Date: Fri, 3 Mar 2006 14:21:58 -0800 (PST)
From: Linus Torvalds <torvalds@osdl.org>
To: Benjamin LaHaise <bcrl@linux.intel.com>
cc: David Howells <dhowells@redhat.com>, Andrew Morton <akpm@osdl.org>,
       ak@suse.de, mingo@redhat.com, jblunck@suse.de, matthew@wil.cx,
       linux-arch@vger.kernel.org, linuxppc64-dev@ozlabs.org,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: Memory barriers and spin_unlock safety
In-Reply-To: <20060303215114.GA13893@linux.intel.com>
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On Fri, 3 Mar 2006, Benjamin LaHaise wrote:
> 
> Actually, no.  At least in testing an implementation of Dekker's and 
> Peterson's algorithms as a replacement for the locked operation in 
> our spinlocks, it is absolutely necessary to have an sfence in the lock 
> to ensure the lock is visible to the other CPU before proceeding.

I suspect you have some bug in your implementation. I think Dekker's 
algorithm depends on the reads and writes being ordered, and you don't 
seem to do that.

The thing is, you pretty much _have_ to be wrong, because the x86-64 
memory ordering rules are _exactly_ the same as for x86, and we've had 
that simple store as an unlock for a long long time.

		Linus
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