linux-kernel - Re: [PATCH 4/4] task_work: use TIF_NOTIFY

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Message-ID: <87362hd6ta.fsf@nanos.tec.linutronix.de>
Date:   Wed, 14 Oct 2020 01:50:25 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org,
        io-uring@...r.kernel.org
Cc:     peterz@...radead.org, oleg@...hat.com,
        Jens Axboe <axboe@...nel.dk>,
        Roman Gershman <romger@...zon.com>
Subject: Re: [PATCH 4/4] task_work: use TIF_NOTIFY_SIGNAL if available

On Thu, Oct 08 2020 at 09:27, Jens Axboe wrote:
> +/*
> + * TWA_SIGNAL signaling - use TIF_NOTIFY_SIGNAL, if available, as it's faster
> + * than TIF_SIGPENDING as there's no dependency on ->sighand. The latter is
> + * shared for threads, and can cause contention on sighand->lock. Even for
> + * the non-threaded case TIF_NOTIFY_SIGNAL is more efficient, as no locking
> + * or IRQ disabling is involved for notification (or running) purposes.
> + */
> +static void task_work_notify_signal(struct task_struct *task)
> +{
> +#ifdef TIF_NOTIFY_SIGNAL
> +	set_notify_signal(task);
> +#else
> +	unsigned long flags;
> +
> +	/*
> +	 * Only grab the sighand lock if we don't already have some
> +	 * task_work pending. This pairs with the smp_store_mb()
> +	 * in get_signal(), see comment there.
> +	 */
> +	if (!(READ_ONCE(task->jobctl) & JOBCTL_TASK_WORK) &&
> +	    lock_task_sighand(task, &flags)) {
> +		task->jobctl |= JOBCTL_TASK_WORK;
> +		signal_wake_up(task, 0);
> +		unlock_task_sighand(task, &flags);
> +	}
> +#endif

Same #ifdeffery comment as before.

Thanks,

        tglx