aio-use-cancellation-list-lazily-fix

aio-use-cancellation-list-lazily-fix

The cancellation changes were fubar - we can't cancel a kiocb if it
doesn't actually have a cancellation callback.

The use of xchg() in aio_complete() was right - there we're marking the
kiocb as completed - but we need to use cmpxchg() in kiocb_cancel() - a
lock isn't sufficient since we're synchronizing with aio_complete() which
isn't taking any locks.

Signed-off-by: Kent Overstreet <koverstreet@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>