rq-qos: use a mb for got_token

Oleg noticed that our checking of data.got_token is unsafe in the
cleanup case, and should really use a memory barrier.  Use a wmb on the
write side, and a rmb() on the read side.  We don't need one in the main
loop since we're saved by set_current_state().

Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-rq-qos.c b/block/blk-rq-qos.c
index c450b89..3954c0d 100644 (file)
--- a/block/blk-rq-qos.c
+++ b/block/blk-rq-qos.c
@@ -202,6 +202,7 @@ static int rq_qos_wake_function(struct wait_queue_entry *curr,
                return -1;
 
        data->got_token = true;
+       smp_wmb();
        list_del_init(&curr->entry);
        wake_up_process(data->task);
        return 1;
@@ -246,6 +247,7 @@ void rq_qos_wait(struct rq_wait *rqw, void *private_data,
        prepare_to_wait_exclusive(&rqw->wait, &data.wq, TASK_UNINTERRUPTIBLE);
        has_sleeper = !wq_has_single_sleeper(&rqw->wait);
        do {
+               /* The memory barrier in set_task_state saves us here. */
                if (data.got_token)
                        break;
                if (!has_sleeper && acquire_inflight_cb(rqw, private_data)) {
@@ -256,6 +258,7 @@ void rq_qos_wait(struct rq_wait *rqw, void *private_data,
                         * which means we now have two. Put our local token
                         * and wake anyone else potentially waiting for one.
                         */
+                       smp_rmb();
                        if (data.got_token)
                                cleanup_cb(rqw, private_data);
                        break;