kern/atomic: fix consume memory order

The consume memory order is known to be problematic in the C11 specification,
and as a result, most compilers alias it to acquire. Based on the assumption
that the kernel doesn't run on architectures with an Alpha-like memory
model, the consume memory order was aliased to relaxed, letting the CPU
identify data dependencies. But there may also be issues at compile time,
making this hack is dangerous. As a result, the actual consume memory order
as implemented by the compiler is now used to define the kernel consume
memory order.

See https://lwn.net/Articles/588300/.

1 files changed, 1 insertions, 12 deletions

diff --git a/kern/atomic.h b/kern/atomic.h
index 6b4ebf3..5d99da9 100644
--- a/kern/atomic.h
+++ b/kern/atomic.h
@@ -31,20 +31,9 @@
 
 /*
  * Supported memory orders.
- *
- * Note that the consume order is aliased to relaxed. This assumes that
- * all supported processors respect data dependencies. The rationale is
- * that the definition for the consume order is confusing enough that
- * most compilers alias it to acquire, which forces the generation of
- * memory barrier instructions even when they're not really needed.
- * Since there is currently no processor where using consume or relaxed
- * would produce different code, it is safe to establish that alias.
- * It serves as explicit documentation for code review, and will easily
- * be replaced with the true consume order once compiler support becomes
- * efficient and reliable.
  */
 #define ATOMIC_RELAXED   __ATOMIC_RELAXED
-#define ATOMIC_CONSUME   __ATOMIC_RELAXED
+#define ATOMIC_CONSUME   __ATOMIC_CONSUME
 #define ATOMIC_ACQUIRE   __ATOMIC_ACQUIRE
 #define ATOMIC_RELEASE   __ATOMIC_RELEASE
 #define ATOMIC_ACQ_REL   __ATOMIC_ACQ_REL