is a potential race with timer deletion here, as the
>  	 * siglock now protects our private firing list.  We have set

I think I would feel more comfortable if this was done at the very
beginning of that function, possibly even with:

> +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.