syzbot

INFO: task kworker/u4:5:8294 blocked for more than 143 seconds.
      Not tainted 5.9.0-rc6-syzkaller #0
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:kworker/u4:5    state:D stack:24640 pid: 8294 ppid:     2 flags:0x00004000
Workqueue: events_unbound flush_to_ldisc
Call Trace:
 context_switch kernel/sched/core.c:3778 [inline]
 __schedule+0xec9/0x2280 kernel/sched/core.c:4527
 schedule+0xd0/0x2a0 kernel/sched/core.c:4602
 schedule_preempt_disabled+0xf/0x20 kernel/sched/core.c:4661
 __mutex_lock_common kernel/locking/mutex.c:1033 [inline]
 __mutex_lock+0x3e2/0x10e0 kernel/locking/mutex.c:1103
 commit_echoes+0x4c/0x210 drivers/tty/n_tty.c:778
 n_tty_receive_char_fast drivers/tty/n_tty.c:1449 [inline]
 n_tty_receive_buf_fast drivers/tty/n_tty.c:1609 [inline]
 __receive_buf drivers/tty/n_tty.c:1644 [inline]
 n_tty_receive_buf_common+0x203f/0x2bc0 drivers/tty/n_tty.c:1742
 tty_ldisc_receive_buf+0xa9/0x190 drivers/tty/tty_buffer.c:461
 tty_port_default_receive_buf+0x6e/0xa0 drivers/tty/tty_port.c:38
 receive_buf drivers/tty/tty_buffer.c:481 [inline]
 flush_to_ldisc+0x20d/0x380 drivers/tty/tty_buffer.c:533
 process_one_work+0x94c/0x1670 kernel/workqueue.c:2269
 worker_thread+0x64c/0x1120 kernel/workqueue.c:2415
 kthread+0x3b5/0x4a0 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

Showing all locks held in the system:
1 lock held by khungtaskd/1173:
 #0: ffffffff8a067f00 (rcu_read_lock){....}-{1:2}, at: debug_show_all_locks+0x53/0x260 kernel/locking/lockdep.c:5852
1 lock held by in:imklog/6764:
 #0: ffff8880a7a94870 (&f->f_pos_lock){+.+.}-{3:3}, at: __fdget_pos+0xe9/0x100 fs/file.c:930
2 locks held by agetty/6775:
 #0: ffff8880a2875098 (&tty->ldisc_sem){++++}-{0:0}, at: tty_ldisc_ref_wait+0x22/0x80 drivers/tty/tty_ldisc.c:267
 #1: ffffc90001f982e8 (&ldata->atomic_read_lock){+.+.}-{3:3}, at: n_tty_read+0x223/0x1a30 drivers/tty/n_tty.c:2156
2 locks held by agetty/6777:
 #0: ffff88809eda9098 (&tty->ldisc_sem){++++}-{0:0}, at: tty_ldisc_ref_wait+0x22/0x80 drivers/tty/tty_ldisc.c:267
 #1: ffffc900018332e8 (&ldata->atomic_read_lock){+.+.}-{3:3}, at: n_tty_read+0x223/0x1a30 drivers/tty/n_tty.c:2156
6 locks held by kworker/u4:5/8294:
 #0: ffff8880aa071138 ((wq_completion)events_unbound){+.+.}-{0:0}, at: arch_atomic64_set arch/x86/include/asm/atomic64_64.h:34 [inline]
 #0: ffff8880aa071138 ((wq_completion)events_unbound){+.+.}-{0:0}, at: atomic64_set include/asm-generic/atomic-instrumented.h:856 [inline]
 #0: ffff8880aa071138 ((wq_completion)events_unbound){+.+.}-{0:0}, at: atomic_long_set include/asm-generic/atomic-long.h:41 [inline]
 #0: ffff8880aa071138 ((wq_completion)events_unbound){+.+.}-{0:0}, at: set_work_data kernel/workqueue.c:616 [inline]
 #0: ffff8880aa071138 ((wq_completion)events_unbound){+.+.}-{0:0}, at: set_work_pool_and_clear_pending kernel/workqueue.c:643 [inline]
 #0: ffff8880aa071138 ((wq_completion)events_unbound){+.+.}-{0:0}, at: process_one_work+0x82b/0x1670 kernel/workqueue.c:2240
 #1: ffffc90015d27da8 ((work_completion)(&buf->work)){+.+.}-{0:0}, at: process_one_work+0x85f/0x1670 kernel/workqueue.c:2244
 #2: ffff8880aa1ff0b8 (&buf->lock){+.+.}-{3:3}, at: flush_to_ldisc+0x3e/0x380 drivers/tty/tty_buffer.c:505
 #3: ffff8880a2875098 (&tty->ldisc_sem){++++}-{0:0}, at: tty_ldisc_ref+0x1d/0x80 drivers/tty/tty_ldisc.c:288
 #4: ffff8880a28752e8 (&tty->termios_rwsem){++++}-{3:3}, at: n_tty_receive_buf_common+0x89/0x2bc0 drivers/tty/n_tty.c:1705
 #5: ffffc90001f98378 (&ldata->output_lock){+.+.}-{3:3}, at: commit_echoes+0x4c/0x210 drivers/tty/n_tty.c:778
5 locks held by syz-executor.3/21286:

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

NMI backtrace for cpu 1
CPU: 1 PID: 1173 Comm: khungtaskd Not tainted 5.9.0-rc6-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x198/0x1fd lib/dump_stack.c:118
 nmi_cpu_backtrace.cold+0x70/0xb1 lib/nmi_backtrace.c:101
 nmi_trigger_cpumask_backtrace+0x1b3/0x223 lib/nmi_backtrace.c:62
 trigger_all_cpu_backtrace include/linux/nmi.h:146 [inline]
 check_hung_uninterruptible_tasks kernel/hung_task.c:209 [inline]
 watchdog+0xd7d/0x1000 kernel/hung_task.c:295
 kthread+0x3b5/0x4a0 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294
Sending NMI from CPU 1 to CPUs 0:
NMI backtrace for cpu 0
CPU: 0 PID: 21286 Comm: syz-executor.3 Not tainted 5.9.0-rc6-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:writeb arch/x86/include/asm/io.h:65 [inline]
RIP: 0010:vga16fb_fillrect+0x9c0/0x193b drivers/video/fbdev/vga16fb.c:923
Code: 86 fd 48 63 44 24 10 45 31 f6 48 89 04 24 e8 f7 33 86 fd 31 ff 89 de 31 ed e8 4c 30 86 fd 85 db 4d 89 ec 74 22 e8 e0 33 86 fd <45> 88 34 24 83 c5 01 89 df 49 83 c4 01 89 ee e8 ac 2f 86 fd 39 eb
RSP: 0018:ffffc90005667778 EFLAGS: 00000246
RAX: 0000000000040000 RBX: 0000000000000001 RCX: ffffc9000e597000
RDX: 0000000000040000 RSI: ffffffff83f00a60 RDI: 0000000000000005
RBP: 0000000000000000 R08: 0000000000000010 R09: 0000000000000008
R10: 0000000000000000 R11: 0000000000000000 R12: ffff8880000a0000
R13: ffff8880000a0000 R14: 0000000000000000 R15: 000000005b1ef446
FS:  00007f534ff61700(0000) GS:ffff8880ae400000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fcf13ca6880 CR3: 0000000051046000 CR4: 00000000001506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 bit_clear+0x386/0x4d0 drivers/video/fbdev/core/bitblit.c:74
 fbcon_clear+0x529/0x5f0 drivers/video/fbdev/core/fbcon.c:1297
 fbcon_scroll+0x1dbb/0x3580 drivers/video/fbdev/core/fbcon.c:1795
 con_scroll+0x5b9/0x6d0 drivers/tty/vt/vt.c:629
 lf+0x26b/0x2c0 drivers/tty/vt/vt.c:1490
 vc_con_write_normal drivers/tty/vt/vt.c:2806 [inline]
 do_con_write+0xefe/0x1dd0 drivers/tty/vt/vt.c:2910
 con_write+0x22/0xb0 drivers/tty/vt/vt.c:3250
 process_output_block drivers/tty/n_tty.c:595 [inline]
 n_tty_write+0x3ce/0xf80 drivers/tty/n_tty.c:2333
 do_tty_write drivers/tty/tty_io.c:962 [inline]
 tty_write+0x4d9/0x870 drivers/tty/tty_io.c:1046
 vfs_write+0x2b0/0x730 fs/read_write.c:576
 ksys_write+0x12d/0x250 fs/read_write.c:631
 do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x45e179
Code: 3d b2 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 0b b2 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f534ff60c78 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00000000000391c0 RCX: 000000000045e179
RDX: 0000000000000078 RSI: 0000000020000380 RDI: 0000000000000003
RBP: 000000000118cf80 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 000000000118cf4c
R13: 00007ffc1aae174f R14: 00007f534ff619c0 R15: 000000000118cf4c