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

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Message-ID: <20061206192939.GX3013@parisc-linux.org>
Date:	Wed, 6 Dec 2006 12:29:39 -0700
From:	Matthew Wilcox <matthew@....cx>
To:	Linus Torvalds <torvalds@...l.org>
Cc:	Al Viro <viro@....linux.org.uk>,
	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, Dec 06, 2006 at 11:25:35AM -0800, Linus Torvalds wrote:
> 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. 

That doesn't help, since assignment can't be guarded by any lock.

> 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.

sparc32 switched over to the parisc way of doing things, so they could
expand their atomic_t to a full 32 bits.  They still have the old
atomic_24_t lying around for their arch-private use, but atomic_t uses a
hashed spinlock.
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