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Patch "powerpc: Make value-returning atomics fully ordered" has been added to the 4.1-stable tree

PATCH [email protected] 1 participant 1 message Jan 27, 2016

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This is a note to let you know that I've just added the patch titled

powerpc: Make value-returning atomics fully ordered

to the 4.1-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
powerpc-make-value-returning-atomics-fully-ordered.patch
and it can be found in the queue-4.1 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

Quoted 1 line

From 49e9cf3f0c04bf76ffa59242254110309554861d Mon Sep 17 00:00:00 2001

From: Boqun Feng <[email protected]>
Date: Mon, 2 Nov 2015 09:30:31 +0800
Subject: powerpc: Make value-returning atomics fully ordered

From: Boqun Feng <[email protected]>

commit 49e9cf3f0c04bf76ffa59242254110309554861d upstream.

According to memory-barriers.txt:

Quoted 4 lines

Any atomic operation that modifies some state in memory and returns information about the state (old or new) implies an SMP-conditional general memory barrier (smp_mb()) on each side of the actual operation ...

Which mean these operations should be fully ordered. However on PPC, PPC_ATOMIC_ENTRY_BARRIER is the barrier before the actual operation, which is currently "lwsync" if SMP=y. The leading "lwsync" can not guarantee fully ordered atomics, according to Paul Mckenney:

https://lkml.org/lkml/2015/10/14/970

To fix this, we define PPC_ATOMIC_ENTRY_BARRIER as "sync" to guarantee the fully-ordered semantics.

This also makes futex atomics fully ordered, which can avoid possible memory ordering problems if userspace code relies on futex system call for fully ordered semantics.

Fixes b97021f85517 ("powerpc: Fix atomic_xxx_return barrier semantics")
Signed-off-by Boqun Feng <[email protected]>
Reviewed-by Paul E. McKenney <[email protected]>
Acked-by Peter Zijlstra (Intel) <[email protected]>
Signed-off-by Michael Ellerman <[email protected]>
Signed-off-by Greg Kroah-Hartman <[email protected]>
---
 arch/powerpc/include/asm/synch.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
arch/powerpc/include/asm/synch.h +1 -1
@@ -44,7 +44,7 @@ static inline void isync(void)
 	MAKE_LWSYNC_SECTION_ENTRY(97, __lwsync_fixup);
 #define PPC_ACQUIRE_BARRIER	 "\n" stringify_in_c(__PPC_ACQUIRE_BARRIER)
 #define PPC_RELEASE_BARRIER	 stringify_in_c(LWSYNC) "\n"
-#define PPC_ATOMIC_ENTRY_BARRIER "\n" stringify_in_c(LWSYNC) "\n"
+#define PPC_ATOMIC_ENTRY_BARRIER "\n" stringify_in_c(sync) "\n"
 #define PPC_ATOMIC_EXIT_BARRIER	 "\n" stringify_in_c(sync) "\n"
 #else
 #define PPC_ACQUIRE_BARRIER

Patches currently in stable-queue which might be from [email protected] are

queue-4.1/powerpc-make-value-returning-atomics-fully-ordered.patch queue-4.1/powerpc-make-cmp-xchg-and-their-atomic_-versions-fully.patch