Successful RMW operations are supposed to be fully ordered, but
Alpha's xchg() and cmpxchg() do not align to this requirement.
Will reported that:
> So MP using xchg:
>
> WRITE_ONCE(x, 1)
> xchg(y, 1)
>
> smp_load_acquire(y) == 1
> READ_ONCE(x) == 0
>
> would be allowed.
(thus violating the above requirement). Amend this by adding a
leading smp_mb() to the implementations of xchg(), cmpxchg().
Reported-by: Will Deacon <will.dea...@arm.com>
Signed-off-by: Andrea Parri <parri.and...@gmail.com>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: Paul E. McKenney <paul...@linux.vnet.ibm.com>
Cc: Alan Stern <st...@rowland.harvard.edu>
Cc: Ivan Kokshaysky <i...@jurassic.park.msu.ru>
Cc: Matt Turner <matts...@gmail.com>
Cc: Richard Henderson <r...@twiddle.net>
Cc: linux-al...@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
---
arch/alpha/include/asm/xchg.h | 21 ++++++++++++++++++---
1 file changed, 18 insertions(+), 3 deletions(-)
diff --git a/arch/alpha/include/asm/xchg.h b/arch/alpha/include/asm/xchg.h
index e1facf6fc2446..e2b59fac5257d 100644
--- a/arch/alpha/include/asm/xchg.h
+++ b/arch/alpha/include/asm/xchg.h
@@ -12,6 +12,10 @@
* 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
@@ -19,6 +23,7 @@ ____xchg(_u8, volatile char *m, unsigned long val)
{
unsigned long ret, tmp, addr64;
+ smp_mb();
__asm__ __volatile__(
" andnot %4,7,%3\n"
" insbl %1,%4,%1\n"
@@ -43,6 +48,7 @@ ____xchg(_u16, volatile short *m, unsigned long val)
{
unsigned long ret, tmp, addr64;
+ smp_mb();
__asm__ __volatile__(
" andnot %4,7,%3\n"
" inswl %1,%4,%1\n"
@@ -67,6 +73,7 @@ ____xchg(_u32, volatile int *m, unsigned long val)
{
unsigned long dummy;
+ smp_mb();
__asm__ __volatile__(
"1: ldl_l %0,%4\n"
" bis $31,%3,%1\n"
@@ -87,6 +94,7 @@ ____xchg(_u64, volatile long *m, unsigned long val)
{
unsigned long dummy;
+ smp_mb();
__asm__ __volatile__(
"1: ldq_l %0,%4\n"
" bis $31,%3,%1\n"
@@ -128,9 +136,12 @@ ____xchg(, volatile void *ptr, unsigned long x, int size)
* 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
@@ -138,6 +149,7 @@ ____cmpxchg(_u8, volatile char *m, unsigned char old,
unsigned char new)
{
unsigned long prev, tmp, cmp, addr64;
+ smp_mb();
__asm__ __volatile__(
" andnot %5,7,%4\n"
" insbl %1,%5,%1\n"
@@ -165,6 +177,7 @@ ____cmpxchg(_u16, volatile short *m, unsigned short old,
unsigned short new)
{
unsigned long prev, tmp, cmp, addr64;
+ smp_mb();
__asm__ __volatile__(
" andnot %5,7,%4\n"
" inswl %1,%5,%1\n"
@@ -192,6 +205,7 @@ ____cmpxchg(_u32, volatile int *m, int old, int new)
{
unsigned long prev, cmp;
+ smp_mb();
__asm__ __volatile__(
"1: ldl_l %0,%5\n"
" cmpeq %0,%3,%1\n"
@@ -215,6 +229,7 @@ ____cmpxchg(_u64, volatile long *m, unsigned long old,
unsigned long new)
{
unsigned long prev, cmp;
+ smp_mb();
__asm__ __volatile__(
"1: ldq_l %0,%5\n"
" cmpeq %0,%3,%1\n"
--
2.7.4
