INFO: task syz.0.20876:14662 blocked for more than 143 seconds.
      Tainted: G             L      syzkaller #0
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:syz.0.20876     state:D stack:26752 pid:14662 tgid:14660 ppid:10542  task_flags:0x400040 flags:0x00080002
Call Trace:
 <TASK>
 __schedule+0x17b4/0x5680
 schedule+0x164/0x360
 schedule_preempt_disabled+0x13/0x30
 __mutex_lock+0x7f7/0x1550
 sync_bdevs+0x27e/0x470
 __ia32_sys_sync+0xc5/0x120
 do_syscall_64+0x15f/0xf80
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f13afb9cdd9
RSP: 002b:00007f13b0a4f028 EFLAGS: 00000246 ORIG_RAX: 00000000000000a2
RAX: ffffffffffffffda RBX: 00007f13afe16090 RCX: 00007f13afb9cdd9
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: 00007f13afe16090 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f13afe16128 R14: 00007f13afe16090 R15: 00007fffdc50de28
 </TASK>

Showing all locks held in the system:
1 lock held by khungtaskd/36:
 #0: ffffffff8e95cda0 (rcu_read_lock){....}-{1:3}, at: debug_show_all_locks+0x2e/0x180
2 locks held by getty/5442:
 #0: ffff8881100890a0 (&tty->ldisc_sem){++++}-{0:0}, at: tty_ldisc_ref_wait+0x25/0x70
 #1: ffffc9000346b2e8 (&ldata->atomic_read_lock){+.+.}-{4:4}, at: n_tty_read+0x45c/0x13a0
3 locks held by kworker/u9:10/32453:
1 lock held by udevd/8945:
 #0: ffff888104b8d350 (&disk->open_mutex){+.+.}-{4:4}, at: bdev_open+0xe0/0xd30
1 lock held by syz.0.20876/14662:
 #0: ffff888104b8d350 (&disk->open_mutex){+.+.}-{4:4}, at: sync_bdevs+0x27e/0x470
1 lock held by syz.3.21257/16118:
 #0: ffff888104b8d350 (
&disk->open_mutex){+.+.}-{4:4}, at: sync_bdevs+0x27e/0x470
1 lock held by syz.6.21265/16141:
 #0: ffff888104b8d350 (&disk->open_mutex){+.+.}-{4:4}, at: sync_bdevs+0x27e/0x470
4 locks held by syz.8.21503/17082:
 #0: ffff8880367fe0d8 (&type->s_umount_key#63/1){+.+.}-{4:4}, at: alloc_super+0x28c/0xab0
 #1: ffff8880367fe600 (sb_internal#3){.+.+}-{0:0}, at: btrfs_rebuild_free_space_tree+0xbb/0x6e0
 #2: ffff888031836640 (btrfs_trans_num_writers){++++}-{0:0}, at: join_transaction+0x154/0xe40
 #3: ffff888031836668 (btrfs_trans_num_extwriters){++++}-{0:0}, at: join_transaction+0x154/0xe40

=============================================

NMI backtrace for cpu 1
CPU: 1 UID: 0 PID: 36 Comm: khungtaskd Tainted: G             L      syzkaller #0 PREEMPT(full) 
Tainted: [L]=SOFTLOCKUP
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
Call Trace:
 <TASK>
 dump_stack_lvl+0xe8/0x150
 nmi_cpu_backtrace+0x274/0x2d0
 nmi_trigger_cpumask_backtrace+0x17a/0x300
 sys_info+0x135/0x170
 watchdog+0xfd3/0x1030
 kthread+0x388/0x470
 ret_from_fork+0x514/0xb70
 ret_from_fork_asm+0x1a/0x30
 </TASK>
Sending NMI from CPU 1 to CPUs 0:
NMI backtrace for cpu 0
CPU: 0 UID: 0 PID: 26565 Comm: kworker/u9:1 Tainted: G             L      syzkaller #0 PREEMPT(full) 
Tainted: [L]=SOFTLOCKUP
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
Workqueue: btrfs-cache btrfs_work_helper
RIP: 0010:__lock_acquire+0x197b/0x2cf0
Code: 14 25 ff 0f 00 00 39 05 cf 1c 9f 12 73 06 89 05 c7 1c 9f 12 48 8b 02 48 89 df 4c 39 e8 0f 84 2c fe ff ff 4c 89 60 30 48 8b 00 <4c> 39 e8 75 f4 48 89 df e9 18 fe ff ff 48 89 fb 48 c7 c7 50 af 82
RSP: 0018:ffffc90003ef7140 EFLAGS: 00000006
RAX: ffffffff93ee68f8 RBX: 00000000000003fd RCX: 0000000000000673
RDX: ffffffff96f66a08 RSI: ffff88802eb44780 RDI: 00000000000003fd
RBP: 1d2c1a09aa9cc677 R08: ffffc90003ef7108 R09: 0000000000000020
R10: 0000000000000003 R11: ffffffff81a20c90 R12: ffffffff970342b8
R13: ffffffff93ee68f8 R14: ffff88802eb43b00 R15: 0000000000000672
FS:  0000000000000000(0000) GS:ffff88818dc90000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000558e90a9cc28 CR3: 0000000111c36000 CR4: 00000000000006f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000600
Call Trace:
 <TASK>
 lock_acquire+0x106/0x350
 seqcount_lockdep_reader_access+0x55/0x100
 ktime_get+0x45/0x220
 clockevents_program_event+0x290/0x630
 __hrtimer_rearm_deferred+0x270/0x450
 irqentry_exit+0x6a0/0x730
 asm_sysvec_apic_timer_interrupt+0x1a/0x20
RIP: 0010:lock_acquire+0x221/0x350
Code: ff ff ff e8 e1 84 06 0a f7 44 24 08 00 02 00 00 0f 84 3a ff ff ff 65 48 8b 05 cb 5f 97 11 48 3b 44 24 58 75 33 fb 48 83 c4 60 <5b> 41 5c 41 5d 41 5e 41 5f 5d c3 cc cc cc cc cc 48 8d 3d 98 68 92
RSP: 0018:ffffc90003ef7590 EFLAGS: 00000286
RAX: bfca11f0a2ac8f00 RBX: 0000000000000000 RCX: 0000000080000001
RDX: 0000000000000004 RSI: ffffffff8e21c2f0 RDI: ffffffff8c28b4e0
RBP: ffffffff83eb8f06 R08: ffffc90003ef73b0 R09: 0000000000000020
R10: ffff888045e92b00 R11: ffffffff81a20c90 R12: 0000000000000001
R13: ffff888031834088 R14: 0000000000000001 R15: 0000000000000246
 _raw_read_lock+0x36/0x50
 btrfs_extent_root+0x196/0x310
 caching_thread+0xd24/0x1d20
 btrfs_work_helper+0x37b/0xc20
 process_scheduled_works+0xb5d/0x1860
 worker_thread+0xa53/0xfc0
 kthread+0x388/0x470
 ret_from_fork+0x514/0xb70
 ret_from_fork_asm+0x1a/0x30
 </TASK>
