Re: [PATCH] bitops: add _local bitops

H. Peter Anvin
2012-05-09T16:25:05Z

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.