linux-kernel - Problem with atomic accesses in pstore on some ARM CPUs

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Date:	Mon, 15 Aug 2016 16:19:13 -0700
From:	Guenter Roeck <groeck@...gle.com>
To:	Kees Cook <keescook@...omium.org>,
	Jeffy Chen <jeffy.chen@...k-chips.com>,
	Colin Cross <ccross@...roid.com>,
	Tony Luck <tony.luck@...el.com>,
	Douglas Anderson <dianders@...omium.org>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-arm-kernel@...ts.infradead.org
Subject: Problem with atomic accesses in pstore on some ARM CPUs

Hi,

we are having a problem with atomic accesses in pstore on some ARM
CPUs (specifically rk3288 and rk3399). With those chips, atomic
accesses fail with both pgprot_noncached and pgprot_writecombine
memory. Atomic accesses do work when selecting PAGE_KERNEL protection.

Debugging on rk3399 shows the following crash.

[    0.912669] Bad mode in Error handler detected, code 0xbf000002 -- SError
[    0.920140] CPU: 4 PID: 1 Comm: swapper/0 Not tainted 4.4.14 #389
[    0.926838] Hardware name: Google Kevin (DT)
[    0.931533] task: ffffffc0edfe0000 ti: ffffffc0edf7c000 task.ti:
ffffffc0edf 7c000
[    0.939780] PC is at __ll_sc___cmpxchg_case_mb_4+0x2c/0x5c
[    0.945811] LR is at 0x1

The "solution" for this problem in various Chrome OS releases is to
disable atomic accesses in pstore entirely, which seems to be a bit
brute-force. Question is what a proper upstream-acceptable solution
might be. Introduce another memory type to select PAGE_KERNEL ? Is
there some means to determine if atomic operations are supported with
a given protection mask, maybe ? Anything else ?

Thanks,
Guenter