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Date: Sun, 3 Mar 2002 18:07:21 +1100
From: Rusty Russell <rusty@rustcorp.com.au>
To: Benjamin LaHaise <bcrl@redhat.com>
Cc: alan@lxorguk.ukuu.org.uk, torvalds@transmeta.com, mingo@elte.hu,
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        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] Lightweight userspace semaphores...
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On Mon, 25 Feb 2002 11:32:40 -0500
Benjamin LaHaise <bcrl@redhat.com> wrote:

> On Mon, Feb 25, 2002 at 04:39:56PM +0000, Alan Cox wrote:
> > _alloca
> > mmap
> > 
> > Still fits on the stack 8)
> 
> Are we sure that forcing semaphore overhead to the size of a page is a 
> good idea?  I'd much rather see a sleep/wakeup mechanism akin to wait 
> queues be exported by the kernel so that userspace can implement a rich 
> set of locking functions on top of that in whatever shared memory is 
> being used.

Unfortunately, no.  You need to know what userspace is using them for so
you can check to avoid the "add to waitqueue" race.

AFAICT a mutex is the simplest useful primitive that can be realistically
exported.

Hope that helps,
Rusty.
-- 
  Anyone who quotes me in their sig is an idiot. -- Rusty Russell.
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