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Date:   Wed, 8 Aug 2018 10:12:27 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     Catalin Marinas <catalin.marinas@....com>
cc:     Will Deacon <will.deacon@....com>,
        Jingoo Han <jingoohan1@...il.com>,
        Joao Pinto <Joao.Pinto@...opsys.com>,
        Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
        libc-alpha@...rceware.org,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Russell King <linux@...linux.org.uk>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Matt Sealey <neko@...uhatsu.net>, linux-pci@...r.kernel.org,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: framebuffer corruption due to overlapping stp instructions on
 arm64



On Wed, 8 Aug 2018, Catalin Marinas wrote:

> On Fri, Aug 03, 2018 at 01:09:02PM -0400, Mikulas Patocka wrote:
> > 	while (1) {
> > 		start = (unsigned)random() % (LEN + 1);
> > 		end = (unsigned)random() % (LEN + 1);
> > 		if (start > end)
> > 			continue;
> > 		for (i = start; i < end; i++)
> > 			data[i] = val++;
> > 		memcpy(map + start, data + start, end - start);
> > 		if (memcmp(map, data, LEN)) {
> 
> It may be worth trying to do a memcmp(map+start, data+start, end-start)
> here to see whether the hazard logic fails when the writes are unaligned
> but the reads are not.
> 
> This problem may as well appear if you do byte writes and read longs
> back (and I consider this a hardware problem on this specific board).

I triad to insert usleep(10000) between the memcpy and memcmp, but the 
same corruption occurs. So, it can't be read-after-write hazard. It is 
caused by the improper handling of hazard between the overlapping writes 
inside memcpy.

Mikulas

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