INFO: task can't die in congestion_wait

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    a804ab08 Add linux-next specific files for 20201006
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=141b7afb900000
kernel config:  https://syzkaller.appspot.com/x/.config?x=26c1b4cc4a62ccb
dashboard link: https://syzkaller.appspot.com/bug?extid=9d67ed950b326f4f35c3
compiler:       gcc (GCC) 10.1.0-syz 20200507

Unfortunately, I don't have any reproducer for this issue yet.

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+9d67ed950b326f4f35c3@xxxxxxxxxxxxxxxxxxxxxxxxx

INFO: task syz-executor.0:8666 can't die for more than 143 seconds.
task:syz-executor.0  state:D stack:26592 pid: 8666 ppid:  6885 flags:0x00004006
Call Trace:
 context_switch kernel/sched/core.c:3772 [inline]
 __schedule+0xec5/0x2200 kernel/sched/core.c:4521
 schedule+0xcf/0x270 kernel/sched/core.c:4599
 schedule_timeout+0x148/0x250 kernel/time/timer.c:1881
 io_schedule_timeout+0xc6/0x140 kernel/sched/core.c:6281
 congestion_wait+0xff/0x450 mm/backing-dev.c:960
 f2fs_get_meta_page_nofail+0x60/0x70 fs/f2fs/checkpoint.c:117
 get_current_nat_page fs/f2fs/node.c:112 [inline]
 __f2fs_build_free_nids+0x46f/0xe10 fs/f2fs/node.c:2403
 f2fs_build_free_nids fs/f2fs/node.c:2446 [inline]
 f2fs_build_node_manager+0x2549/0x3310 fs/f2fs/node.c:3180
 f2fs_fill_super+0x3b7b/0x7410 fs/f2fs/super.c:3695
 mount_bdev+0x32e/0x3f0 fs/super.c:1419
 legacy_get_tree+0x105/0x220 fs/fs_context.c:592
 vfs_get_tree+0x89/0x2f0 fs/super.c:1549
 do_new_mount fs/namespace.c:2896 [inline]
 path_mount+0x12ae/0x1e70 fs/namespace.c:3227
 __do_sys_mount fs/namespace.c:3438 [inline]
 __se_sys_mount fs/namespace.c:3411 [inline]
 __x64_sys_mount+0x278/0x2f0 fs/namespace.c:3411
 do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x46087a
Code: Unable to access opcode bytes at RIP 0x460850.
RSP: 002b:00007f4166abba88 EFLAGS: 00000202 ORIG_RAX: 00000000000000a5
RAX: ffffffffffffffda RBX: 00007f4166abbb20 RCX: 000000000046087a
RDX: 0000000020000000 RSI: 0000000020000100 RDI: 00007f4166abbae0
RBP: 00007f4166abbae0 R08: 00007f4166abbb20 R09: 0000000020000000
R10: 0000000000000000 R11: 0000000000000202 R12: 0000000020000000
R13: 0000000020000100 R14: 0000000020000200 R15: 0000000020012400

Showing all locks held in the system:
3 locks held by kworker/u4:3/39:
 #0: ffff8880ae436058 (&rq->lock){-.-.}-{2:2}, at: rq_lock kernel/sched/sched.h:1292 [inline]
 #0: ffff8880ae436058 (&rq->lock){-.-.}-{2:2}, at: __schedule+0x284/0x2200 kernel/sched/core.c:4439
 #1: ffff8880ae420ec8 (&per_cpu_ptr(group->pcpu, cpu)->seq){-.-.}-{0:0}, at: psi_task_switch+0x305/0x440 kernel/sched/psi.c:833
 #2: ffffffff8a554da0 (rcu_read_lock){....}-{1:2}, at: batadv_nc_purge_orig_hash net/batman-adv/network-coding.c:405 [inline]
 #2: ffffffff8a554da0 (rcu_read_lock){....}-{1:2}, at: batadv_nc_worker+0xf3/0xe50 net/batman-adv/network-coding.c:718
1 lock held by khungtaskd/1181:
 #0: ffffffff8a554da0 (rcu_read_lock){....}-{1:2}, at: debug_show_all_locks+0x53/0x260 kernel/locking/lockdep.c:6242
1 lock held by in:imklog/6770:
 #0: ffff8880a75216b0 (&f->f_pos_lock){+.+.}-{3:3}, at: __fdget_pos+0xe9/0x100 fs/file.c:930
2 locks held by agetty/6781:
 #0: ffff8880973f4098 (&tty->ldisc_sem){++++}-{0:0}, at: tty_ldisc_ref_wait+0x22/0x80 drivers/tty/tty_ldisc.c:266
 #1: ffffc900010cc2e8 (&ldata->atomic_read_lock){+.+.}-{3:3}, at: n_tty_read+0x223/0x1a70 drivers/tty/n_tty.c:2156
3 locks held by syz-executor.0/8666:

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