[LEDE-DEV] libubox, procd: init process hangs

[Xinxing Hu]

Hi Guys,

I have another idea about this issue. Maybe it is not kernel, but uloop 
related. I read procd and libubox code a little bit, and it seems there 
is a potential issue existing in uloop_run().

In general, uloop_run() is running in a while loop:

while()
         1, Process timeouts list

         2, Handle terminated child processes

         3, uloop_run_events(timeout) => calls epoll_wait()
done

During boot, procd_inittab_run("sysinit") is called in Step1, which 
calls add_initd(). add_initd() would add an entry in timeouts list, 
whose callback function is to execute an rc.d/S* script.

When the while loop goes back to Step1 again, the timeouts list would be 
processed, and an rc.d/S* script would be executed in a child process 
while the parent process remains in the while loop. If everything goes 
fine, when the child process is terminated, the parent process will 
handle terminated child process by calling waitpid() in the while loop. 
A process callback function will also be called, which adds another 
timeout entry in timeouts list. This new entry corresponds to the next 
rc.d/S* script to be executed. When the while loop reaches Step1 again, 
the next rc.d/S* script would be invoked.

Everything looks OK till now. However, due to process scheduling, 
problems might happen when uloop_run_events(uloop_get_next_timeout(&tv)) 
is called. For instance: if the child process is still running when 
uloop_get_next_timeout(&tv) is called, then the timeouts list is already 
empty at that time, so the return value of uloop_get_next_timeout(&tv) 
would be -1. Furthermore, if the child process is terminated and signal 
handler is executed before epoll_wait() is called, then epoll_wait will 
block the parent process forever until some other events it is listening 
to arrive. In this sense, other events arriving just hide this issue. 
During the boot, as long as /etc/rc.d/S* is not finished executing, 
epoll_wait() should never be blocked.

I think, a potential solution might be: during initialization, we let 
uloop listens to a kind of 'dummy' event. Every time when the child 
process finishes executing a rc.d/S* script, we send a 'dummy' event. In 
this case, epoll_wait would never be blocked during booting.

Best Regards,
Xinxing




On 2016/5/17 18:03, Mats Karrman wrote:
Hi Felix, others,

I have been experiencing problems with the init scripts dispatch
suddenly stopping (indefinitely).
This happens maybe once in 100 reboots.
After inserting a new start script that launches another daemon
(cgrulesengd) very early in the boot process, the failures started to
come a lot more frequently, maybe once in 10 reboots, making this a real
issue.
I'm normally using the versions of procd and libubox selected by OpenWRT
BB branch but I have tested the latest versions from the git repos with
the same result.
So far I have only got this to happen on a quite fast board (ARM dual
CorexA9 @ 1GHz).
Inserting trace prints in libubox changes behavior, also suggesting the
problem is timing dependent.

When init hangs:
- it is still possible to log in on console
- there is always a zombie start script, e.g. S11sysctl.
- by killing a process (e.g. ubusd or cgrulesengd) the init process
continues.
- otherwise generating an event, e.g inserting something into a USB port
also makes the init continue.

I have traced the problem down to the "epoll_wait" call in
libubox::uloop.c::uloop_fetch_events().
The following patch makes sure epoll_wait is never called without a timeout.
My tests show that this solves the problem.
I have been able to observe the case when the boot gets stuck and then
continues after the 8s timeout.
However I'm not sure that this is the correct fix for the problem as
there may be other reasons that there is no event in the first place.
Your feedback would be welcome!

BR // Mats
Currently working for Inteno Broadband Technology AB

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