[PATCH 6.18 292/314] block: use trylock to avoid lockdep circular dependency in sysfs

Sasha Levin
2026-03-10T11:30:02Z

From: Ming Lei <ming.lei@redhat.com>

[ Upstream commit ce8ee8583ed83122405eabaa8fb351be4d9dc65c ]

Use trylock instead of blocking lock acquisition for update_nr_hwq_lock
in queue_requests_store() and elv_iosched_store() to avoid circular lock
dependency with kernfs active reference during concurrent disk deletion:

update_nr_hwq_lock -> kn->active (via del_gendisk -> kobject_del)
kn->active -> update_nr_hwq_lock (via sysfs write path)

Return -EBUSY when the lock is not immediately available.

Reported-and-tested-by: Yi Zhang <yi.zhang@redhat.com>
Closes: https://lore.kernel.org/linux-block/CAHj4cs-em-4acsHabMdT=jJhXkCzjnprD-aQH1OgrZo4nTnmMw@mail.gmail.com/
Fixes: 626ff4f8ebcb ("blk-mq: convert to serialize updating nr_requests with update_nr_hwq_lock")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Tested-by: Yi Zhang <yi.zhang@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
block/blk-sysfs.c | 8 +++++++-
block/elevator.c | 12 +++++++++++-
2 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index e0a70d26972b3..af12526d866a9 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -78,8 +78,14 @@ queue_requests_store(struct gendisk *disk, const char *page, size_t count)
/*
* Serialize updating nr_requests with concurrent queue_requests_store()
* and switching elevator.
+ *
+ * Use trylock to avoid circular lock dependency with kernfs active
+ * reference during concurrent disk deletion:
+ * update_nr_hwq_lock -> kn->active (via del_gendisk -> kobject_del)
+ * kn->active -> update_nr_hwq_lock (via this sysfs write path)
*/
- down_write(&set->update_nr_hwq_lock);
+ if (!down_write_trylock(&set->update_nr_hwq_lock))
+ return -EBUSY;

if (nr == q->nr_requests)
goto unlock;
diff --git a/block/elevator.c b/block/elevator.c
index a2f8b2251dc6e..7a97998cd8bd7 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -806,7 +806,16 @@ ssize_t elv_iosched_store(struct gendisk *disk, const char *buf,
elv_iosched_load_module(ctx.name);
ctx.type = elevator_find_get(ctx.name);

- down_read(&set->update_nr_hwq_lock);
+ /*
+ * Use trylock to avoid circular lock dependency with kernfs active
+ * reference during concurrent disk deletion:
+ * update_nr_hwq_lock -> kn->active (via del_gendisk -> kobject_del)
+ * kn->active -> update_nr_hwq_lock (via this sysfs write path)
+ */
+ if (!down_read_trylock(&set->update_nr_hwq_lock)) {
+ ret = -EBUSY;
+ goto out;
+ }
if (!blk_queue_no_elv_switch(q)) {
ret = elevator_change(q, &ctx);
if (!ret)
@@ -816,6 +825,7 @@ ssize_t elv_iosched_store(struct gendisk *disk, const char *buf,
}
up_read(&set->update_nr_hwq_lock);

+out:
if (ctx.type)
elevator_put(ctx.type);
return ret;
--
2.51.0