Re: [RCU] zombie task hung in synchronize_rcu_expedited

[Oleg Nesterov]

Hi Wei, thanks for more info.

On 06/06, Wei Fu wrote:
>
> > Well, due to unfortunate design zap_pid_ns_processes() can hang "forever"
> > if this namespace has a (zombie) task injected from the parent ns, this
> > task should be reaped by its parent.
>
> That zombie task was cloned by pid-1 process in that pid namespace. In my last
> reproduced log, the process tree in that pid namespace looks like

OK,

> ```
> # unshare(CLONE_NEWPID | CLONE_NEWNS)
>
> npm start (pid 2522045)
>     |__npm run zombie (pid 2522605)
>        |__ sh -c "whle true; do echo zombie; sleep 1; done" (pid 2522869)
> ```

only 3 processes? nothing is running? Is the last process 2522869 a
zombie too?

Could you show your .config? In particular, CONFIG_PREEMPT...

> The `npm start (pid 2522045)` was stuck in kernel_wait4. And its child,

so this is the init task in this namespace,

> `npm run zombie (pid 2522605)`, has two threads. One of them was in D status.
...
> $ sudo cat /proc/2522605/task/*/stack
> [<0>] synchronize_rcu_expedited+0x177/0x1f0
> [<0>] namespace_unlock+0xd6/0x1b0
> [<0>] put_mnt_ns+0x73/0xa0
> [<0>] free_nsproxy+0x1c/0x1b0
> [<0>] switch_task_namespaces+0x5d/0x70
> [<0>] exit_task_namespaces+0x10/0x20
> [<0>] do_exit+0x2ce/0x500
> [<0>] io_sq_thread+0x48e/0x5a0
> [<0>] ret_from_fork+0x3c/0x60
> [<0>] ret_from_fork_asm+0x1b/0x30

so I guess this is the trace of its sub-thread 2522645.

What about the process 2522605? Has it exited too?

> > But zap_pid_ns_processes() shouldn't cause the soft-lockup, it should
> > sleep in kernel_wait4().
>
> I run `cat /proc/2522045/status` and found that the status was kept switching
> between running and sleeping.

OK, this shouldn't happen in this case. So it really looks like it spins
in a busy-wait loop because TIF_NOTIFY_SIGNAL is not cleared. It can be
reported as sleeping because do_wait() sets/clears TASK_INTERRUPTIBLE,
although the window is small...

Oleg.