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Subject[PATCH 4.9 136/329] locking/xchg/alpha: Fix xchg() and cmpxchg() memory ordering bugs
Date
4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Andrea Parri <parri.andrea@gmail.com>

[ Upstream commit 472e8c55cf6622d1c112dc2bc777f68bbd4189db ]

Successful RMW operations are supposed to be fully ordered, but
Alpha's xchg() and cmpxchg() do not meet this requirement.

Will Deacon noticed the bug:

> So MP using xchg:
>
> WRITE_ONCE(x, 1)
> xchg(y, 1)
>
> smp_load_acquire(y) == 1
> READ_ONCE(x) == 0
>
> would be allowed.

... which thus violates the above requirement.

Fix it by adding a leading smp_mb() to the xchg() and cmpxchg() implementations.

Reported-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Andrea Parri <parri.andrea@gmail.com>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matt Turner <mattst88@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-alpha@vger.kernel.org
Link: http://lkml.kernel.org/r/1519291488-5752-1-git-send-email-parri.andrea@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/alpha/include/asm/xchg.h | 21 ++++++++++++++++++---
1 file changed, 18 insertions(+), 3 deletions(-)

--- a/arch/alpha/include/asm/xchg.h
+++ b/arch/alpha/include/asm/xchg.h
@@ -11,6 +11,10 @@
* Atomic exchange.
* Since it can be used to implement critical sections
* it must clobber "memory" (also for interrupts in UP).
+ *
+ * The leading and the trailing memory barriers guarantee that these
+ * operations are fully ordered.
+ *
*/

static inline unsigned long
@@ -18,6 +22,7 @@ ____xchg(_u8, volatile char *m, unsigned
{
unsigned long ret, tmp, addr64;

+ smp_mb();
__asm__ __volatile__(
" andnot %4,7,%3\n"
" insbl %1,%4,%1\n"
@@ -42,6 +47,7 @@ ____xchg(_u16, volatile short *m, unsign
{
unsigned long ret, tmp, addr64;

+ smp_mb();
__asm__ __volatile__(
" andnot %4,7,%3\n"
" inswl %1,%4,%1\n"
@@ -66,6 +72,7 @@ ____xchg(_u32, volatile int *m, unsigned
{
unsigned long dummy;

+ smp_mb();
__asm__ __volatile__(
"1: ldl_l %0,%4\n"
" bis $31,%3,%1\n"
@@ -86,6 +93,7 @@ ____xchg(_u64, volatile long *m, unsigne
{
unsigned long dummy;

+ smp_mb();
__asm__ __volatile__(
"1: ldq_l %0,%4\n"
" bis $31,%3,%1\n"
@@ -127,9 +135,12 @@ ____xchg(, volatile void *ptr, unsigned
* store NEW in MEM. Return the initial value in MEM. Success is
* indicated by comparing RETURN with OLD.
*
- * The memory barrier is placed in SMP unconditionally, in order to
- * guarantee that dependency ordering is preserved when a dependency
- * is headed by an unsuccessful operation.
+ * The leading and the trailing memory barriers guarantee that these
+ * operations are fully ordered.
+ *
+ * The trailing memory barrier is placed in SMP unconditionally, in
+ * order to guarantee that dependency ordering is preserved when a
+ * dependency is headed by an unsuccessful operation.
*/

static inline unsigned long
@@ -137,6 +148,7 @@ ____cmpxchg(_u8, volatile char *m, unsig
{
unsigned long prev, tmp, cmp, addr64;

+ smp_mb();
__asm__ __volatile__(
" andnot %5,7,%4\n"
" insbl %1,%5,%1\n"
@@ -164,6 +176,7 @@ ____cmpxchg(_u16, volatile short *m, uns
{
unsigned long prev, tmp, cmp, addr64;

+ smp_mb();
__asm__ __volatile__(
" andnot %5,7,%4\n"
" inswl %1,%5,%1\n"
@@ -191,6 +204,7 @@ ____cmpxchg(_u32, volatile int *m, int o
{
unsigned long prev, cmp;

+ smp_mb();
__asm__ __volatile__(
"1: ldl_l %0,%5\n"
" cmpeq %0,%3,%1\n"
@@ -214,6 +228,7 @@ ____cmpxchg(_u64, volatile long *m, unsi
{
unsigned long prev, cmp;

+ smp_mb();
__asm__ __volatile__(
"1: ldq_l %0,%5\n"
" cmpeq %0,%3,%1\n"

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