[syzbot] INFO: task hung in flush_to_ldisc (3)

[syzbot] INFO: task hung in flush_to_ldisc (3)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    6498f6151825 Merge tag 'riscv-for-linus-5.14-rc3' of git:/..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1085167a300000
kernel config:  https://syzkaller.appspot.com/x/.config?x=42748795a8952874
dashboard link: https://syzkaller.appspot.com/bug?extid=5864be6ce31bdf05e8ee
compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.1
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=147459f4300000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=146889b2300000

Bisection is inconclusive: the issue happens on the oldest tested release.

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=14e2c32e300000
final oops:     https://syzkaller.appspot.com/x/report.txt?x=16e2c32e300000
console output: https://syzkaller.appspot.com/x/log.txt?x=12e2c32e300000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+5864be6ce31bdf05e8ee@syzkaller.appspotmail.com

INFO: task kworker/u4:0:8 blocked for more than 143 seconds.
      Not tainted 5.14.0-rc2-syzkaller #0
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:kworker/u4:0    state:D stack:24104 pid:    8 ppid:     2 flags:0x00004000
Workqueue: events_unbound flush_to_ldisc
Call Trace:
 context_switch kernel/sched/core.c:4683 [inline]
 __schedule+0x93a/0x26f0 kernel/sched/core.c:5940
 schedule+0xd3/0x270 kernel/sched/core.c:6019
 schedule_preempt_disabled+0xf/0x20 kernel/sched/core.c:6078
 __mutex_lock_common kernel/locking/mutex.c:1036 [inline]
 __mutex_lock+0x7b6/0x10a0 kernel/locking/mutex.c:1104
 flush_to_ldisc+0x3e/0x380 drivers/tty/tty_buffer.c:515
 process_one_work+0x98d/0x1630 kernel/workqueue.c:2276
 worker_thread+0x658/0x11f0 kernel/workqueue.c:2422
 kthread+0x3e5/0x4d0 kernel/kthread.c:319
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295
INFO: task syz-executor035:8505 blocked for more than 143 seconds.
      Not tainted 5.14.0-rc2-syzkaller #0
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:syz-executor035 state:D stack:28176 pid: 8505 ppid:  8447 flags:0x00004004
Call Trace:
 context_switch kernel/sched/core.c:4683 [inline]
 __schedule+0x93a/0x26f0 kernel/sched/core.c:5940
 schedule+0xd3/0x270 kernel/sched/core.c:6019
 schedule_preempt_disabled+0xf/0x20 kernel/sched/core.c:6078
 __mutex_lock_common kernel/locking/mutex.c:1036 [inline]
 __mutex_lock+0x7b6/0x10a0 kernel/locking/mutex.c:1104
 set_selection_kernel drivers/tty/vt/selection.c:357 [inline]
 set_selection_user+0x9f/0x1b0 drivers/tty/vt/selection.c:185
 tioclinux+0x114/0x560 drivers/tty/vt/vt.c:3160
 vt_ioctl+0x225f/0x2ac0 drivers/tty/vt/vt_ioctl.c:760
 tty_ioctl+0xe4d/0x1600 drivers/tty/tty_io.c:2805
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:1069 [inline]
 __se_sys_ioctl fs/ioctl.c:1055 [inline]
 __x64_sys_ioctl+0x193/0x200 fs/ioctl.c:1055
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae
RIP: 0033:0x4412a9
RSP: 002b:00007ffd8b8eef98 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 0000000000172c20 RCX: 00000000004412a9
RDX: 0000000020000000 RSI: 000000000000541c RDI: 0000000000000003
RBP: 0000000000000000 R08: 000000000000000e R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007ffd8b8eefdc
R13: 00007ffd8b8ef010 R14: 00007ffd8b8eeff0 R15: 0000000000000015

Showing all locks held in the system:
3 locks held by kworker/u4:0/8:
 #0: ffff888010869138 ((wq_completion)events_unbound){+.+.}-{0:0}, at: arch_atomic64_set arch/x86/include/asm/atomic64_64.h:34 [inline]
 #0: ffff888010869138 ((wq_completion)events_unbound){+.+.}-{0:0}, at: atomic64_set include/asm-generic/atomic-instrumented.h:620 [inline]
 #0: ffff888010869138 ((wq_completion)events_unbound){+.+.}-{0:0}, at: atomic_long_set include/asm-generic/atomic-long.h:41 [inline]
 #0: ffff888010869138 ((wq_completion)events_unbound){+.+.}-{0:0}, at: set_work_data kernel/workqueue.c:617 [inline]
 #0: ffff888010869138 ((wq_completion)events_unbound){+.+.}-{0:0}, at: set_work_pool_and_clear_pending kernel/workqueue.c:644 [inline]
 #0: ffff888010869138 ((wq_completion)events_unbound){+.+.}-{0:0}, at: process_one_work+0x871/0x1630 kernel/workqueue.c:2247
 #1: ffffc90000cd7db0 ((work_completion)(&buf->work)){+.+.}-{0:0}, at: process_one_work+0x8a5/0x1630 kernel/workqueue.c:2251
 #2: ffff8880210310b8 (&buf->lock){+.+.}-{3:3}, at: flush_to_ldisc+0x3e/0x380 drivers/tty/tty_buffer.c:515
1 lock held by khungtaskd/1651:
 #0: ffffffff8b97b9c0 (rcu_read_lock){....}-{1:2}, at: debug_show_all_locks+0x53/0x260 kernel/locking/lockdep.c:6446
1 lock held by in:imklog/8139:
 #0: ffff88802aa319f0 (&f->f_pos_lock){+.+.}-{3:3}, at: __fdget_pos+0xe9/0x100 fs/file.c:974
3 locks held by syz-executor035/8503:
1 lock held by syz-executor035/8505:
 #0: ffffffff8c3855c8 (vc_sel.lock){+.+.}-{3:3}, at: set_selection_kernel drivers/tty/vt/selection.c:357 [inline]
 #0: ffffffff8c3855c8 (vc_sel.lock){+.+.}-{3:3}, at: set_selection_user+0x9f/0x1b0 drivers/tty/vt/selection.c:185

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

NMI backtrace for cpu 0
CPU: 0 PID: 1651 Comm: khungtaskd Not tainted 5.14.0-rc2-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:88 [inline]
 dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:105
 nmi_cpu_backtrace.cold+0x44/0xd7 lib/nmi_backtrace.c:105
 nmi_trigger_cpumask_backtrace+0x1b3/0x230 lib/nmi_backtrace.c:62
 trigger_all_cpu_backtrace include/linux/nmi.h:146 [inline]
 check_hung_uninterruptible_tasks kernel/hung_task.c:210 [inline]
 watchdog+0xd0a/0xfc0 kernel/hung_task.c:295
 kthread+0x3e5/0x4d0 kernel/kthread.c:319
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295
Sending NMI from CPU 0 to CPUs 1:
NMI backtrace for cpu 1
CPU: 1 PID: 8503 Comm: syz-executor035 Not tainted 5.14.0-rc2-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:paste_selection+0x2c1/0x4c0 drivers/tty/vt/selection.c:396
Code: be 08 00 00 00 48 89 df e8 3c 40 8d fd 41 80 3e 00 0f 85 c3 01 00 00 48 8b 03 31 ff 83 e0 04 49 89 c7 48 89 c6 e8 5f 11 47 fd <4d> 85 ff 0f 85 88 00 00 00 e8 f1 0b 47 fd 4d 8d bd 38 04 00 00 be
RSP: 0018:ffffc900016ffa88 EFLAGS: 00000246
RAX: 0000000000000000 RBX: ffff8880246cd4c0 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffff8880246cd4c0 RDI: 0000000000000003
RBP: 0000000000000000 R08: 0000000000000000 R09: ffff8880246cd4c7
R10: ffffffff842e7cd1 R11: 0000000000000000 R12: dffffc0000000000
R13: ffff888028323000 R14: ffffed10048d9a98 R15: 0000000000000000
FS:  00000000016363c0(0000) GS:ffff8880b9d00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000001636384 CR3: 00000000211b9000 CR4: 0000000000350ee0
Call Trace:
 tioclinux+0x126/0x560 drivers/tty/vt/vt.c:3164
 vt_ioctl+0x225f/0x2ac0 drivers/tty/vt/vt_ioctl.c:760
 tty_ioctl+0xe4d/0x1600 drivers/tty/tty_io.c:2805
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:1069 [inline]
 __se_sys_ioctl fs/ioctl.c:1055 [inline]
 __x64_sys_ioctl+0x193/0x200 fs/ioctl.c:1055
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae
RIP: 0033:0x4412a9
Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 11 15 00 00 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 73 01 c3 48 c7 c1 c4 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffd8b8eef98 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 0000000000000063 RCX: 00000000004412a9
RDX: 0000000020000000 RSI: 000000000000541c RDI: 0000000000000003
RBP: 0000000000000063 R08: 000000000000000e R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007ffd8b8eefdc
R13: 00007ffd8b8ef010 R14: 00007ffd8b8eeff0 R15: 0000000000000014


---
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For information about bisection process see: https://goo.gl/tpsmEJ#bisection
syzbot can test patches for this issue, for details see:
https://goo.gl/tpsmEJ#testing-patches