Elevator switch code is another `nr_hw_queue` reader in non-fast-IO code
path, so it can't be done if updating `nr_hw_queues` is in-progress.
Take same approach with not allowing add/del disk when updating
nr_hw_queues is in-progress, by grabbing read lock of
set->update_nr_hwq_sema.
Take the nested variant for avoiding the following false positive
splat[1], and this way is correct because:
- the read lock in elv_iosched_store() is not overlapped with the read lock
in adding/deleting disk:
- kobject attribute is only available after the kobject is added and
before it is deleted
-> #4 (&q->q_usage_counter(queue){++++}-{0:0}:
-> #3 (&q->limits_lock){+.+.}-{4:4}:
-> #2 (&disk->open_mutex){+.+.}-{4:4}:
-> #1 (&set->update_nr_hwq_lock){.+.+}-{4:4}:
-> #0 (kn->active#103){++++}-{0:0}:
Link: https://lore.kernel.org/linux-block/aAWv3NPtNIKKvJZc@fedora/ [1]
Reported-by: Shinichiro Kawasaki <shinichiro.kawasaki@xxxxxxx>
Closes: https://lore.kernel.org/linux-block/mz4t4tlwiqjijw3zvqnjb7ovvvaegkqganegmmlc567tt5xj67@xal5ro544cnc/
Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
---
block/elevator.c | 3 +++
1 file changed, 3 insertions(+)
diff --git a/block/elevator.c b/block/elevator.c
index 4400eb8fe54f..56da6ab7691a 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -723,6 +723,7 @@ ssize_t elv_iosched_store(struct gendisk *disk, const char *buf,
int ret;
unsigned int memflags;
struct request_queue *q = disk->queue;
+ struct blk_mq_tag_set *set = q->tag_set;
/*
* If the attribute needs to load a module, do it before freezing the
@@ -734,6 +735,7 @@ ssize_t elv_iosched_store(struct gendisk *disk, const char *buf,
elv_iosched_load_module(name);
+ down_read_nested(&set->update_nr_hwq_sema, 1);
memflags = blk_mq_freeze_queue(q);
mutex_lock(&q->elevator_lock);
ret = elevator_change(q, name);
@@ -741,6 +743,7 @@ ssize_t elv_iosched_store(struct gendisk *disk, const char *buf,
ret = count;
mutex_unlock(&q->elevator_lock);
blk_mq_unfreeze_queue(q, memflags);
+ up_read(&set->update_nr_hwq_sema);
return ret;
}
--
2.47.0
