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Date: Thu, 19 Apr 2001 14:24:09 -0400 (EDT)
From: Alexander Viro <viro@math.psu.edu>
To: Linus Torvalds <torvalds@transmeta.com>
cc: Abramo Bagnara <abramo@alsa-project.org>, Alon Ziv <alonz@nolaviz.org>,
        Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Mike Kravetz <mkravetz@sequent.com>,
        Ulrich Drepper <drepper@cygnus.com>
Subject: Re: light weight user level semaphores
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Message-ID: <Pine.GSO.4.21.0104191409270.16930-100000@weyl.math.psu.edu>
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On Thu, 19 Apr 2001, Linus Torvalds wrote:

> 
> 
> On Thu, 19 Apr 2001, Alexander Viro wrote:
> >
> > Ehh... Non-lazy variant is just read() and write() as down_failed() and
> > up_wakeup() Lazy... How about
> 
> Looks good to me. Anybody want to try this out and test some benchmarks?

Ugh. It doesn't look good for me. s/MAX_INT/MAX_INT>>1/ or we will
get into trouble on anything that goes into spin_and_lose. Window is
pretty narrow (notice that lost_it is OK - we only need to worry
about somebody coming in after winner drives Lock from 1 to 0
and before it gets it from 0 to MAX_INT), but we can get into serious
trouble if schedule() will hit that window.

MAX_INT/2 should be enough to deal with that, AFAICS.

However, I would _really_ like to get that code reviewed from the memory
access ordering POV. Warning: right now I'm half-asleep, so the thing can
very well be completely bogus in that area. Extra eyes would be certainly
welcome.

								Al

PS: ->Lock should be set to 1 when we initialize semaphore. Destroying
semaphore should do
	if (sem->Lock > 1) {
		close(sem->writer);
		close(sem->reader);
	}

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