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

[Chris Wilson <chris@chris-wilson.co.uk>]

On Sat, Jan 09, 2016 at 02:36:03PM -0800, Andy Lutomirski wrote:
> On Sat, Jan 9, 2016 at 12:01 AM, Chris Wilson <chris@chris-wilson.co.uk> wrote:
> > 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?
> >
> > s/sync_cpu()/sync_core()/
> >
> > 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?
> 
> No clue, but I don't know much about the underlying architecture.
> 
> Can you try clflush_cache_ranging one cacheline less and then manually
> doing clflushopt; mb on the last cache line, just to make sure that
> the helper is really doing the right thing?  You could also try
> clflush instead of clflushopt to see if that makes a difference.

I had looked at increasing the range over which clflush_cache_range()
runs (using roundup/rounddown by cache lines), but it took something
like +/- 256 bytes to pass all the tests. And also did
s/clflushopt/clflush/ to confirm that made no differnce.

Bizarrely,

diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c
index 6000ad7..cf074400 100644
--- a/arch/x86/mm/pageattr.c
+++ b/arch/x86/mm/pageattr.c
@@ -141,6 +141,7 @@ void clflush_cache_range(void *vaddr, unsigned int size)
        for (; p < vend; p += clflush_size)
                clflushopt(p);
 
+       clflushopt(vend-1);
        mb();
 }
 EXPORT_SYMBOL_GPL(clflush_cache_range);

works like a charm.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre