Date: Thu, 26 May 2005 08:44:37 -0700 (PDT)
From: Linus Torvalds <torvalds@osdl.org>
To: Robin Holt <holt@sgi.com>
cc: Simon Derr <Simon.Derr@bull.net>, Paul Jackson <pj@sgi.com>,
       Andrew Morton <akpm@osdl.org>, Dinakar Guniguntala <dino@in.ibm.com>,
       linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2.6.12-rc4] cpuset exit NULL dereference fix
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On Thu, 26 May 2005, Robin Holt wrote:
> 
> Why not change the atomic into a lock and a refcount.  Grab the lock before
> each increment/decrement of the refcount and only continue with the removal
> code when the refcount reaches 0.

For this, there is a nice 

	atomic_dec_and_lock()

function that is the same as "atomic_dec_and_test()", except it grabs the 
lock if the count decrements to zero.

I'm surprised people haven't picked up on it - it's been around for a
while, the VFS layer uses it for some quite fundamental data structures
(inode and dcache refcounts), and it's even documented in "atomic_ops.txt"

And it's _designed_ for refcountign things like this.

Basic rule of kernel programming: if a globally visible object does not 
have a refcount, IT IS A BUG.

		Linus
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