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authorBoqun Feng <boqun.feng@gmail.com>2015-11-02 09:30:31 +0800
committerMichael Ellerman <mpe@ellerman.id.au>2015-12-14 20:38:18 +1100
commit49e9cf3f0c04bf76ffa59242254110309554861d (patch)
treea8dad7893126a02ebb18c88ea0d374502cf68beb
parent4dcbd88eb600d52ce52a75c5075c2eff2f6849e6 (diff)
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op-kernel-dev-49e9cf3f0c04bf76ffa59242254110309554861d.tar.gz
powerpc: Make value-returning atomics fully ordered
According to memory-barriers.txt: > 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") Cc: stable@vger.kernel.org # 3.2+ Signed-off-by: Boqun Feng <boqun.feng@gmail.com> Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
-rw-r--r--arch/powerpc/include/asm/synch.h2
1 files changed, 1 insertions, 1 deletions
diff --git a/arch/powerpc/include/asm/synch.h b/arch/powerpc/include/asm/synch.h
index e682a71..c508686 100644
--- a/arch/powerpc/include/asm/synch.h
+++ b/arch/powerpc/include/asm/synch.h
@@ -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
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