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

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Date:	Thu, 7 Dec 2006 16:51:42 +0000
From:	Ralf Baechle <ralf@...ux-mips.org>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	Russell King <rmk+lkml@....linux.org.uk>,
	Christoph Lameter <clameter@....com>,
	David Howells <dhowells@...hat.com>, torvalds@...l.org,
	akpm@...l.org, linux-arm-kernel@...ts.arm.linux.org.uk,
	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 Thu, Dec 07, 2006 at 08:31:08PM +1100, Nick Piggin wrote:

> Wrong. Your ll/sc implementation with cmpxchg is buggy. The cmpxchg
> load_locked is not locked at all, and there can be interleaving writes
> between the load and cmpxchg which do not cause the store_conditional
> to fail.
> 
> It might be reasonable to implement this watered down version, but:
> don't some architectures have restrictions on what instructions can
> be issued between the ll and the sc?

On MIPS the restriction is no loads or stores or sync instructions between
ll/sc.  Also there may be at most 2048 bytes between the address of the
ll and sc instructions.  Which means ll/sc sequences should better be
written in assembler or gcc might do a bit too creative things ...

  Ralf
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