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metag: fix memory barriers 

Volatile access doesn't really imply the compiler barrier. Volatile access
is only ordered with respect to other volatile accesses, it isn't ordered
with respect to general memory accesses. Gcc may reorder memory accesses
around volatile access, as we can see in this simple example (if we
compile it with optimization, both increments of *b will be collapsed to
just one):

void fn(volatile int *a, long *b)
{
	(*b)++;
	*a = 10;
	(*b)++;
}

Consequently, we need the compiler barrier after a write to the volatile
variable, to make sure that the compiler doesn't reorder the volatile
write with something else.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@vger.kernel.org
Acked-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: James Hogan <james.hogan@imgtec.com>
This commit is contained in:
Mikulas Patocka 2014-05-08 15:51:37 -04:00 committed by James Hogan
parent d6d211db37
commit 2425ce8402
1 changed files with 3 additions and 0 deletions
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3
arch/metag/include/asm/barrier.h
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@ -15,6 +15,7 @@ static inline void wr_fence(void)
volatile int *flushptr = (volatile int *) LINSYSEVENT_WR_FENCE; volatile int *flushptr = (volatile int *) LINSYSEVENT_WR_FENCE;
barrier(); barrier();
*flushptr = 0; *flushptr = 0;
barrier();
} }
#else /* CONFIG_METAG_META21 */ #else /* CONFIG_METAG_META21 */
@ -35,6 +36,7 @@ static inline void wr_fence(void)
*flushptr = 0; *flushptr = 0;
*flushptr = 0; *flushptr = 0;
*flushptr = 0; *flushptr = 0;
barrier();
} }
#endif /* !CONFIG_METAG_META21 */ #endif /* !CONFIG_METAG_META21 */
@ -68,6 +70,7 @@ static inline void fence(void)
volatile int *flushptr = (volatile int *) LINSYSEVENT_WR_ATOMIC_UNLOCK; volatile int *flushptr = (volatile int *) LINSYSEVENT_WR_ATOMIC_UNLOCK;
barrier(); barrier();
*flushptr = 0; *flushptr = 0;
barrier();
} }
#define smp_mb() fence() #define smp_mb() fence()
#define smp_rmb() fence() #define smp_rmb() fence()