My load tests on PowerPC freeze within minutes in __slab_free(). I

My load tests on PowerPC freeze within minutes in __slab_free().  I
happened to try PPC first, didn't try without this fix on x86.

It looks as if the author was interrupted while devising the new
cmpxchg_double_slab() version of __slab_free(): its decision to
spin_lock_irqsave() depends on several uninitialized fields, and fixing
that (by copying page to new) mostly fixes it.

But I didn't think about it very much, and this may well not be what the
author intends; and I have seen a couple of much rarer freezes in
__slab_free() on PPC (not yet on x86) even after applying this.

Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: Christoph Lameter <cl@linux.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>