From 153847586b0aaa9482e42bc7e95b24adb87a1859 Mon Sep 17 00:00:00 2001 From: Chris Metcalf Date: Tue, 26 Apr 2016 09:54:56 -0400 Subject: 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 --- arch/tile/include/asm/atomic_64.h | 17 +++++++++++++++-- 1 file changed, 15 insertions(+), 2 deletions(-) (limited to 'arch/tile/include') 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; } -- cgit v1.2.3