Re: [PATCH] bitops: add _local bitops

[H. Peter Anvin]

On 05/09/2012 08:47 AM, Michael S. Tsirkin wrote:
> 
> By the way, clear_bit on x86 does not seem to contain
> an optimization barrier - is my reading correct?
> Lock prefix does not affect the compiler, right?

Yes, as it clearly states in the comment:

 * clear_bit() is atomic and may not be reordered.  However, it does
 * not contain a memory barrier, so if it is used for locking purposes,
 * you should call smp_mb__before_clear_bit() and/or
smp_mb__after_clear_bit()
 * in order to ensure changes are visible on other processors.

There is clear_bit_unlock() which has the barrier semantics.

	-hpa


-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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