Re: [PATCH] x86: Add an explicit barrier() to clflushopt()

From: Chris Wilson
Date: Sat Jan 09 2016 - 03:02:27 EST

On Thu, Jan 07, 2016 at 02:32:23PM -0800, H. Peter Anvin wrote:
> On 01/07/16 14:29, H. Peter Anvin wrote:
> >
> > I would be very interested in knowing if replacing the final clflushopt
> > with a clflush would resolve your problems (in which case the last mb()
> > shouldn't be necessary either.)
> >
> Nevermind. CLFLUSH is not ordered with regards to CLFLUSHOPT to the
> same cache line.
> Could you add a sync_cpu(); call to the end (can replace the final mb())
> and see if that helps your case?


No. I still see failures on Baytrail and Braswell (Pineview is not
affected) with the final mb() replaced with sync_core(). I can reproduce
failures on Pineview by tweaking the clflush_cache_range() parameters,
so I am fairly confident that it is validating the current code.

iirc sync_core() is cpuid, a heavy serialising instruction, an
alternative to mfence. Is there anything that else I can infer about
the nature of my bug from this result?

Chris Wilson, Intel Open Source Technology Centre