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Subject: Re: INFO: task can't die in io_sq_thread_stop
To:     syzbot <syzbot+03beeb595f074db9cfd1@syzkaller.appspotmail.com>,
        axboe@kernel.dk, io-uring@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        syzkaller-bugs@googlegroups.com, viro@zeniv.linux.org.uk
References: <00000000000038569805b4211287@google.com>
From:   Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
Message-ID: <39be8d01-6550-ee8a-5a8d-2707b372b711@linux.alibaba.com>
Date:   Mon, 16 Nov 2020 17:32:33 +0800
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hi jens,

> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    6dd65e60 Add linux-next specific files for 20201110
> git tree:       linux-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=14727d42500000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=4fab43daf5c54712
> dashboard link: https://syzkaller.appspot.com/bug?extid=03beeb595f074db9cfd1
> 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+03beeb595f074db9cfd1@syzkaller.appspotmail.com
> 
> INFO: task syz-executor.2:12399 can't die for more than 143 seconds.
> task:syz-executor.2  state:D stack:28744 pid:12399 ppid:  8504 flags:0x00004004
> Call Trace:
>   context_switch kernel/sched/core.c:3773 [inline]
>   __schedule+0x893/0x2170 kernel/sched/core.c:4522
>   schedule+0xcf/0x270 kernel/sched/core.c:4600
>   schedule_timeout+0x1d8/0x250 kernel/time/timer.c:1847
>   do_wait_for_common kernel/sched/completion.c:85 [inline]
>   __wait_for_common kernel/sched/completion.c:106 [inline]
>   wait_for_common kernel/sched/completion.c:117 [inline]
>   wait_for_completion+0x163/0x260 kernel/sched/completion.c:138
>   kthread_stop+0x17a/0x720 kernel/kthread.c:596
>   io_put_sq_data fs/io_uring.c:7193 [inline]
>   io_sq_thread_stop+0x452/0x570 fs/io_uring.c:7290
>   io_finish_async fs/io_uring.c:7297 [inline]
>   io_sq_offload_create fs/io_uring.c:8015 [inline]
>   io_uring_create fs/io_uring.c:9433 [inline]
>   io_uring_setup+0x19b7/0x3730 fs/io_uring.c:9507
>   do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
>   entry_SYSCALL_64_after_hwframe+0x44/0xa9
I also don't have a reproducer yet, but seems that there is a race
in current codes:                  |
=> io_put_sq_data                  |
==> kthread_park(sqd->thread);     |
                                    | T1: sq thread is parked now.
==> kthread_stop(sqd->thread);     |
===> kthread_unpark(k);            |
                                    | T2: sq thread is now unpared, can run again
                                    |
                                    | T3: sq thread is now preempted out.
                                    |
===> wake_up_process(k);           |
                                    |
                                    | T4: Since sqd ctx list is empty, needs_sched will be true,
                                    | then sq thread sets task state to TASK_INTERRUPTIBLE,
                                    | and schedule, now sq thread will never be waken up.
===> wait_for_completion           |

I have artificially used mdelay() to simulate above race, will get same stack like
this syzbot report.

-               if (kthread_should_park())
+               if (kthread_should_park()) {
                         kthread_parkme();
+                       if (kthread_should_stop())
+                               break;
+               }
this diff can fix this issue, and if ctx_list is empty, we don't need to call schedule().

Regards,
Xiaoguang Wang


> RIP: 0033:0x45deb9
> Code: Unable to access opcode bytes at RIP 0x45de8f.
> RSP: 002b:00007f174e51ac78 EFLAGS: 00000246 ORIG_RAX: 00000000000001a9
> RAX: ffffffffffffffda RBX: 0000000000008640 RCX: 000000000045deb9
> RDX: 0000000000000000 RSI: 0000000020000140 RDI: 00000000000050e5
> RBP: 000000000118bf58 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 000000000118bf2c
> R13: 00007ffed9ca723f R14: 00007f174e51b9c0 R15: 000000000118bf2c
> INFO: task syz-executor.2:12399 blocked for more than 143 seconds.
>        Not tainted 5.10.0-rc3-next-20201110-syzkaller #0
> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> task:syz-executor.2  state:D stack:28744 pid:12399 ppid:  8504 flags:0x00004004
> Call Trace:
>   context_switch kernel/sched/core.c:3773 [inline]
>   __schedule+0x893/0x2170 kernel/sched/core.c:4522
>   schedule+0xcf/0x270 kernel/sched/core.c:4600
>   schedule_timeout+0x1d8/0x250 kernel/time/timer.c:1847
>   do_wait_for_common kernel/sched/completion.c:85 [inline]
>   __wait_for_common kernel/sched/completion.c:106 [inline]
>   wait_for_common kernel/sched/completion.c:117 [inline]
>   wait_for_completion+0x163/0x260 kernel/sched/completion.c:138
>   kthread_stop+0x17a/0x720 kernel/kthread.c:596
>   io_put_sq_data fs/io_uring.c:7193 [inline]
>   io_sq_thread_stop+0x452/0x570 fs/io_uring.c:7290
>   io_finish_async fs/io_uring.c:7297 [inline]
>   io_sq_offload_create fs/io_uring.c:8015 [inline]
>   io_uring_create fs/io_uring.c:9433 [inline]
>   io_uring_setup+0x19b7/0x3730 fs/io_uring.c:9507
>   do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
>   entry_SYSCALL_64_after_hwframe+0x44/0xa9
> RIP: 0033:0x45deb9
> Code: Unable to access opcode bytes at RIP 0x45de8f.
> RSP: 002b:00007f174e51ac78 EFLAGS: 00000246 ORIG_RAX: 00000000000001a9
> RAX: ffffffffffffffda RBX: 0000000000008640 RCX: 000000000045deb9
> RDX: 0000000000000000 RSI: 0000000020000140 RDI: 00000000000050e5
> RBP: 000000000118bf58 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 000000000118bf2c
> R13: 00007ffed9ca723f R14: 00007f174e51b9c0 R15: 000000000118bf2c
> 
> Showing all locks held in the system:
> 1 lock held by khungtaskd/1653:
>   #0: ffffffff8b3386a0 (rcu_read_lock){....}-{1:2}, at: debug_show_all_locks+0x53/0x260 kernel/locking/lockdep.c:6253
> 1 lock held by systemd-journal/4873:
> 1 lock held by in:imklog/8167:
>   #0: ffff88801c86e0f0 (&f->f_pos_lock){+.+.}-{3:3}, at: __fdget_pos+0xe9/0x100 fs/file.c:932
> 
> =============================================
> 
> NMI backtrace for cpu 1
> CPU: 1 PID: 1653 Comm: khungtaskd Not tainted 5.10.0-rc3-next-20201110-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+0x107/0x163 lib/dump_stack.c:118
>   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:147 [inline]
>   check_hung_uninterruptible_tasks kernel/hung_task.c:253 [inline]
>   watchdog+0xd89/0xf30 kernel/hung_task.c:338
>   kthread+0x3af/0x4a0 kernel/kthread.c:292
>   ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:296
> Sending NMI from CPU 1 to CPUs 0:
> NMI backtrace for cpu 0
> CPU: 0 PID: 5 Comm: kworker/0:0 Not tainted 5.10.0-rc3-next-20201110-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> Workqueue: events nsim_dev_trap_report_work
> RIP: 0010:mark_lock+0x30/0x24c0 kernel/locking/lockdep.c:4371
> Code: 41 54 41 89 d4 48 ba 00 00 00 00 00 fc ff df 55 53 48 81 ec 18 01 00 00 48 8d 5c 24 38 48 89 3c 24 48 c7 44 24 38 b3 8a b5 41 <48> c1 eb 03 48 c7 44 24 40 30 1b c6 8a 48 8d 04 13 48 c7 44 24 48
> RSP: 0018:ffffc90000ca7988 EFLAGS: 00000096
> RAX: 0000000000000004 RBX: ffffc90000ca79c0 RCX: ffffffff8155b947
> RDX: dffffc0000000000 RSI: ffff888010d20918 RDI: ffff888010d20000
> RBP: 0000000000000006 R08: 0000000000000000 R09: ffffffff8ebb477f
> R10: fffffbfff1d768ef R11: 000000004fb6aa4b R12: 0000000000000006
> R13: dffffc0000000000 R14: ffff888010d20918 R15: 0000000000000022
> FS:  0000000000000000(0000) GS:ffff8880b9e00000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007f8ffcf99000 CR3: 000000001b2e7000 CR4: 00000000001506f0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>   mark_held_locks+0x9f/0xe0 kernel/locking/lockdep.c:4011
>   __trace_hardirqs_on_caller kernel/locking/lockdep.c:4037 [inline]
>   lockdep_hardirqs_on_prepare kernel/locking/lockdep.c:4097 [inline]
>   lockdep_hardirqs_on_prepare+0x28b/0x400 kernel/locking/lockdep.c:4049
>   trace_hardirqs_on+0x5b/0x1c0 kernel/trace/trace_preemptirq.c:49
>   __raw_spin_unlock_irqrestore include/linux/spinlock_api_smp.h:160 [inline]
>   _raw_spin_unlock_irqrestore+0x42/0x50 kernel/locking/spinlock.c:191
>   extract_crng drivers/char/random.c:1026 [inline]
>   _get_random_bytes+0x229/0x670 drivers/char/random.c:1562
>   nsim_dev_trap_skb_build drivers/net/netdevsim/dev.c:538 [inline]
>   nsim_dev_trap_report drivers/net/netdevsim/dev.c:568 [inline]
>   nsim_dev_trap_report_work+0x740/0xbd0 drivers/net/netdevsim/dev.c:609
>   process_one_work+0x933/0x15a0 kernel/workqueue.c:2272
>   worker_thread+0x64c/0x1120 kernel/workqueue.c:2418
>   kthread+0x3af/0x4a0 kernel/kthread.c:292
>   ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:296
> 
> 
> ---
> This report is generated by a bot. It may contain errors.
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> 
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