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Date: Fri, 22 Dec 2000 18:33:00 +1100
From: Andrew Morton <andrewm@uow.edu.au>
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To: Andrea Arcangeli <andrea@suse.de>
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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Andrea Arcangeli wrote:
> 
> > > Other thing about your patch, adding TASK_EXCLUSIVE to
> > > wake_up/wake_up_interruptible is useless.
> >
> > This enables wake_up_all().
> 
> It is useless as it is in 2.2.19pre2: there's no wake_up_all in 2.2.19pre2.

#define wake_up_all(x) __wake_up((x),TASK_UNINTERRUPTIBLE | TASK_INTERRUPTIBLE)

> > Anyway, this is all just noise.
> >
> > The key question is: which of the following do we want?
> >
> > a) A simple, specific accept()-accelerator, and 2.2 remains without
> >    an exclusive wq API or
> 
> To make the accellerator we need a minimal wake-one support. So a) doesn't
> make sense to me.

It makes heaps of sense.  We've introduced into 2.2 an API
which has the same appearance as one in 2.4, but which is
subtly broken wrt the 2.4 one.

I suggest you change the names to something other than
add_waitqueue_exclusive() and TASK_EXCLUSIVE, add a
cautionary comment and then go ahead with your patch.

Except for this bit, which looks slightly fatal:

	/*
         * We can drop the read-lock early if this
         * is the only/last process.
         */
        if (next == head) {
                 read_unlock(&waitqueue_lock);
                 wake_up_process(p);
                 goto out;
        }

Once the waitqueue_lock has been dropped, the task at `p'
is free to remove itself from the waitqueue and exit.  This
CPU can then try to wake up a non-existent task, no?

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