generic/648, on latest misc-next (that has this patch integrated),
also triggers the same type of lockdep warning involving the same two
locks:

  This lockdep warning is fixed by the yet to merge patch:

   [PATCH v2 3/7] btrfs: do not read super look for a device path


Thanks, Anand

[19738.081729] ======================================================
[19738.082620] WARNING: possible circular locking dependency detected
[19738.083511] 5.15.0-rc2-btrfs-next-99 #1 Not tainted
[19738.084234] ------------------------------------------------------
[19738.085149] umount/508378 is trying to acquire lock:
[19738.085884] ffff97a34c161d48 ((wq_completion)loop0){+.+.}-{0:0},
at: flush_workqueue+0x8b/0x5b0
[19738.087180]
                but task is already holding lock:
[19738.088048] ffff97a31f64d4a0 (&lo->lo_mutex){+.+.}-{3:3}, at:
__loop_clr_fd+0x5a/0x680 [loop]
[19738.089274]
                which lock already depends on the new lock.

[19738.090287]
                the existing dependency chain (in reverse order) is:
[19738.091216]
                -> #8 (&lo->lo_mutex){+.+.}-{3:3}:
[19738.091959]        __mutex_lock+0x92/0x900
[19738.092473]        lo_open+0x28/0x60 [loop]
[19738.093018]        blkdev_get_whole+0x28/0x90
[19738.093650]        blkdev_get_by_dev.part.0+0x142/0x320
[19738.094298]        blkdev_open+0x5e/0xa0
[19738.094790]        do_dentry_open+0x163/0x390
[19738.095425]        path_openat+0x3f0/0xa80
[19738.096041]        do_filp_open+0xa9/0x150
[19738.096657]        do_sys_openat2+0x97/0x160
[19738.097299]        __x64_sys_openat+0x54/0x90
[19738.097914]        do_syscall_64+0x3b/0xc0
[19738.098433]        entry_SYSCALL_64_after_hwframe+0x44/0xae
[19738.099243]
                -> #7 (&disk->open_mutex){+.+.}-{3:3}:
[19738.100259]        __mutex_lock+0x92/0x900
[19738.100865]        blkdev_get_by_dev.part.0+0x56/0x320
[19738.101530]        swsusp_check+0x19/0x150
[19738.102046]        software_resume.part.0+0xb8/0x150
[19738.102678]        resume_store+0xaf/0xd0
[19738.103181]        kernfs_fop_write_iter+0x140/0x1e0
[19738.103799]        new_sync_write+0x122/0x1b0
[19738.104341]        vfs_write+0x29e/0x3d0
[19738.104831]        ksys_write+0x68/0xe0
[19738.105309]        do_syscall_64+0x3b/0xc0
[19738.105823]        entry_SYSCALL_64_after_hwframe+0x44/0xae
[19738.106524]
                -> #6 (system_transition_mutex/1){+.+.}-{3:3}:
[19738.107393]        __mutex_lock+0x92/0x900
[19738.107911]        software_resume.part.0+0x18/0x150
[19738.108537]        resume_store+0xaf/0xd0
[19738.109057]        kernfs_fop_write_iter+0x140/0x1e0
[19738.109675]        new_sync_write+0x122/0x1b0
[19738.110218]        vfs_write+0x29e/0x3d0
[19738.110711]        ksys_write+0x68/0xe0
[19738.111190]        do_syscall_64+0x3b/0xc0
[19738.111699]        entry_SYSCALL_64_after_hwframe+0x44/0xae
[19738.112388]
                -> #5 (&of->mutex){+.+.}-{3:3}:
[19738.113089]        __mutex_lock+0x92/0x900
[19738.113600]        kernfs_seq_start+0x2a/0xb0
[19738.114141]        seq_read_iter+0x101/0x4d0
[19738.114679]        new_sync_read+0x11b/0x1a0
[19738.115212]        vfs_read+0x128/0x1c0
[19738.115691]        ksys_read+0x68/0xe0
[19738.116159]        do_syscall_64+0x3b/0xc0
[19738.116670]        entry_SYSCALL_64_after_hwframe+0x44/0xae
[19738.117382]
                -> #4 (&p->lock){+.+.}-{3:3}:
[19738.118062]        __mutex_lock+0x92/0x900
[19738.118580]        seq_read_iter+0x51/0x4d0
[19738.119102]        proc_reg_read_iter+0x48/0x80
[19738.119651]        generic_file_splice_read+0x102/0x1b0
[19738.120301]        splice_file_to_pipe+0xbc/0xd0
[19738.120879]        do_sendfile+0x14e/0x5a0
[19738.121389]        do_syscall_64+0x3b/0xc0
[19738.121901]        entry_SYSCALL_64_after_hwframe+0x44/0xae
[19738.122597]
                -> #3 (&pipe->mutex/1){+.+.}-{3:3}:
[19738.123339]        __mutex_lock+0x92/0x900
[19738.123850]        iter_file_splice_write+0x98/0x440
[19738.124475]        do_splice+0x36b/0x880
[19738.124981]        __do_splice+0xde/0x160
[19738.125483]        __x64_sys_splice+0x92/0x110
[19738.126037]        do_syscall_64+0x3b/0xc0
[19738.126553]        entry_SYSCALL_64_after_hwframe+0x44/0xae
[19738.127245]
                -> #2 (sb_writers#14){.+.+}-{0:0}:
[19738.127978]        lo_write_bvec+0xea/0x2a0 [loop]
[19738.128576]        loop_process_work+0x257/0xdb0 [loop]
[19738.129224]        process_one_work+0x24c/0x5b0
[19738.129789]        worker_thread+0x55/0x3c0
[19738.130311]        kthread+0x155/0x180
[19738.130783]        ret_from_fork+0x22/0x30
[19738.131296]
                -> #1 ((work_completion)(&lo->rootcg_work)){+.+.}-{0:0}:
[19738.132262]        process_one_work+0x223/0x5b0
[19738.132827]        worker_thread+0x55/0x3c0
[19738.133365]        kthread+0x155/0x180
[19738.133834]        ret_from_fork+0x22/0x30
[19738.134350]
                -> #0 ((wq_completion)loop0){+.+.}-{0:0}:
[19738.135153]        __lock_acquire+0x130e/0x2210
[19738.135715]        lock_acquire+0xd7/0x310
[19738.136224]        flush_workqueue+0xb5/0x5b0
[19738.136766]        drain_workqueue+0xa0/0x110
[19738.137308]        destroy_workqueue+0x36/0x280
[19738.137870]        __loop_clr_fd+0xb4/0x680 [loop]
[19738.138473]        blkdev_put+0xc7/0x220
[19738.138964]        close_fs_devices+0x95/0x220 [btrfs]
[19738.139685]        btrfs_close_devices+0x48/0x160 [btrfs]
[19738.140379]        generic_shutdown_super+0x74/0x110
[19738.141011]        kill_anon_super+0x14/0x30
[19738.141542]        btrfs_kill_super+0x12/0x20 [btrfs]
[19738.142189]        deactivate_locked_super+0x31/0xa0
[19738.142812]        cleanup_mnt+0x147/0x1c0
[19738.143322]        task_work_run+0x5c/0xa0
[19738.143831]        exit_to_user_mode_prepare+0x20c/0x210
[19738.144487]        syscall_exit_to_user_mode+0x27/0x60
[19738.145125]        do_syscall_64+0x48/0xc0
[19738.145636]        entry_SYSCALL_64_after_hwframe+0x44/0xae
[19738.146466]
                other info that might help us debug this:

[19738.147602] Chain exists of:
                  (wq_completion)loop0 --> &disk->open_mutex --> &lo->lo_mutex

[19738.149221]  Possible unsafe locking scenario:

[19738.149952]        CPU0                    CPU1
[19738.150520]        ----                    ----
[19738.151082]   lock(&lo->lo_mutex);
[19738.151508]                                lock(&disk->open_mutex);
[19738.152276]                                lock(&lo->lo_mutex);
[19738.153010]   lock((wq_completion)loop0);
[19738.153510]
                 *** DEADLOCK ***

[19738.154241] 4 locks held by umount/508378:
[19738.154756]  #0: ffff97a30dd9c0e8
(&type->s_umount_key#62){++++}-{3:3}, at: deactivate_super+0x2c/0x40
[19738.155900]  #1: ffffffffc0ac5f10 (uuid_mutex){+.+.}-{3:3}, at:
btrfs_close_devices+0x40/0x160 [btrfs]
[19738.157094]  #2: ffff97a31bc6d928 (&disk->open_mutex){+.+.}-{3:3},
at: blkdev_put+0x3a/0x220
[19738.158137]  #3: ffff97a31f64d4a0 (&lo->lo_mutex){+.+.}-{3:3}, at:
__loop_clr_fd+0x5a/0x680 [loop]
[19738.159244]
                stack backtrace:
[19738.159784] CPU: 2 PID: 508378 Comm: umount Not tainted
5.15.0-rc2-btrfs-next-99 #1
[19738.160723] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
[19738.162132] Call Trace:
[19738.162448]  dump_stack_lvl+0x57/0x72
[19738.162908]  check_noncircular+0xf3/0x110
[19738.163411]  __lock_acquire+0x130e/0x2210
[19738.163912]  lock_acquire+0xd7/0x310
[19738.164358]  ? flush_workqueue+0x8b/0x5b0
[19738.164859]  ? lockdep_init_map_type+0x51/0x260
[19738.165437]  ? lockdep_init_map_type+0x51/0x260
[19738.165999]  flush_workqueue+0xb5/0x5b0
[19738.166481]  ? flush_workqueue+0x8b/0x5b0
[19738.166990]  ? __mutex_unlock_slowpath+0x45/0x280
[19738.167574]  drain_workqueue+0xa0/0x110
[19738.168052]  destroy_workqueue+0x36/0x280
[19738.168551]  __loop_clr_fd+0xb4/0x680 [loop]
[19738.169084]  blkdev_put+0xc7/0x220
[19738.169510]  close_fs_devices+0x95/0x220 [btrfs]
[19738.170109]  btrfs_close_devices+0x48/0x160 [btrfs]
[19738.170745]  generic_shutdown_super+0x74/0x110
[19738.171300]  kill_anon_super+0x14/0x30
[19738.171760]  btrfs_kill_super+0x12/0x20 [btrfs]
[19738.172342]  deactivate_locked_super+0x31/0xa0
[19738.172880]  cleanup_mnt+0x147/0x1c0
[19738.173343]  task_work_run+0x5c/0xa0
[19738.173781]  exit_to_user_mode_prepare+0x20c/0x210
[19738.174381]  syscall_exit_to_user_mode+0x27/0x60
[19738.174957]  do_syscall_64+0x48/0xc0
[19738.175407]  entry_SYSCALL_64_after_hwframe+0x44/0xae
[19738.176037] RIP: 0033:0x7f4d7104fee7
[19738.176487] Code: ff 0b 00 f7 d8 64 89 01 48 83 c8 ff c3 66 0f 1f
44 00 00 31 f6 e9 09 00 00 00 66 0f 1f 84 00 00 00 00 00 b8 a6 00 00
00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 79 ff 0b 00 f7 d8 64 89
01 48
[19738.178787] RSP: 002b:00007ffeca2fd758 EFLAGS: 00000246 ORIG_RAX:
00000000000000a6
[19738.179722] RAX: 0000000000000000 RBX: 00007f4d71175264 RCX: 00007f4d7104fee7
[19738.180601] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 00005615eb38bdd0
[19738.181496] RBP: 00005615eb38bba0 R08: 0000000000000000 R09: 00007ffeca2fc4d0
[19738.182376] R10: 00005615eb38bdf0 R11: 0000000000000246 R12: 0000000000000000
[19738.183249] R13: 00005615eb38bdd0 R14: 00005615eb38bcb0 R15: 0000000000000000