[3.8.y.z extended stable] Patch "ARM: 7955/1: spinlock: ensure we have a compiler barrier before sev" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

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

to the linux-3.8.y-queue branch of the 3.8.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.8.y-queue

This patch is scheduled to be released in version 3.8.13.20.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.8.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From b40a702efef22eed3d41f35c2f219d587c22afb0 Mon Sep 17 00:00:00 2001
From: Will Deacon <will.deacon at arm.com>
Date: Fri, 7 Feb 2014 19:12:32 +0100
Subject: 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 at arm.com>
Signed-off-by: Will Deacon <will.deacon at arm.com>
Signed-off-by: Russell King <rmk+kernel at arm.linux.org.uk>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 arch/arm/include/asm/spinlock.h | 15 +++------------
 1 file changed, 3 insertions(+), 12 deletions(-)

diff --git a/arch/arm/include/asm/spinlock.h b/arch/arm/include/asm/spinlock.h
index b4ca707..d429475 100644
--- a/arch/arm/include/asm/spinlock.h
+++ b/arch/arm/include/asm/spinlock.h
@@ -44,18 +44,9 @@

 static inline void dsb_sev(void)
 {
-#if __LINUX_ARM_ARCH__ >= 7
-	__asm__ __volatile__ (
-		"dsb\n"
-		SEV
-	);
-#else
-	__asm__ __volatile__ (
-		"mcr p15, 0, %0, c7, c10, 4\n"
-		SEV
-		: : "r" (0)
-	);
-#endif
+
+	dsb(ishst);
+	__asm__(SEV);
 }

 /*
--
1.8.3.2