linux-kernel - Re: [PATCH 3/3] WorkStruct: Use direct assignment rather than cmpxchg()

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Date:	Thu, 7 Dec 2006 23:42:51 +0000
From:	Russell King <rmk+lkml@....linux.org.uk>
To:	David Howells <dhowells@...hat.com>
Cc:	Andrew Morton <akpm@...l.org>, torvalds@...l.org,
	davem@...emloft.com, wli@...omorphy.com, matthew@....cx,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH 3/3] WorkStruct: Use direct assignment rather than cmpxchg()

On Thu, Dec 07, 2006 at 08:06:39PM +0000, David Howells wrote:
> Andrew Morton <akpm@...l.org> wrote:
> 
> > and we can assume (and ensure) that a failing test_and_set_bit() will not
> > write to the affected word at all.
> 
> You may not assume that; and indeed that is not so in the generic
> spinlock-based bitops or ARM pre-v6 or PA-RISC or sparc32 or ...

Incorrect.  pre-v6 ARM bitops for test_and_xxx_bit() all do:

	save and disable irqs
	load value
	test bit
	if not in desired state, alter bit and write it back
	restore irqs

but I don't guarantee that we'll always do that - indeed, post-armv6
bitops always write back even if the bit was in the desired state.

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:
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