block: decrement block_rq_qos static key in rq_qos_del()

rq_qos_add() increments the block_rq_qos static key when a QoS
policy is attached. When a QoS policy is removed via rq_qos_del(),
we must symmetrically decrement the static key. If this removal drops
the last QoS policy from the queue (q->rq_qos becomes NULL), the
static branch can be disabled and the jump label patched to a NOP,
avoiding overhead on the hot path.

This change ensures rq_qos_add()/rq_qos_del() keep the
block_rq_qos static key balanced and prevents leaving the branch
permanently enabled after the last policy is removed.

Fixes: 033b667a823e ("block: blk-rq-qos: guard rq-qos helpers by static key")
Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20250814082612.500845-3-nilay@linux.ibm.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-rq-qos.c b/block/blk-rq-qos.c
index 848591fb3c57b6f068389a9aff988bd2970ec9ab..b1e24bb85ad2afb82a59d4b500f5852f8a54a0c9 100644 (file)
--- a/block/blk-rq-qos.c
+++ b/block/blk-rq-qos.c
@@ -374,6 +374,7 @@ void rq_qos_del(struct rq_qos *rqos)
        for (cur = &q->rq_qos; *cur; cur = &(*cur)->next) {
                if (*cur == rqos) {
                        *cur = rqos->next;
+                       static_branch_dec(&block_rq_qos);
                        break;
                }
        }