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Date:   Wed, 28 Mar 2018 02:56:01 -0700
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Subject: Re: INFO: task hung in ucma_destroy_id
From:   syzbot <syzbot+449737930e1faf08523e@syzkaller.appspotmail.com>
To:     dasaratharaman.chandramouli@intel.com, dledford@redhat.com,
        ira.weiny@intel.com, jgg@ziepe.ca, leonro@mellanox.com,
        linux-kernel@vger.kernel.org, linux-rdma@vger.kernel.org,
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        viro@zeniv.linux.org.uk
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syzbot has found reproducer for the following crash on upstream commit
3eb2ce825ea1ad89d20f7a3b5780df850e4be274 (Sun Mar 25 22:44:30 2018 +0000)
Linux 4.16-rc7
syzbot dashboard link:  
https://syzkaller.appspot.com/bug?extid=449737930e1faf08523e

So far this crash happened 38 times on upstream.
C reproducer: https://syzkaller.appspot.com/x/repro.c?id=6522989826801664
syzkaller reproducer:  
https://syzkaller.appspot.com/x/repro.syz?id=4513717152645120
Raw console output:  
https://syzkaller.appspot.com/x/log.txt?id=6625862883475456
Kernel config:  
https://syzkaller.appspot.com/x/.config?id=-8440362230543204781
compiler: gcc (GCC) 7.1.1 20170620

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+449737930e1faf08523e@syzkaller.appspotmail.com
It will help syzbot understand when the bug is fixed.

INFO: task syzkaller681645:4295 blocked for more than 120 seconds.
       Not tainted 4.16.0-rc7+ #3
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
syzkaller681645 D20744  4295   4293 0x00000000
Call Trace:
  context_switch kernel/sched/core.c:2862 [inline]
  __schedule+0x8fb/0x1ec0 kernel/sched/core.c:3440
  schedule+0xf5/0x430 kernel/sched/core.c:3499
  schedule_timeout+0x1a3/0x230 kernel/time/timer.c:1777
  do_wait_for_common kernel/sched/completion.c:86 [inline]
  __wait_for_common kernel/sched/completion.c:107 [inline]
  wait_for_common kernel/sched/completion.c:118 [inline]
  wait_for_completion+0x415/0x770 kernel/sched/completion.c:139
  ucma_destroy_id+0x2f0/0x500 drivers/infiniband/core/ucma.c:611
  ucma_write+0x2d6/0x3d0 drivers/infiniband/core/ucma.c:1649
  __vfs_write+0xef/0x970 fs/read_write.c:480
  vfs_write+0x189/0x510 fs/read_write.c:544
  SYSC_write fs/read_write.c:589 [inline]
  SyS_write+0xef/0x220 fs/read_write.c:581
  do_syscall_64+0x281/0x940 arch/x86/entry/common.c:287
  entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x440719
RSP: 002b:00007ffc7e451f28 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007ffc7e451f50 RCX: 0000000000440719
RDX: 0000000000000018 RSI: 0000000020000080 RDI: 0000000000000003
RBP: 0000000000000000 R08: 00007ffc7e451fa0 R09: 00007ffc7e451fa0
R10: 00007ffc7e451fa0 R11: 0000000000000246 R12: 0000000000402040
R13: 00000000004020d0 R14: 0000000000000000 R15: 0000000000000000

Showing all locks held in the system:
2 locks held by khungtaskd/868:
  #0:  (rcu_read_lock){....}, at: [<00000000dbcbeec6>]  
check_hung_uninterruptible_tasks kernel/hung_task.c:175 [inline]
  #0:  (rcu_read_lock){....}, at: [<00000000dbcbeec6>] watchdog+0x1c5/0xd60  
kernel/hung_task.c:249
  #1:  (tasklist_lock){.+.+}, at: [<000000003f35a062>]  
debug_show_all_locks+0xd3/0x3d0 kernel/locking/lockdep.c:4470
1 lock held by rsyslogd/4176:
  #0:  (&f->f_pos_lock){+.+.}, at: [<00000000337d089e>]  
__fdget_pos+0x12b/0x190 fs/file.c:765
2 locks held by getty/4266:
  #0:  (&tty->ldisc_sem){++++}, at: [<0000000091eb7f81>]  
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
  #1:  (&ldata->atomic_read_lock){+.+.}, at: [<00000000c6c6bab6>]  
n_tty_read+0x2ef/0x1a40 drivers/tty/n_tty.c:2131
2 locks held by getty/4267:
  #0:  (&tty->ldisc_sem){++++}, at: [<0000000091eb7f81>]  
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
  #1:  (&ldata->atomic_read_lock){+.+.}, at: [<00000000c6c6bab6>]  
n_tty_read+0x2ef/0x1a40 drivers/tty/n_tty.c:2131
2 locks held by getty/4268:
  #0:  (&tty->ldisc_sem){++++}, at: [<0000000091eb7f81>]  
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
  #1:  (&ldata->atomic_read_lock){+.+.}, at: [<00000000c6c6bab6>]  
n_tty_read+0x2ef/0x1a40 drivers/tty/n_tty.c:2131
2 locks held by getty/4269:
  #0:  (&tty->ldisc_sem){++++}, at: [<0000000091eb7f81>]  
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
  #1:  (&ldata->atomic_read_lock){+.+.}, at: [<00000000c6c6bab6>]  
n_tty_read+0x2ef/0x1a40 drivers/tty/n_tty.c:2131
2 locks held by getty/4270:
  #0:  (&tty->ldisc_sem){++++}, at: [<0000000091eb7f81>]  
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
  #1:  (&ldata->atomic_read_lock){+.+.}, at: [<00000000c6c6bab6>]  
n_tty_read+0x2ef/0x1a40 drivers/tty/n_tty.c:2131
2 locks held by getty/4271:
  #0:  (&tty->ldisc_sem){++++}, at: [<0000000091eb7f81>]  
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
  #1:  (&ldata->atomic_read_lock){+.+.}, at: [<00000000c6c6bab6>]  
n_tty_read+0x2ef/0x1a40 drivers/tty/n_tty.c:2131
2 locks held by getty/4272:
  #0:  (&tty->ldisc_sem){++++}, at: [<0000000091eb7f81>]  
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
  #1:  (&ldata->atomic_read_lock){+.+.}, at: [<00000000c6c6bab6>]  
n_tty_read+0x2ef/0x1a40 drivers/tty/n_tty.c:2131

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

NMI backtrace for cpu 0
CPU: 0 PID: 868 Comm: khungtaskd Not tainted 4.16.0-rc7+ #3
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
Call Trace:
  __dump_stack lib/dump_stack.c:17 [inline]
  dump_stack+0x194/0x24d lib/dump_stack.c:53
  nmi_cpu_backtrace+0x1d2/0x210 lib/nmi_backtrace.c:103
  nmi_trigger_cpumask_backtrace+0x123/0x180 lib/nmi_backtrace.c:62
  arch_trigger_cpumask_backtrace+0x14/0x20 arch/x86/kernel/apic/hw_nmi.c:38
  trigger_all_cpu_backtrace include/linux/nmi.h:138 [inline]
  check_hung_task kernel/hung_task.c:132 [inline]
  check_hung_uninterruptible_tasks kernel/hung_task.c:190 [inline]
  watchdog+0x90c/0xd60 kernel/hung_task.c:249
  kthread+0x33c/0x400 kernel/kthread.c:238
  ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:406
Sending NMI from CPU 0 to CPUs 1:
NMI backtrace for cpu 1 skipped: idling at native_safe_halt+0x6/0x10  
arch/x86/include/asm/irqflags.h:54