linux-kernel - Re: [RFC PATCH 02/33] Add sync bitops

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Message-Id: <7534c2861b892e1b42bc0880801a4121@cl.cam.ac.uk>
Date:	Tue, 18 Jul 2006 11:18:45 +0100
From:	Keir Fraser <Keir.Fraser@...cam.ac.uk>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Ian Pratt <ian.pratt@...source.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	xen-devel@...ts.xensource.com, linux-kernel@...r.kernel.org,
	virtualization@...ts.osdl.org,
	Christoph Lameter <clameter@....com>,
	Chris Wright <chrisw@...s-sol.org>,
	Andrew Morton <akpm@...l.org>
Subject: Re: [RFC PATCH 02/33] Add sync bitops


On 18 Jul 2006, at 10:56, Arjan van de Ven wrote:

>> plain text document attachment (synch-ops)
>> Add "always lock'd" implementations of set_bit, clear_bit and
>> change_bit and the corresponding test_and_ functions.  Also add
>> "always lock'd" implementation of cmpxchg.  These give guaranteed
>> strong synchronisation and are required for non-SMP kernels running on
>> an SMP hypervisor.
>
> Hi,
>
> this sounds really like the wrong approach; you know you're compiling
> for xen, so why not just make set_bit() and the others use the lock'd
> instructions at compile time?

Then all users of bitops would unnecessarily pay the price when running 
in a single-CPU guest. Only a few of our bitops callers absolutely 
require the lock prefix in all cases.

   -- Keir

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