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Message-ID: <3A42AA60.80FA07F7@innominate.de>
Date: Fri, 22 Dec 2000 02:12:00 +0100
From: Daniel Phillips <phillips@innominate.de>
Organization: innominate
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To: Paul Cassella <pwc@sgi.com>, linux-kernel@vger.kernel.org
Subject: Re: [RFC] Semaphores used for daemon wakeup
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Paul Cassella wrote:
> > int atomic_read_and_clear(atomic_t *p)
> > {
> >         int n = atomic_read(p);
> >         atomic_sub(p, n);
> >         return n;
> > }
> 
> I don't think this will work; consider two callers doing the atomic_read()
> at the same time, or someone else doing an atomic_dec() after the
> atomic_read().

Oh yes, mea culpa, this is a terrible primitive, yet it works for this
application.  1) We don't have two callers 2) We only have atomic_inc
from the other processes, and it's ok for the atomic_inc to occur after
the atomic_read because that means the atomic_inc'er will then proceed
to up() the atomic_sub'ers semaphore, and it won't block.

I much preferred my original waiters = xchg(&sem.count, 0), but as noted
it doesn't work with sparc.  A satisfying approach would be to create
the new primitive up_down, which simplifies everything dramatically.

--
Daniel
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