From: Will Deacon <will.deacon@arm.com>
Date: Fri, 7 Feb 2014 18:12:32 +0000 (+0100)
Subject: ARM: 7955/1: spinlock: ensure we have a compiler barrier before sev
X-Git-Tag: v3.2.56~101
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=8c8e77aa65fd685e09804e7a6709c82baa3648d1;p=pandora-kernel.git

ARM: 7955/1: spinlock: ensure we have a compiler barrier before sev

commit 7c8746a9eb287642deaad0e7c2cdf482dce5e4be upstream.

When unlocking a spinlock, we require the following, strictly ordered
sequence of events:

	<barrier>	/* dmb */
	<unlock>
	<barrier>	/* dsb */
	<sev>

Whilst the code does indeed reflect this in terms of the architecture,
the final <barrier> + <sev> have been contracted into a single inline
asm without a "memory" clobber, therefore the compiler is at liberty to
reorder the unlock to the end of the above sequence. In such a case,
a waiting CPU may be woken up before the lock has been unlocked, leading
to extremely poor performance.

This patch reworks the dsb_sev() function to make use of the dsb()
macro and ensure ordering against the unlock.

Reported-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
[bwh: Backported to 3.2: 'ishst' variant is not used here]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---

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