Commits
Peter Zijlstra committed 3c8ed889744
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>