On 28/07/2021 05:01, Josef Bacik wrote:

We got the following lockdep splat while running xfstests (specifically
btrfs/003 and btrfs/020 in a row) with the new rc.  This was uncovered
by 87579e9b7d8d ("loop: use worker per cgroup instead of kworker") which
converted loop to using workqueues, which comes with lockdep
annotations that don't exist with kworkers.  The lockdep splat is as
follows

======================================================
WARNING: possible circular locking dependency detected
5.14.0-rc2-custom+ #34 Not tainted
------------------------------------------------------
losetup/156417 is trying to acquire lock:
ffff9c7645b02d38 ((wq_completion)loop0){+.+.}-{0:0}, at: flush_workqueue+0x84/0x600

but task is already holding lock:
ffff9c7647395468 (&lo->lo_mutex){+.+.}-{3:3}, at: __loop_clr_fd+0x41/0x650 [loop]

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #5 (&lo->lo_mutex){+.+.}-{3:3}:
        __mutex_lock+0xba/0x7c0
        lo_open+0x28/0x60 [loop]
        blkdev_get_whole+0x28/0xf0
        blkdev_get_by_dev.part.0+0x168/0x3c0
        blkdev_open+0xd2/0xe0
        do_dentry_open+0x163/0x3a0
        path_openat+0x74d/0xa40
        do_filp_open+0x9c/0x140
        do_sys_openat2+0xb1/0x170
        __x64_sys_openat+0x54/0x90
        do_syscall_64+0x3b/0x90
        entry_SYSCALL_64_after_hwframe+0x44/0xae

-> #4 (&disk->open_mutex){+.+.}-{3:3}:
        __mutex_lock+0xba/0x7c0
        blkdev_get_by_dev.part.0+0xd1/0x3c0
        blkdev_get_by_path+0xc0/0xd0
        btrfs_scan_one_device+0x52/0x1f0 [btrfs]
        btrfs_control_ioctl+0xac/0x170 [btrfs]
        __x64_sys_ioctl+0x83/0xb0
        do_syscall_64+0x3b/0x90
        entry_SYSCALL_64_after_hwframe+0x44/0xae

-> #3 (uuid_mutex){+.+.}-{3:3}:
        __mutex_lock+0xba/0x7c0
        btrfs_rm_device+0x48/0x6a0 [btrfs]
        btrfs_ioctl+0x2d1c/0x3110 [btrfs]
        __x64_sys_ioctl+0x83/0xb0
        do_syscall_64+0x3b/0x90
        entry_SYSCALL_64_after_hwframe+0x44/0xae

-> #2 (sb_writers#11){.+.+}-{0:0}:
        lo_write_bvec+0x112/0x290 [loop]
        loop_process_work+0x25f/0xcb0 [loop]
        process_one_work+0x28f/0x5d0
        worker_thread+0x55/0x3c0
        kthread+0x140/0x170
        ret_from_fork+0x22/0x30

-> #1 ((work_completion)(&lo->rootcg_work)){+.+.}-{0:0}:
        process_one_work+0x266/0x5d0
        worker_thread+0x55/0x3c0
        kthread+0x140/0x170
        ret_from_fork+0x22/0x30

-> #0 ((wq_completion)loop0){+.+.}-{0:0}:
        __lock_acquire+0x1130/0x1dc0
        lock_acquire+0xf5/0x320
        flush_workqueue+0xae/0x600
        drain_workqueue+0xa0/0x110
        destroy_workqueue+0x36/0x250
        __loop_clr_fd+0x9a/0x650 [loop]
        lo_ioctl+0x29d/0x780 [loop]
        block_ioctl+0x3f/0x50
        __x64_sys_ioctl+0x83/0xb0
        do_syscall_64+0x3b/0x90
        entry_SYSCALL_64_after_hwframe+0x44/0xae

other info that might help us debug this:
Chain exists of:
   (wq_completion)loop0 --> &disk->open_mutex --> &lo->lo_mutex
  Possible unsafe locking scenario:
        CPU0                    CPU1
        ----                    ----
   lock(&lo->lo_mutex);
                                lock(&disk->open_mutex);
                                lock(&lo->lo_mutex);
   lock((wq_completion)loop0);

  *** DEADLOCK ***
1 lock held by losetup/156417:
  #0: ffff9c7647395468 (&lo->lo_mutex){+.+.}-{3:3}, at: __loop_clr_fd+0x41/0x650 [loop]

stack backtrace:
CPU: 8 PID: 156417 Comm: losetup Not tainted 5.14.0-rc2-custom+ #34
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
Call Trace:
  dump_stack_lvl+0x57/0x72
  check_noncircular+0x10a/0x120
  __lock_acquire+0x1130/0x1dc0
  lock_acquire+0xf5/0x320
  ? flush_workqueue+0x84/0x600
  flush_workqueue+0xae/0x600
  ? flush_workqueue+0x84/0x600
  drain_workqueue+0xa0/0x110
  destroy_workqueue+0x36/0x250
  __loop_clr_fd+0x9a/0x650 [loop]
  lo_ioctl+0x29d/0x780 [loop]
  ? __lock_acquire+0x3a0/0x1dc0
  ? update_dl_rq_load_avg+0x152/0x360
  ? lock_is_held_type+0xa5/0x120
  ? find_held_lock.constprop.0+0x2b/0x80
  block_ioctl+0x3f/0x50
  __x64_sys_ioctl+0x83/0xb0
  do_syscall_64+0x3b/0x90
  entry_SYSCALL_64_after_hwframe+0x44/0xae
RIP: 0033:0x7f645884de6b

Usually the uuid_mutex exists to protect the fs_devices that map
together all of the devices that match a specific uuid.  In rm_device
we're messing with the uuid of a device, so it makes sense to protect
that here.

However in doing that it pulls in a whole host of lockdep dependencies,
as we call mnt_may_write() on the sb before we grab the uuid_mutex, thus
we end up with the dependency chain under the uuid_mutex being added
under the normal sb write dependency chain, which causes problems with
loop devices.

We don't need the uuid mutex here however.  If we call
btrfs_scan_one_device() before we scratch the super block we will find
the fs_devices and not find the device itself and return EBUSY because
the fs_devices is open.  If we call it after the scratch happens it will
not appear to be a valid btrfs file system.

We do not need to worry about other fs_devices modifying operations here
because we're protected by the exclusive operations locking.

So drop the uuid_mutex here in order to fix the lockdep splat.

I think uuid_mutex should stay. Here is why.

  While thread A takes %device at line 816 and deref at line 880.
  Thread B can completely remove and free that %device.
  As of now these threads are mutual exclusive using uuid_mutex.

Thread A

btrfs_control_ioctl()
   mutex_lock(&uuid_mutex);
     btrfs_scan_one_device()
       device_list_add()
       {
  815                 mutex_lock(&fs_devices->device_list_mutex);

  816                 device = btrfs_find_device(fs_devices, devid,
  817                                 disk_super->dev_item.uuid, NULL);

  880         } else if (!device->name || strcmp(device->name->str, path)) {

  933                         if (device->bdev->bd_dev != path_dev) {

  982         mutex_unlock(&fs_devices->device_list_mutex);
        }


Thread B

btrfs_rm_device()

2069         mutex_lock(&uuid_mutex);  <-- proposed to remove

2150         mutex_lock(&fs_devices->device_list_mutex);

2172         mutex_unlock(&fs_devices->device_list_mutex);

2180                 btrfs_scratch_superblocks(fs_info, device->bdev,
2181                                           device->name->str);

2183         btrfs_close_bdev(device);
2184         synchronize_rcu();
2185         btrfs_free_device(device);

2194         mutex_unlock(&uuid_mutex);  <-- proposed to remove


Well, I don't have a better option to fix this issue as of now.