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

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Date:	Thu, 07 Dec 2006 20:06:39 +0000
From:	David Howells <dhowells@...hat.com>
To:	Andrew Morton <akpm@...l.org>
Cc:	David Howells <dhowells@...hat.com>, 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() 

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 ...

Remember: if you have to put a conditional jump in there, it's going to fail
one way or the other a certain percentage of the time, and that's going to
cause a pipeline stall, and these ops are used quite a lot.

OTOH, I don't know that the stall would be that bad since the spin_lock and
spin_unlock may cause a stall anyway.

David
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