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Message-Id: <20160127180939.562799022@linuxfoundation.org>
Date: Wed, 27 Jan 2016 10:13:15 -0800
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-kernel@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
stable@...r.kernel.org, Boqun Feng <boqun.feng@...il.com>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
"Peter Zijlstra (Intel)" <peterz@...radead.org>,
Michael Ellerman <mpe@...erman.id.au>
Subject: [PATCH 4.3 128/157] powerpc: Make value-returning atomics fully ordered
4.3-stable review patch. If anyone has any objections, please let me know.
------------------
From: Boqun Feng <boqun.feng@...il.com>
commit 49e9cf3f0c04bf76ffa59242254110309554861d upstream.
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")
Signed-off-by: Boqun Feng <boqun.feng@...il.com>
Reviewed-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Signed-off-by: Michael Ellerman <mpe@...erman.id.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
arch/powerpc/include/asm/synch.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
--- 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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