bfq: Fix the missing barrier in __bfq_entity_update_weight_prio

The comment of bfq_group_set_weight says the reading of prio_changed
should happen before the reading of weight, but a memory barrier is
missing here. Add it now, to match the smp_wmb() there.

Signed-off-by: Fam Zheng <zhengfeiran@bytedance.com>
Reviewed-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bfq-wf2q.c

@@ -744,6 +744,8 @@ __bfq_entity_update_weight_prio(struct bfq_service_tree *old_st,
 		}
 #endif
 
+		/* Matches the smp_wmb() in bfq_group_set_weight. */
+		smp_rmb();
 		old_st->wsum -= entity->weight;
 
 		if (entity->new_weight != entity->orig_weight) {