block: split .sysfs_lock into two locks

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.

So split .sysfs_lock into two: one is still named as .sysfs_lock for
covering sync .store, the other one is named as .sysfs_dir_lock
for covering kobjects and related status change.

sysfs itself can handle the race between add/remove kobjects and
showing/storing attributes under kobjects. For switching scheduler
via storing to 'queue/scheduler', we use the queue flag of
QUEUE_FLAG_REGISTERED with .sysfs_lock for avoiding the race, then
we can avoid to hold .sysfs_lock during removing/adding kobjects.

[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: Christoph Hellwig <hch@infradead.org>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Greg KH <gregkh@linuxfoundation.org>
Cc: Mike Snitzer <snitzer@redhat.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 28 insertions, 18 deletions

diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index 5b0b5224cfd4..107513495220 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -938,6 +938,7 @@ int blk_register_queue(struct gendisk *disk)
 	int ret;
 	struct device *dev = disk_to_dev(disk);
 	struct request_queue *q = disk->queue;
+	bool has_elevator = false;
 
 	if (WARN_ON(!q))
 		return -ENXIO;
@@ -945,7 +946,6 @@ int blk_register_queue(struct gendisk *disk)
 	WARN_ONCE(blk_queue_registered(q),
 		  "%s is registering an already registered queue\n",
 		  kobject_name(&dev->kobj));
-	blk_queue_flag_set(QUEUE_FLAG_REGISTERED, q);
 
 	/*
 	 * SCSI probing may synchronously create and destroy a lot of
@@ -965,8 +965,7 @@ int blk_register_queue(struct gendisk *disk)
 	if (ret)
 		return ret;
 
-	/* Prevent changes through sysfs until registration is completed. */
-	mutex_lock(&q->sysfs_lock);
+	mutex_lock(&q->sysfs_dir_lock);
 
 	ret = kobject_add(&q->kobj, kobject_get(&dev->kobj), "%s", "queue");
 	if (ret < 0) {
@@ -987,26 +986,36 @@ int blk_register_queue(struct gendisk *disk)
 		blk_mq_debugfs_register(q);
 	}
 
-	kobject_uevent(&q->kobj, KOBJ_ADD);
-
-	wbt_enable_default(q);
-
-	blk_throtl_register_queue(q);
-
+	/*
+	 * The flag of QUEUE_FLAG_REGISTERED isn't set yet, so elevator
+	 * switch won't happen at all.
+	 */
 	if (q->elevator) {
-		ret = elv_register_queue(q);
+		ret = elv_register_queue(q, false);
 		if (ret) {
-			mutex_unlock(&q->sysfs_lock);
-			kobject_uevent(&q->kobj, KOBJ_REMOVE);
+			mutex_unlock(&q->sysfs_dir_lock);
 			kobject_del(&q->kobj);
 			blk_trace_remove_sysfs(dev);
 			kobject_put(&dev->kobj);
 			return ret;
 		}
+		has_elevator = true;
 	}
+
+	mutex_lock(&q->sysfs_lock);
+	blk_queue_flag_set(QUEUE_FLAG_REGISTERED, q);
+	wbt_enable_default(q);
+	blk_throtl_register_queue(q);
+
+	/* Now everything is ready and send out KOBJ_ADD uevent */
+	kobject_uevent(&q->kobj, KOBJ_ADD);
+	if (has_elevator)
+		kobject_uevent(&q->elevator->kobj, KOBJ_ADD);
+	mutex_unlock(&q->sysfs_lock);
+
 	ret = 0;
 unlock:
-	mutex_unlock(&q->sysfs_lock);
+	mutex_unlock(&q->sysfs_dir_lock);
 	return ret;
 }
 EXPORT_SYMBOL_GPL(blk_register_queue);
@@ -1021,6 +1030,7 @@ 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;
@@ -1035,25 +1045,25 @@ 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);
 
+	mutex_lock(&q->sysfs_dir_lock);
 	/*
 	 * Remove the sysfs attributes before unregistering the queue data
 	 * structures that can be modified through sysfs.
 	 */
 	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);
+	mutex_unlock(&q->sysfs_dir_lock);
 
 	kobject_put(&disk_to_dev(disk)->kobj);
 }