[systemd-devel] systemd kills mdmon if it was started manually by user

[Williams, Dan J]

On Wed, Nov 2, 2011 at 8:17 AM, Lennart Poettering
<lennart at poettering.net> wrote:
> On Wed, 02.11.11 15:33, Kay Sievers (kay.sievers at vrfy.org) wrote:
>
>> > The big thing is that if things are done that way you'll always have the
>> > chicken and egg problem: you really need to shut down mdmon before
>> > unmounting root, but currently you require us to do it in the other
>> > order too.
>>
>> Yeah, that's just madness.
>>
>> I talked to Harald, and the currently preferred idea is the version
>> where we start mdmon in the initramfs and never touch it again, and it
>> runs until the initramfs unmounts the rootfs and shuts down the box.
>>
>> In that picture, the mdmon process is conceptually more like a kernel
>> thread than a userspace process. It can not be updated, can not be
>> restarted. The only way to update it is to rebuild initramfs and
>> reboot the box.
>
> OK, I guess that means we'll need to define a way how we can recognize
> the process then, to avoid killing it by systemd, similar to how we
> exclude kernel threads from killing.
>
> Kernel threads we detect by checking whether /proc/$PID/cmdline is
> empty, hence I'd suggest we use the first char of argv[0][0] here, to
> detect whether something is a process to avoid killing. Question is
> which char to choose for that. I am tempted to use '@'.
>
> That means we'd:
>
> a) patch systemd to check whether argv[0][0] of a process is '@' and
> owned by root and exclude it from killing on shutdown.
>
> b) patch mdmon to set argv[0][0] of itself to '@' iff it is running from
> the initrd. If it is run from the main system it should not set that and
> just be shut down like any other service.

Well, there are two cases to consider:

1/ user starts the array manually and stops it with mdadm -Ss (mdmon
automatically shuts down).  No need for a service mdmon just follows
the lifespan of the array.

2/ user starts the array but then expects it to be around until system shutdown

In the latter case let the initramfs-mdmon takeover all arrays with
"mdmon --takeover --all".  But if all arrays may eventually be
re-parented to an mdmon instance from /run, why not always start mdmon
from there?