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Subject: Re: [PATCH 4/4] task_work: use TIF_NOTIFY_SIGNAL if available
To:     Thomas Gleixner <tglx@linutronix.de>, linux-kernel@vger.kernel.org,
        io-uring@vger.kernel.org
Cc:     peterz@infradead.org, oleg@redhat.com,
        Roman Gershman <romger@amazon.com>
References: <20201008152752.218889-1-axboe@kernel.dk>
 <20201008152752.218889-5-axboe@kernel.dk>
 <87362hd6ta.fsf@nanos.tec.linutronix.de>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <327d0ab5-a77e-8f9c-95da-d0dccece6ad8@kernel.dk>
Date:   Tue, 13 Oct 2020 17:55:07 -0600
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On 10/13/20 5:50 PM, Thomas Gleixner wrote:
> 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.

Fixed up.

-- 
Jens Axboe