The kernfs built-in lock of 'kn->count' is held in sysfs .show/.store path. Meantime, inside block's .show/.store callback, q->sysfs_lock is required. However, when mq & iosched kobjects are removed via blk_mq_unregister_dev() & elv_unregister_queue(), q->sysfs_lock is held too. This way causes AB-BA lock because the kernfs built-in lock of 'kn-count' is required inside kobject_del() too, see the lockdep warning[1]. On the other hand, it isn't necessary to acquire q->sysfs_lock for both blk_mq_unregister_dev() & elv_unregister_queue() because clearing REGISTERED flag prevents storing to 'queue/scheduler' from being happened. Also sysfs write(store) is exclusive, so no necessary to hold the lock for elv_unregister_queue() when it is called in switching elevator path. Fixes the issue by not holding the q->sysfs_lock for blk_mq_unregister_dev() & elv_unregister_queue(). [1] lockdep warning ====================================================== WARNING: possible circular locking dependency detected 5.3.0-rc3-00044-g73277fc75ea0 #1380 Not tainted ------------------------------------------------------ rmmod/777 is trying to acquire lock: 00000000ac50e981 (kn->count#202){++++}, at: kernfs_remove_by_name_ns+0x59/0x72 but task is already holding lock: 00000000fb16ae21 (&q->sysfs_lock){+.+.}, at: blk_unregister_queue+0x78/0x10b which lock already depends on the new lock. the existing dependency chain (in reverse order) is: -> #1 (&q->sysfs_lock){+.+.}: __lock_acquire+0x95f/0xa2f lock_acquire+0x1b4/0x1e8 __mutex_lock+0x14a/0xa9b blk_mq_hw_sysfs_show+0x63/0xb6 sysfs_kf_seq_show+0x11f/0x196 seq_read+0x2cd/0x5f2 vfs_read+0xc7/0x18c ksys_read+0xc4/0x13e do_syscall_64+0xa7/0x295 entry_SYSCALL_64_after_hwframe+0x49/0xbe -> #0 (kn->count#202){++++}: check_prev_add+0x5d2/0xc45 validate_chain+0xed3/0xf94 __lock_acquire+0x95f/0xa2f lock_acquire+0x1b4/0x1e8 __kernfs_remove+0x237/0x40b kernfs_remove_by_name_ns+0x59/0x72 remove_files+0x61/0x96 sysfs_remove_group+0x81/0xa4 sysfs_remove_groups+0x3b/0x44 kobject_del+0x44/0x94 blk_mq_unregister_dev+0x83/0xdd blk_unregister_queue+0xa0/0x10b del_gendisk+0x259/0x3fa null_del_dev+0x8b/0x1c3 [null_blk] null_exit+0x5c/0x95 [null_blk] __se_sys_delete_module+0x204/0x337 do_syscall_64+0xa7/0x295 entry_SYSCALL_64_after_hwframe+0x49/0xbe other info that might help us debug this: Possible unsafe locking scenario: CPU0 CPU1 ---- ---- lock(&q->sysfs_lock); lock(kn->count#202); lock(&q->sysfs_lock); lock(kn->count#202); *** DEADLOCK *** 2 locks held by rmmod/777: #0: 00000000e69bd9de (&lock){+.+.}, at: null_exit+0x2e/0x95 [null_blk] #1: 00000000fb16ae21 (&q->sysfs_lock){+.+.}, at: blk_unregister_queue+0x78/0x10b stack backtrace: CPU: 0 PID: 777 Comm: rmmod Not tainted 5.3.0-rc3-00044-g73277fc75ea0 #1380 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS ?-20180724_192412-buildhw-07.phx4 Call Trace: dump_stack+0x9a/0xe6 check_noncircular+0x207/0x251 ? print_circular_bug+0x32a/0x32a ? find_usage_backwards+0x84/0xb0 check_prev_add+0x5d2/0xc45 validate_chain+0xed3/0xf94 ? check_prev_add+0xc45/0xc45 ? mark_lock+0x11b/0x804 ? check_usage_forwards+0x1ca/0x1ca __lock_acquire+0x95f/0xa2f lock_acquire+0x1b4/0x1e8 ? kernfs_remove_by_name_ns+0x59/0x72 __kernfs_remove+0x237/0x40b ? kernfs_remove_by_name_ns+0x59/0x72 ? kernfs_next_descendant_post+0x7d/0x7d ? strlen+0x10/0x23 ? strcmp+0x22/0x44 kernfs_remove_by_name_ns+0x59/0x72 remove_files+0x61/0x96 sysfs_remove_group+0x81/0xa4 sysfs_remove_groups+0x3b/0x44 kobject_del+0x44/0x94 blk_mq_unregister_dev+0x83/0xdd blk_unregister_queue+0xa0/0x10b del_gendisk+0x259/0x3fa ? disk_events_poll_msecs_store+0x12b/0x12b ? check_flags+0x1ea/0x204 ? mark_held_locks+0x1f/0x7a null_del_dev+0x8b/0x1c3 [null_blk] null_exit+0x5c/0x95 [null_blk] __se_sys_delete_module+0x204/0x337 ? free_module+0x39f/0x39f ? blkcg_maybe_throttle_current+0x8a/0x718 ? rwlock_bug+0x62/0x62 ? __blkcg_punt_bio_submit+0xd0/0xd0 ? trace_hardirqs_on_thunk+0x1a/0x20 ? mark_held_locks+0x1f/0x7a ? do_syscall_64+0x4c/0x295 do_syscall_64+0xa7/0x295 entry_SYSCALL_64_after_hwframe+0x49/0xbe RIP: 0033:0x7fb696cdbe6b Code: 73 01 c3 48 8b 0d 1d 20 0c 00 f7 d8 64 89 01 48 83 c8 ff c3 66 2e 0f 1f 84 00 00 008 RSP: 002b:00007ffec9588788 EFLAGS: 00000206 ORIG_RAX: 00000000000000b0 RAX: ffffffffffffffda RBX: 0000559e589137c0 RCX: 00007fb696cdbe6b RDX: 000000000000000a RSI: 0000000000000800 RDI: 0000559e58913828 RBP: 0000000000000000 R08: 00007ffec9587701 R09: 0000000000000000 R10: 00007fb696d4eae0 R11: 0000000000000206 R12: 00007ffec95889b0 R13: 00007ffec95896b3 R14: 0000559e58913260 R15: 0000559e589137c0 Cc: Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> Cc: Mike Snitzer <snitzer@xxxxxxxxxx> Cc: Bart Van Assche <bvanassche@xxxxxxx> Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx> --- block/blk-mq-sysfs.c | 4 ++-- block/blk-sysfs.c | 9 ++++----- block/elevator.c | 16 +++++++++++++--- 3 files changed, 19 insertions(+), 10 deletions(-) diff --git a/block/blk-mq-sysfs.c b/block/blk-mq-sysfs.c index e0b97c22726c..d5e805281130 100644 --- a/block/blk-mq-sysfs.c +++ b/block/blk-mq-sysfs.c @@ -247,8 +247,6 @@ void blk_mq_unregister_dev(struct device *dev, struct request_queue *q) struct blk_mq_hw_ctx *hctx; int i; - lockdep_assert_held(&q->sysfs_lock); - queue_for_each_hw_ctx(q, hctx, i) blk_mq_unregister_hctx(hctx); @@ -256,7 +254,9 @@ void blk_mq_unregister_dev(struct device *dev, struct request_queue *q) kobject_del(q->mq_kobj); kobject_put(&dev->kobj); + mutex_lock(&q->sysfs_lock); q->mq_sysfs_init_done = false; + mutex_unlock(&q->sysfs_lock); } void blk_mq_hctx_kobj_init(struct blk_mq_hw_ctx *hctx) diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c index 977c659dcd18..46f033b48917 100644 --- a/block/blk-sysfs.c +++ b/block/blk-sysfs.c @@ -1021,6 +1021,7 @@ EXPORT_SYMBOL_GPL(blk_register_queue); void blk_unregister_queue(struct gendisk *disk) { struct request_queue *q = disk->queue; + bool has_elevator; if (WARN_ON(!q)) return; @@ -1035,8 +1036,9 @@ void blk_unregister_queue(struct gendisk *disk) * concurrent elv_iosched_store() calls. */ mutex_lock(&q->sysfs_lock); - blk_queue_flag_clear(QUEUE_FLAG_REGISTERED, q); + has_elevator = q->elevator; + mutex_unlock(&q->sysfs_lock); /* * Remove the sysfs attributes before unregistering the queue data @@ -1044,16 +1046,13 @@ void blk_unregister_queue(struct gendisk *disk) */ if (queue_is_mq(q)) blk_mq_unregister_dev(disk_to_dev(disk), q); - mutex_unlock(&q->sysfs_lock); kobject_uevent(&q->kobj, KOBJ_REMOVE); kobject_del(&q->kobj); blk_trace_remove_sysfs(disk_to_dev(disk)); - mutex_lock(&q->sysfs_lock); - if (q->elevator) + if (has_elevator) elv_unregister_queue(q); - mutex_unlock(&q->sysfs_lock); kobject_put(&disk_to_dev(disk)->kobj); } diff --git a/block/elevator.c b/block/elevator.c index 2f17d66d0e61..4a12c60ba946 100644 --- a/block/elevator.c +++ b/block/elevator.c @@ -495,16 +495,17 @@ int elv_register_queue(struct request_queue *q) void elv_unregister_queue(struct request_queue *q) { - lockdep_assert_held(&q->sysfs_lock); - if (q) { struct elevator_queue *e = q->elevator; kobject_uevent(&e->kobj, KOBJ_REMOVE); kobject_del(&e->kobj); + + mutex_lock(&q->sysfs_lock); e->registered = 0; /* Re-enable throttling in case elevator disabled it */ wbt_enable_default(q); + mutex_unlock(&q->sysfs_lock); } } @@ -567,8 +568,17 @@ int elevator_switch_mq(struct request_queue *q, lockdep_assert_held(&q->sysfs_lock); if (q->elevator) { - if (q->elevator->registered) + if (q->elevator->registered) { + /* + * sysfs write is exclusively, release + * sysfs_lock for avoiding deadlock with + * sysfs built-in lock which is required + * in either .show or .store path. + */ + mutex_unlock(&q->sysfs_lock); elv_unregister_queue(q); + mutex_lock(&q->sysfs_lock); + } ioc_clear_queue(q); elevator_exit(q, q->elevator); } -- 2.20.1