Hi,

On Sat, Dec 10, 2011 at 6:43 PM, Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> 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

IMO, the added two barriers are pairs of the implicit barrier in kref_put, so
that we can order between kref_init/kref_get and kref_put.

> 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: adobriyan@gmail.com
> Cc: eric.dumazet@gmail.com
> Cc: mingo@elte.hu
> Cc: Oliver Neukum <oneukum@suse.de>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> ---
>  include/linux/kref.h |    2 --
>  1 file changed, 2 deletions(-)
>
> Index: linux-2.6/include/linux/kref.h
> ===================================================================
> --- linux-2.6.orig/include/linux/kref.h
> +++ linux-2.6/include/linux/kref.h
> @@ -28,7 +28,6 @@ struct kref {
>  static inline void kref_init(struct kref *kref)
>  {
>        atomic_set(&kref->refcount, 1);
> -       smp_mb();
>  }
>
>  /**
> @@ -39,7 +38,6 @@ static inline void kref_get(struct kref
>  {
>        WARN_ON(!atomic_read(&kref->refcount));
>        atomic_inc(&kref->refcount);
> -       smp_mb__after_atomic_inc();
>  }
>
>  /**
>
>
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thanks,
-- 
Ming Lei
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