From 3c8ed88974472b928489e3943616500ce2ad0cd8 Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Sat, 10 Dec 2011 11:43:44 +0100 Subject: [PATCH] kref: Remove the memory barriers Commit 1b0b3b9980e ("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 Cc: Eric Dumazet Cc: Ingo Molnar Cc: Oliver Neukum Signed-off-by: Peter Zijlstra Signed-off-by: Greg Kroah-Hartman --- include/linux/kref.h | 2 -- 1 file changed, 2 deletions(-) diff --git a/include/linux/kref.h b/include/linux/kref.h index fa9907a541e2..d66c88a3b48c 100644 --- a/include/linux/kref.h +++ b/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(); } /** -- 2.20.1