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>

diff --git a/arch/tile/include/asm/atomic_64.h b/arch/tile/include/asm/atomic_64.h
index 51cabc26e387c32fd368d8058563eae117782ff2..b0531a623653a702544c475c110e8d19cb7b10eb 100644 (file)
--- 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;
 }