Re: [RFC PATCH 01/13] futex2: Implement wait and wake functions

[André Almeida]

Hi Peter,

Às 06:02 de 16/02/21, Peter Zijlstra escreveu:
> On Mon, Feb 15, 2021 at 12:23:52PM -0300, André Almeida wrote:
> > +static int __futex_wait(struct futexv_head *futexv, unsigned int nr_futexes,
> > +			struct hrtimer_sleeper *timeout)
> > +{
> > +	int ret;
> > +
> > +	while (1) {
> > +		int awakened = -1;
> > +
>
> Might be easier to understand if the set_current_state() is here,
> instead of squirreled away in futex_enqueue().

I placed set_current_state() inside futex_enqueue() because we need to 
set RUNNING and then INTERRUPTIBLE again for the retry path.

> > +		ret = futex_enqueue(futexv, nr_futexes, &awakened);
> > +
> > +		if (ret) {
> > +			if (awakened >= 0)
> > +				return awakened;
> > +			return ret;
> > +		}
> > +
> > +		/* Before sleeping, check if someone was woken */
> > +		if (!futexv->hint && (!timeout || timeout->task))
> > +			freezable_schedule();
> > +
> > +		__set_current_state(TASK_RUNNING);
>
> This is typically after the loop.

Sorry, which loop?

> > +
> > +		/*
> > +		 * One of those things triggered this wake:
> > +		 *
> > +		 * * We have been removed from the bucket. futex_wake() woke
> > +		 *   us. We just need to dequeue and return 0 to userspace.
> > +		 *
> > +		 * However, if no futex was dequeued by a futex_wake():
> > +		 *
> > +		 * * If the there's a timeout and it has expired,
> > +		 *   return -ETIMEDOUT.
> > +		 *
> > +		 * * If there is a signal pending, something wants to kill our
> > +		 *   thread, return -ERESTARTSYS.
> > +		 *
> > +		 * * If there's no signal pending, it was a spurious wake
> > +		 *   (scheduler gave us a change to do some work, even if we
>
> chance?

Indeed, fixed.

> > +		 *   don't want to). We need to remove ourselves from the
> > +		 *   bucket and add again, to prevent losing wakeups in the
> > +		 *   meantime.
> > +		 */
>
> Anyway, doing a dequeue and enqueue for spurious wakes is a bit of an
> anti-pattern that can lead to starvation. I've not actually looked at
> much detail yet as this is my first read-through, but did figure I'd
> mention it.

So we could just leave everything enqueued for spurious wake? I was 
expecting that we would need to do all the work again (including 
rechecking *uaddr == val) to see if we didn't miss a futex_wake() 
between the kernel thread waking (spuriously) and going to sleep again.

> > +
> > +		ret = futex_dequeue_multiple(futexv, nr_futexes);
> > +
> > +		/* Normal wake */
> > +		if (ret >= 0)
> > +			return ret;
> > +
> > +		if (timeout && !timeout->task)
> > +			return -ETIMEDOUT;
> > +
> > +		if (signal_pending(current))
> > +			return -ERESTARTSYS;
> > +
> > +		/* Spurious wake, do everything again */
> > +	}
> > +}

Thanks,
	André