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

[H. Peter Anvin]

On January 11, 2016 3:28:01 AM PST, Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> wrote:
>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@xxxxxxxxxxxxxxxxxx> 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

That clflushopt touches a cache line already touched and therefore serializes with it.
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