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authorChris Metcalf <cmetcalf@mellanox.com>2016-04-26 09:54:56 -0400
committerChris Metcalf <cmetcalf@mellanox.com>2016-04-26 09:54:56 -0400
commit153847586b0aaa9482e42bc7e95b24adb87a1859 (patch)
tree1bd99e332e83378d2067ede0d6d981d4a3e2fe78 /arch/tile
parent85f5251792abcd6dae897df8eb4ca0e890bc5882 (diff)
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tile: clarify barrier semantics of atomic_add_return
A recent discussion on LKML made it clear that the one-line comment previously in atomic_add_return() was not clear enough: https://lkml.kernel.org/r/571E87E2.3010306@mellanox.com Signed-off-by: Chris Metcalf <cmetcalf@mellanox.com>
Diffstat (limited to 'arch/tile')
-rw-r--r--arch/tile/include/asm/atomic_64.h17
1 files changed, 15 insertions, 2 deletions
diff --git a/arch/tile/include/asm/atomic_64.h b/arch/tile/include/asm/atomic_64.h
index 51cabc26e387..b0531a623653 100644
--- a/arch/tile/include/asm/atomic_64.h
+++ b/arch/tile/include/asm/atomic_64.h
@@ -37,12 +37,25 @@ static inline void atomic_add(int i, atomic_t *v)
__insn_fetchadd4((void *)&v->counter, i);
}
+/*
+ * Note a subtlety of the locking here. We are required to provide a
+ * full memory barrier before and after the operation. However, we
+ * only provide an explicit mb before the operation. After the
+ * operation, we use barrier() to get a full mb for free, because:
+ *
+ * (1) The barrier directive to the compiler prohibits any instructions
+ * being statically hoisted before the barrier;
+ * (2) the microarchitecture will not issue any further instructions
+ * until the fetchadd result is available for the "+ i" add instruction;
+ * (3) the smb_mb before the fetchadd ensures that no other memory
+ * operations are in flight at this point.
+ */
static inline int atomic_add_return(int i, atomic_t *v)
{
int val;
smp_mb(); /* barrier for proper semantics */
val = __insn_fetchadd4((void *)&v->counter, i) + i;
- barrier(); /* the "+ i" above will wait on memory */
+ barrier(); /* equivalent to smp_mb(); see block comment above */
return val;
}
@@ -95,7 +108,7 @@ static inline long atomic64_add_return(long i, atomic64_t *v)
int val;
smp_mb(); /* barrier for proper semantics */
val = __insn_fetchadd((void *)&v->counter, i) + i;
- barrier(); /* the "+ i" above will wait on memory */
+ barrier(); /* equivalent to smp_mb; see atomic_add_return() */
return val;
}