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SubjectRe: [RFC PATCH 02/33] Add sync bitops
Date

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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