Re: [RCU] zombie task hung in synchronize_rcu_expedited

[Wei Fu]

> 
> > ```
> > # 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?

Yes. The pid-2522045 sent SIGKILL to all the processes in that pid namespace,
when it exited. The last process 2522869 was zombie as well. Sometimes, 
`npm start` could exit before `npm run zombie` forks `sh`. You might see there
are only two processes in that pid namespace.


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

I'm using [6.5.0-1021-azure][1] kernel and preempt is disabled.
Highlight part of .config.

```
$ cat /boot/config-6.5.0-1021-azure | grep _RCU
CONFIG_TREE_RCU=y
# CONFIG_RCU_EXPERT is not set
CONFIG_TASKS_RCU_GENERIC=y
CONFIG_TASKS_RUDE_RCU=y
CONFIG_TASKS_TRACE_RCU=y
CONFIG_RCU_STALL_COMMON=y
CONFIG_RCU_NEED_SEGCBLIST=y
CONFIG_RCU_NOCB_CPU=y
# CONFIG_RCU_NOCB_CPU_DEFAULT_ALL is not set
# CONFIG_RCU_LAZY is not set
CONFIG_MMU_GATHER_RCU_TABLE_FREE=y
# CONFIG_RCU_SCALE_TEST is not set
# CONFIG_RCU_TORTURE_TEST is not set
# CONFIG_RCU_REF_SCALE_TEST is not set
CONFIG_RCU_CPU_STALL_TIMEOUT=60
CONFIG_RCU_EXP_CPU_STALL_TIMEOUT=0
CONFIG_RCU_CPU_STALL_CPUTIME=y
# CONFIG_RCU_TRACE is not set
# CONFIG_RCU_EQS_DEBUG is not set

$ cat /boot/config-6.5.0-1021-azure | grep _PREEMPT
CONFIG_PREEMPT_VOLUNTARY_BUILD=y
# CONFIG_PREEMPT_NONE is not set
CONFIG_PREEMPT_VOLUNTARY=y
# CONFIG_PREEMPT is not set
# CONFIG_PREEMPT_DYNAMIC is not set
CONFIG_HAVE_PREEMPT_DYNAMIC=y
CONFIG_HAVE_PREEMPT_DYNAMIC_CALL=y
CONFIG_PREEMPT_NOTIFIERS=y
CONFIG_DRM_I915_PREEMPT_TIMEOUT=640
CONFIG_DRM_I915_PREEMPT_TIMEOUT_COMPUTE=7500
# CONFIG_PREEMPTIRQ_DELAY_TEST is not set

$ cat /boot/config-6.5.0-1021-azure | grep HZ
CONFIG_NO_HZ_COMMON=y
# CONFIG_HZ_PERIODIC is not set
# CONFIG_NO_HZ_IDLE is not set
CONFIG_NO_HZ_FULL=y
CONFIG_NO_HZ=y
# CONFIG_HZ_100 is not set
CONFIG_HZ_250=y
# CONFIG_HZ_300 is not set
# CONFIG_HZ_1000 is not set
CONFIG_HZ=250
CONFIG_MACHZ_WDT=m
```

> 
> > The `npm start (pid 2522045)` was stuck in kernel_wait4. And its child,
> 
> so this is the init task in this namespace,

Yes~

> 
> > `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.

Sorry for unclear message.
Yes~

> 
> What about the process 2522605? Has it exited too?

The process-2522605 has two threads. The main thread-2522605 was in zombie
status. Yes. That main thread has exited as well. Only thread-2522645 was
stuck in synchronize_rcu_expedited.

> 
> > > 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...
> 

I can reproduce this issue in v5.15, v6.1, v6.5, v6.8, v6.9 and v6.10-rc2.
All the kernels disable CONFIG_PREEMPT and PREEMPT_RCU. And it's very easy to
reproduce this in v5.15.x with 8 vcores in few minutes. For the other versions
of kernel, it could take 30 minutes or few hours.

Rachel provides [golang-repro][2] which is similar to docker repro. It can be
built as static binary which is friendly to reproduce.

Hope this information can help.

Thanks,
Wei

[1]: https://gist.github.com/fuweid/ae8bad349fee3e00a4f1ce82397831ac
[2]: https://github.com/rlmenge/rcu-soft-lock-issue-repro?tab=readme-ov-file#golang-repro