linux-kernel - Re: [PATCH] WorkStruct: Implement generic UP cmpxchg() where an arch doesn't support it

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Message-ID: <Pine.LNX.4.64.0612061122120.3542@woody.osdl.org>
Date:	Wed, 6 Dec 2006 11:25:35 -0800 (PST)
From:	Linus Torvalds <torvalds@...l.org>
To:	Al Viro <viro@....linux.org.uk>
cc:	Christoph Lameter <clameter@....com>,
	Russell King <rmk+lkml@....linux.org.uk>,
	David Howells <dhowells@...hat.com>,
	Andrew Morton <akpm@...l.org>,
	linux-arm-kernel@...ts.arm.linux.org.uk,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-arch@...r.kernel.org
Subject: Re: [PATCH] WorkStruct: Implement generic UP cmpxchg() where an arch
 doesn't support it

On Wed, 6 Dec 2006, Al Viro wrote:
> 
> No.  sparc32 doesn't have one, for instance.

Ok. For SMP-safety, it's important that any architecture that can't do 
this needs to _share_ the same spinlock (on SMP only, of course) that it 
uses for the bitops. 

It would be good (but perhaps not as strict a requirement) if the atomic 
counters also use the same lock. But that is probably impossible on 
sparc32 (since it has a per-counter "lock"-like thing, iirc). So doing a 
cmpxchg() on an atomic_t would be a bug.

		Linus
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