lockref: allow relaxed cmpxchg64 variant for lockless updates

The 64-bit cmpxchg operation on the lockref is ordered by virtue of
hazarding between the cmpxchg operation and the reference count
manipulation. On weakly ordered memory architectures (such as ARM), it
can be of great benefit to omit the barrier instructions where they are
not needed.

This patch moves the lockless lockref code over to a cmpxchg64_relaxed
operation, which doesn't provide barrier semantics. If the operation
isn't defined, we simply #define it as the usual 64-bit cmpxchg macro.

Cc: Waiman Long <Waiman.Long@hp.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/lib/lockref.c b/lib/lockref.c
index 677d036cf3c70d6d72b9aa5f108470dd890752cd..e294ae445c9ab4b7e97dd1a46efc7dc7d6a1848d 100644 (file)
--- a/lib/lockref.c
+++ b/lib/lockref.c
@@ -3,6 +3,14 @@
 
 #ifdef CONFIG_CMPXCHG_LOCKREF
 
+/*
+ * Allow weakly-ordered memory architectures to provide barrier-less
+ * cmpxchg semantics for lockref updates.
+ */
+#ifndef cmpxchg64_relaxed
+# define cmpxchg64_relaxed cmpxchg64
+#endif
+
 /*
  * Note that the "cmpxchg()" reloads the "old" value for the
  * failure case.
@@ -14,8 +22,9 @@
        while (likely(arch_spin_value_unlocked(old.lock.rlock.raw_lock))) {     \
                struct lockref new = old, prev = old;                           \
                CODE                                                            \
-               old.lock_count = cmpxchg64(&lockref->lock_count,                \
-                                          old.lock_count, new.lock_count);     \
+               old.lock_count = cmpxchg64_relaxed(&lockref->lock_count,        \
+                                                  old.lock_count,              \
+                                                  new.lock_count);             \
                if (likely(old.lock_count == prev.lock_count)) {                \
                        SUCCESS;                                                \
                }                                                               \