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

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Date:	Mon, 11 Dec 2006 11:04:00 +0000
From:	David Howells <dhowells@...hat.com>
To:	Russell King <rmk+lkml@....linux.org.uk>
Cc:	Linus Torvalds <torvalds@...l.org>,
	David Howells <dhowells@...hat.com>,
	Christoph Lameter <clameter@....com>,
	Nick Piggin <nickpiggin@...oo.com.au>, 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 

Russell King <rmk+lkml@....linux.org.uk> wrote:

> Yes you can.  Well, you can on ARM at least.  Between the load exclusive
> you can do anything you like until you hit the store exclusive.

How come atomic_set() on arm6 is implemented as:

	static inline void atomic_set(atomic_t *v, int i)
	{
		unsigned long tmp;

		__asm__ __volatile__("@ atomic_set\n"
	"1:	ldrex	%0, [%1]\n"
	"	strex	%0, %2, [%1]\n"
	"	teq	%0, #0\n"
	"	bne	1b"
		: "=&r" (tmp)
		: "r" (&v->counter), "r" (i)
		: "cc");
	}

Why LDREX/STREX and not direct assignment?

David
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