kref: Remove the memory barriers

Commit 1b0b3b9980 ("kref: fix CPU ordering with respect to krefs")
wrongly adds memory barriers to kref.

It states:

  some atomic operations are only atomic, not ordered. Thus a CPU is allowed
  to reorder memory references to an object to before the reference is
  obtained. This fixes it.

While true, it fails to show why this is a problem. I say it is not a
problem because if there is a race with kref_put() such that we could
end up referencing a free'd object without this memory barrier, we
would still have that race with the memory barrier.

The kref_put() in question could complete (and free the object) before
the atomic_inc() and we'd still be up shit creek.

The kref_init() case is even worse, if your object is published at this
time you're so wrong the memory barrier won't make a difference what
so ever. If its not published, the act of publishing should include
the needed barriers/locks to make sure all writes prior to the act of
publishing are complete such that others will only observe a complete
object.

Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Oliver Neukum <oneukum@suse.de>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

include/linux/kref.h

@@ -29,7 +29,6 @@ struct kref {
 static inline void kref_init(struct kref *kref)
 {
 	atomic_set(&kref->refcount, 1);
-	smp_mb();
 }
 
 /**
@@ -40,7 +39,6 @@ static inline void kref_get(struct kref *kref)
 {
 	WARN_ON(!atomic_read(&kref->refcount));
 	atomic_inc(&kref->refcount);
-	smp_mb__after_atomic_inc();
 }
 
 /**