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Date: Sun, 24 Dec 2000 15:43:02 +0100
From: Andrea Arcangeli <andrea@suse.de>
To: Andrew Morton <andrewm@uow.edu.au>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>, linux-kernel@vger.kernel.org,
        "David S. Miller" <davem@redhat.com>
Subject: Re: Linux 2.2.19pre2
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In-Reply-To: <3A4586D6.EF357D62@uow.edu.au>; from andrewm@uow.edu.au on Sun, Dec 24, 2000 at 04:17:10PM +1100
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On Sun, Dec 24, 2000 at 04:17:10PM +1100, Andrew Morton wrote:
> I was talking about a different scenario:
> 
>         add_wait_queue_exclusive(&q->wait_for_request, &wait);
>         for (;;) {
>                 __set_current_state(TASK_UNINTERRUPTIBLE);
> 	/* WINDOW */
>                 spin_lock_irq(&io_request_lock);
>                 rq = get_request(q, rw);
>                 spin_unlock_irq(&io_request_lock);
>                 if (rq)
>                         break;
>                 generic_unplug_device(q);
>                 schedule();
>         }
>         remove_wait_queue(&q->wait_for_request, &wait);
> 
> Suppose there are two tasks sleeping in the schedule().
> 
> A wakeup comes.  One task wakes.  It loops aound and reaches
> the window.  At this point in time, another wakeup gets sent
> to the waitqueue. It gets directed to the task which just
> woke up![..]

Ok, this is a very minor window compared to the current one, but yes, that
could happen too in test4.

> I assume this is because this waitqueue gets lots of wakeups sent to it.

It only gets the strictly necessary number of wakeups.

> Linus suggested at one point that we clear the waitqueue's
> WQ_FLAG_EXCLUSIVE bit when we wake it up, [..]

.. and then set it after checking if a new request is available, just
before schedule(). That would avoid the above race (and the one
I mentioned in previous email) but it doesn't address the lost wakeups
for example when setting USE_RW_WAIT_QUEUE_SPINLOCK to 1.

Considering wakeups only the ones that moves the task to the runqueue will get
rid of the races all together and it looks right conceptually so I prefer it.

Andrea
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