diff options
| author | Peter Zijlstra <a.p.zijlstra@chello.nl> | 2011-12-10 11:43:44 +0100 |
|---|---|---|
| committer | Greg Kroah-Hartman <gregkh@suse.de> | 2011-12-13 09:11:19 -0800 |
| commit | 3c8ed88974472b928489e3943616500ce2ad0cd8 (patch) | |
| tree | f13010d3417f86b3909cd55858ee589b7de366c9 | |
| parent | 47dbd7d90ad80edb67822f327241edcab8f3f46f (diff) | |
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 <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>
| -rw-r--r-- | include/linux/kref.h | 2 |
1 files changed, 0 insertions, 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(); } /** |