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[Xen-devel] 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.


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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