From: Russell King <rmk+kernel@arm.linux.org.uk>
Date: Sat, 18 Dec 2010 10:53:12 +0000 (+0000)
Subject: ARM: Fix subtle race in CPU pen_release hotplug code
X-Git-Tag: v2.6.38-rc1~471^2~2^2^2~10
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=3705ff6da538aff6dba535e2e9cbcbb9456d0d53;p=pandora-kernel.git

ARM: Fix subtle race in CPU pen_release hotplug code

There is a subtle race in the CPU hotplug code, where a CPU which has
been offlined can online itself before being requested, which results
in things going astray on the next online/offline cycle.

What happens in the normal online/offline/online cycle is:

	CPU0			CPU3
	requests boot of CPU3
	pen_release = 3
	flush cache line
				checks pen_release, reads 3
				starts boot
				pen_release = -1
	... requests CPU3 offline ...
				... dies ...
				checks pen_release, reads -1
	requests boot of CPU3
	pen_release = 3
	flush cache line
				checks pen_release, reads 3
				starts boot
				pen_release = -1

However, as the write of -1 of pen_release is not fully flushed back to
memory, and the checking of pen_release is done with caches disabled,
this allows CPU3 the opportunity to read the old value of pen_release:

	CPU0			CPU3
	requests boot of CPU3
	pen_release = 3
	flush cache line
				checks pen_release, reads 3
				starts boot
				pen_release = -1
	... requests CPU3 offline ...
				... dies ...
				checks pen_release, reads 3
				starts boot
				pen_release = -1
	requests boot of CPU3
	pen_release = 3
	flush cache line

Fix this by grouping the write of pen_release along with its cache line
flushing code to ensure that any update to pen_release is always pushed
out to physical memory.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
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