ARM: 7955/1: spinlock: ensure we have a compiler barrier before sev
authorWill Deacon <will.deacon@arm.com>
Fri, 7 Feb 2014 18:12:32 +0000 (19:12 +0100)
committerRussell King <rmk+kernel@arm.linux.org.uk>
Mon, 10 Feb 2014 11:44:50 +0000 (11:44 +0000)
commit7c8746a9eb287642deaad0e7c2cdf482dce5e4be
treed8740bb7222926df4bae9f93a6564839b08dc6c3
parentbae0ca2bc550d1ec6a118fb8f2696f18c4da3d8e
ARM: 7955/1: spinlock: ensure we have a compiler barrier before sev

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.

Cc: <stable@vger.kernel.org>
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>
arch/arm/include/asm/spinlock.h