commit 115e12e58dbc055e98c965e3255aed7b20214f95 upstream.

The memory operations before reading cq head are unrelated and we
don't care about their order.

Document that the control dependency in combination with READ_ONCE and
WRITE_ONCE forms a barrier we need.
Signed-off-by: NStefan Bühler <source@stbuehler.de>
Signed-off-by: NJens Axboe <axboe@kernel.dk>
Signed-off-by: NJoseph Qi <joseph.qi@linux.alibaba.com>
Reviewed-by: NJeffle Xu <jefflexu@linux.alibaba.com>
Acked-by: NCaspar Zhang <caspar@linux.alibaba.com>