INFO: task hung in commit_echoes

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: 71fce1ed UPSTREAM: tracing: always define trace_{irq,preem..
git tree: android-4.9
console output: https://syzkaller.appspot.com/x/log.txt?x=125918eb800000
kernel config: https://syzkaller.appspot.com/x/.config?x=a54f56879744de40
dashboard link: https://syzkaller.appspot.com/bug?extid=94592dd3bbe74e2cba7f
compiler: gcc (GCC) 8.0.1 20180413 (experimental)
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=178df247800000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=17a537bb800000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+94592d...@syzkaller.appspotmail.com

INFO: task kworker/u4:2:3780 blocked for more than 120 seconds.
Not tainted 4.9.96-g71fce1e #10
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
kworker/u4:2 D28600 3780 2 0x00000000
Workqueue: events_unbound flush_to_ldisc
ffff8801d8139800 0000000000000000 ffff8801cac21f80 ffff8801d00b4800
ffff8801db221b98 ffff8801d56af8c8 ffffffff839e240d ffff8801d813a0c8
ffffed003b027418 ffff8801d8139800 00fffc0000000000 ffff8801db222468
Call Trace:
[<ffffffff839e3a0f>] schedule+0x7f/0x1b0 kernel/sched/core.c:3557
[<ffffffff839e4393>] schedule_preempt_disabled+0x13/0x20
kernel/sched/core.c:3590
[<ffffffff839e8816>] __mutex_lock_common kernel/locking/mutex.c:582
[inline]
[<ffffffff839e8816>] mutex_lock_nested+0x326/0x870
kernel/locking/mutex.c:621
[<ffffffff82119f60>] commit_echoes+0x120/0x1b0 drivers/tty/n_tty.c:764
[<ffffffff82122bb8>] n_tty_receive_char_fast drivers/tty/n_tty.c:1418
[inline]
[<ffffffff82122bb8>] n_tty_receive_buf_fast drivers/tty/n_tty.c:1578
[inline]
[<ffffffff82122bb8>] __receive_buf drivers/tty/n_tty.c:1613 [inline]
[<ffffffff82122bb8>] n_tty_receive_buf_common+0x1878/0x2300
drivers/tty/n_tty.c:1711
[<ffffffff82123673>] n_tty_receive_buf2+0x33/0x40 drivers/tty/n_tty.c:1746
[<ffffffff8212917f>] tty_ldisc_receive_buf+0xaf/0x190
drivers/tty/tty_buffer.c:455
[<ffffffff8212a183>] receive_buf drivers/tty/tty_buffer.c:474 [inline]
[<ffffffff8212a183>] flush_to_ldisc+0x253/0x370
drivers/tty/tty_buffer.c:533
[<ffffffff8118ae31>] process_one_work+0x7e1/0x1500 kernel/workqueue.c:2092
[<ffffffff8118bc26>] worker_thread+0xd6/0x10a0 kernel/workqueue.c:2226
[<ffffffff8119ad5d>] kthread+0x26d/0x300 kernel/kthread.c:211
[<ffffffff839f34dc>] ret_from_fork+0x5c/0x70 arch/x86/entry/entry_64.S:373

Showing all locks held in the system:
2 locks held by khungtaskd/518:
#0: (rcu_read_lock){......}, at: [<ffffffff813646ec>]
check_hung_uninterruptible_tasks kernel/hung_task.c:168 [inline]
#0: (rcu_read_lock){......}, at: [<ffffffff813646ec>]
watchdog+0x11c/0xa20 kernel/hung_task.c:239
#1: (tasklist_lock){.+.+..}, at: [<ffffffff81423ce0>]
debug_show_all_locks+0x79/0x218 kernel/locking/lockdep.c:4336
2 locks held by getty/3631:
#0: (&tty->ldisc_sem){++++++}, at: [<ffffffff839f14b2>]
ldsem_down_read+0x32/0x40 drivers/tty/tty_ldsem.c:367
#1: (&ldata->atomic_read_lock){+.+...}, at: [<ffffffff8211c792>]
n_tty_read+0x202/0x16e0 drivers/tty/n_tty.c:2133
6 locks held by kworker/u4:2/3780:
#0: ("events_unbound"){.+.+.+}, at: [<ffffffff8118ad3e>] work_static
include/linux/workqueue.h:186 [inline]
#0: ("events_unbound"){.+.+.+}, at: [<ffffffff8118ad3e>] set_work_data
kernel/workqueue.c:617 [inline]
#0: ("events_unbound"){.+.+.+}, at: [<ffffffff8118ad3e>]
set_work_pool_and_clear_pending kernel/workqueue.c:644 [inline]
#0: ("events_unbound"){.+.+.+}, at: [<ffffffff8118ad3e>]
process_one_work+0x6ee/0x1500 kernel/workqueue.c:2085
#1: ((&buf->work)){+.+...}, at: [<ffffffff8118ad78>]
process_one_work+0x728/0x1500 kernel/workqueue.c:2089
#2: (&tty->ldisc_sem){++++++}, at: [<ffffffff82127940>]
tty_ldisc_ref+0x20/0x80 drivers/tty/tty_ldisc.c:297
#3: (&port->buf.lock/1){+.+...}, at: [<ffffffff82129fb6>]
flush_to_ldisc+0x86/0x370 drivers/tty/tty_buffer.c:505
#4: (&o_tty->termios_rwsem/1){++++..}, at: [<ffffffff821213cb>]
n_tty_receive_buf_common+0x8b/0x2300 drivers/tty/n_tty.c:1674
#5: (&ldata->output_lock){+.+...}, at: [<ffffffff82119f60>]
commit_echoes+0x120/0x1b0 drivers/tty/n_tty.c:764
6 locks held by kworker/u4:6/5106:
#0: ("events_unbound"){.+.+.+}, at: [<ffffffff8118ad3e>] work_static
include/linux/workqueue.h:186 [inline]
#0: ("events_unbound"){.+.+.+}, at: [<ffffffff8118ad3e>] set_work_data
kernel/workqueue.c:617 [inline]
#0: ("events_unbound"){.+.+.+}, at: [<ffffffff8118ad3e>]
set_work_pool_and_clear_pending kernel/workqueue.c:644 [inline]
#0: ("events_unbound"){.+.+.+}, at: [<ffffffff8118ad3e>]
process_one_work+0x6ee/0x1500 kernel/workqueue.c:2085
#1: ((&buf->work)){+.+...}, at: [<ffffffff8118ad78>]
process_one_work+0x728/0x1500 kernel/workqueue.c:2089
#2: (&tty->ldisc_sem){++++++}, at: [<ffffffff82127940>]
tty_ldisc_ref+0x20/0x80 drivers/tty/tty_ldisc.c:297
#3: (&port->buf.lock/1){+.+...}, at: [<ffffffff82129fb6>]
flush_to_ldisc+0x86/0x370 drivers/tty/tty_buffer.c:505
#4: (&o_tty->termios_rwsem/1){++++..}, at: [<ffffffff821213cb>]
n_tty_receive_buf_common+0x8b/0x2300 drivers/tty/n_tty.c:1674
#5: (&ldata->output_lock){+.+...}, at: [<ffffffff82119f60>]
commit_echoes+0x120/0x1b0 drivers/tty/n_tty.c:764
6 locks held by kworker/u4:7/5390:
#0: ("events_unbound"){.+.+.+}, at: [<ffffffff8118ad3e>] work_static
include/linux/workqueue.h:186 [inline]
#0: ("events_unbound"){.+.+.+}, at: [<ffffffff8118ad3e>] set_work_data
kernel/workqueue.c:617 [inline]
#0: ("events_unbound"){.+.+.+}, at: [<ffffffff8118ad3e>]
set_work_pool_and_clear_pending kernel/workqueue.c:644 [inline]
#0: ("events_unbound"){.+.+.+}, at: [<ffffffff8118ad3e>]
process_one_work+0x6ee/0x1500 kernel/workqueue.c:2085
#1: ((&buf->work)){+.+...}, at: [<ffffffff8118ad78>]
process_one_work+0x728/0x1500 kernel/workqueue.c:2089
#2: (&tty->ldisc_sem){++++++}, at: [<ffffffff82127940>]
tty_ldisc_ref+0x20/0x80 drivers/tty/tty_ldisc.c:297
#3: (&port->buf.lock/1){+.+...}, at: [<ffffffff82129fb6>]
flush_to_ldisc+0x86/0x370 drivers/tty/tty_buffer.c:505
#4: (&o_tty->termios_rwsem/1){++++..}, at: [<ffffffff821213cb>]
n_tty_receive_buf_common+0x8b/0x2300 drivers/tty/n_tty.c:1674
#5: (&ldata->output_lock){+.+...}, at: [<ffffffff82119f60>]
commit_echoes+0x120/0x1b0 drivers/tty/n_tty.c:764
2 locks held by syz-executor226/32325:
#0: (&tty->ldisc_sem){++++++}, at: [<ffffffff839f14b2>]
ldsem_down_read+0x32/0x40 drivers/tty/tty_ldsem.c:367
#1: (&tty->atomic_write_lock){+.+.+.}, at: [<ffffffff8210a8a1>]
tty_write_lock+0x21/0x60 drivers/tty/tty_io.c:1107
2 locks held by syz-executor226/32324:
#0: (&tty->ldisc_sem){++++++}, at: [<ffffffff839f14b2>]
ldsem_down_read+0x32/0x40 drivers/tty/tty_ldsem.c:367
#1: (&tty->atomic_write_lock){+.+.+.}, at: [<ffffffff8210a8a1>]
tty_write_lock+0x21/0x60 drivers/tty/tty_io.c:1107
2 locks held by syz-executor226/32330:
#0: (&tty->ldisc_sem){++++++}, at: [<ffffffff839f14b2>]
ldsem_down_read+0x32/0x40 drivers/tty/tty_ldsem.c:367
#1: (&tty->atomic_write_lock){+.+.+.}, at: [<ffffffff8210a8a1>]
tty_write_lock+0x21/0x60 drivers/tty/tty_io.c:1107

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

NMI backtrace for cpu 0
CPU: 0 PID: 518 Comm: khungtaskd Not tainted 4.9.96-g71fce1e #10
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
ffff8801d860fd08 ffffffff81eb0b69 0000000000000000 0000000000000000
0000000000000000 0000000000000001 ffffffff810b7d60 ffff8801d860fd40
ffffffff81ebbe97 0000000000000000 0000000000000000 0000000000000003
Call Trace:
[<ffffffff81eb0b69>] __dump_stack lib/dump_stack.c:15 [inline]
[<ffffffff81eb0b69>] dump_stack+0xc1/0x128 lib/dump_stack.c:51
[<ffffffff81ebbe97>] nmi_cpu_backtrace.cold.2+0x48/0x87
lib/nmi_backtrace.c:99
[<ffffffff81ebbe2a>] nmi_trigger_cpumask_backtrace+0x12a/0x14f
lib/nmi_backtrace.c:60
[<ffffffff810b7e64>] arch_trigger_cpumask_backtrace+0x14/0x20
arch/x86/kernel/apic/hw_nmi.c:37
[<ffffffff81364c84>] trigger_all_cpu_backtrace include/linux/nmi.h:58
[inline]
[<ffffffff81364c84>] check_hung_task kernel/hung_task.c:125 [inline]
[<ffffffff81364c84>] check_hung_uninterruptible_tasks
kernel/hung_task.c:182 [inline]
[<ffffffff81364c84>] watchdog+0x6b4/0xa20 kernel/hung_task.c:239
[<ffffffff8119ad5d>] kthread+0x26d/0x300 kernel/kthread.c:211
[<ffffffff839f34dc>] ret_from_fork+0x5c/0x70 arch/x86/entry/entry_64.S:373
Sending NMI from CPU 0 to CPUs 1:
NMI backtrace for cpu 1
CPU: 1 PID: 30775 Comm: syz-executor226 Not tainted 4.9.96-g71fce1e #10
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
task: ffff8801c42f6000 task.stack: ffff8801c1f60000
RIP: 0010:[<ffffffff82118620>] c [<ffffffff82118620>] echo_buf
drivers/tty/n_tty.c:146 [inline]
RIP: 0010:[<ffffffff82118620>] c [<ffffffff82118620>]
__process_echoes+0x5b0/0x780 drivers/tty/n_tty.c:734
RSP: 0018:ffff8801c1f67980 EFLAGS: 00000293
RAX: ffff8801c42f6000 RBX: ffffc900003bb000 RCX: 0000000000000001
RDX: 0000000000000000 RSI: ffffffff821186be RDI: ffffc900003bd24a
RBP: ffff8801c1f679e8 R08: ffff8801c42f6938 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000188395fe9
R13: 0000000000001f7a R14: dffffc0000000000 R15: 0000000188395feb
FS: 00007fdc03009700(0000) GS:ffff8801db300000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000004c26e4 CR3: 00000001da38d000 CR4: 00000000001606f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Stack:
ffff8801c1f679c0 c 0000000000000000 c ffffc900003bd278 c ffffed003868ec25 c
ffff8801c347612c c ffffc900003bb020 c 0000000000001700 c ffff8801c3475d80 c
ffffc900003bd300 c ffffc900003bb000 c dffffc0000000000 c ffffc900003bb020 c
Call Trace:
[<ffffffff82122074>] flush_echoes drivers/tty/n_tty.c:801 [inline]
[<ffffffff82122074>] __receive_buf drivers/tty/n_tty.c:1617 [inline]
[<ffffffff82122074>] n_tty_receive_buf_common+0xd34/0x2300
drivers/tty/n_tty.c:1711
[<ffffffff821236b0>] n_tty_receive_buf+0x30/0x40 drivers/tty/n_tty.c:1740
[<ffffffff82113aef>] tiocsti drivers/tty/tty_io.c:2314 [inline]
[<ffffffff82113aef>] tty_ioctl+0xc8f/0x2270 drivers/tty/tty_io.c:2905
[<ffffffff815b051c>] vfs_ioctl fs/ioctl.c:43 [inline]
[<ffffffff815b051c>] file_ioctl fs/ioctl.c:493 [inline]
[<ffffffff815b051c>] do_vfs_ioctl+0x1ac/0x11a0 fs/ioctl.c:677
[<ffffffff815b159f>] SYSC_ioctl fs/ioctl.c:694 [inline]
[<ffffffff815b159f>] SyS_ioctl+0x8f/0xc0 fs/ioctl.c:685
[<ffffffff81006316>] do_syscall_64+0x1a6/0x490 arch/x86/entry/common.c:282
[<ffffffff839f3313>] entry_SYSCALL_64_after_swapgs+0x5d/0xdb
Code: c00 c00 c00 c00 cfc cff cdf c48 cc1 cea c03 c80 c3c c02
c00 c0f c85 c47 c01 c00 c00 c8b c4d ccc c4c c8b c6b c20 c29
c4d cc8 c49 cbe c00 c00 c00 c00 c00 cfc cff cdf ceb c03
c<4d> c89 cfc ce8 cd8 c43 c24 cff c4c c89 ce8 c4c c29 ce0
c48 c3d cdf c0e c00 c00 c0f c


---
This bug is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzk...@googlegroups.com.

syzbot will keep track of this bug report. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
syzbot can test patches for this bug, for details see:
https://goo.gl/tpsmEJ#testing-patches