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

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Message-ID: <20160109080138.GG652@nuc-i3427.alporthouse.com>
Date:	Sat, 9 Jan 2016 08:01:38 +0000
From:	Chris Wilson <chris@...is-wilson.co.uk>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	"H . Peter Anvin" <hpa@...ux.intel.com>,
	Borislav Petkov <bp@...en8.de>,
	Brian Gerst <brgerst@...il.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Imre Deak <imre.deak@...el.com>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	DRI <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH] x86: Add an explicit barrier() to clflushopt()

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?
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre