order of executing MOD_LOAD and registering module sysctl-s

[Hans Petter Selasky]

On 08/02/17 17:49, John Baldwin wrote:
> On Wednesday, August 02, 2017 12:39:36 PM Hans Petter Selasky wrote:
>> On 08/02/17 12:13, Andriy Gapon wrote:
>>>
>>> As far as I understand a module initialization routine is executed via the
>>> sysinit mechanism.  Specifically, module_register_init is set up as the sysinit
>>> function for every module and it calls MOD_EVENT(mod, MOD_LOAD) to invoke the
>>> module event handler.
>>>
>>> In linker_load_file() I see the following code:
>>>                           linker_file_register_sysctls(lf);
>>>                           linker_file_sysinit(lf);
>>>
>>> I think that this means that any statically declared sysctl-s in the module
>>> would be registered before the module receives the MOD_LOAD event.
>>> It's possible that some of the sysctl-s could have procedures as handlers and
>>> they might access data that is supposed to be initialized by the module event
>>> handler.
>>>
>>> So, for example, running sysctl -a at just the right moment during the loading
>>> of a module might end up in an expected behavior (including a crash).
>>>
>>> Is my interpretation of how the code works correct?
>>> Can the order of linker_file_sysinit and linker_file_register_sysctls be changed
>>> without a great risk?
>>>
>>> Thank you!
>>>
>>> P.S.
>>> The same applies to:
>>>                   linker_file_sysuninit(file);
>>>                   linker_file_unregister_sysctls(file);
>>>
>>
>> Hi,
>>
>> Not sure if this answers your question.
>>

Hi,

>> If a SYSCTL() is TUNABLE, it's procedure can be called when the sysctl
>> is created. Else the SYSCTL() procedure callback might be called right
>> after it's registered. I think there is an own subsystem in sys/kernel.h
>> which takes care of the actual SYSCTL() creation/destruction - after the
>> linker is involved.
> 
> sysctl nodes are created explicitly via linker_file_register_sysctls, not via
> SYSINITs, so you can't order them with respect to other init functions.

For GENERIC (non-modules) the SYSCTLS() are registered by 
sysctl_register_all() at SYSINIT(sysctl, SI_SUB_KMEM, SI_ORDER_FIRST, 
sysctl_register_all, 0);

> 
> I think Andriy's suggestion of doing sysctls "inside" sysinits (so they are
> registered last and unregistered first) is probably better than the current
> state and is a simpler fix than changing all sysctls to use SYSINITs.
> 

If the module provided SYSCTLS's could use the same SI_SUB_KMEM it would 
be compatible.

You have three cases to think about:

1) SYSCTLS's in modules loaded before the kernel is booted
2) SYSCTLS's in modules after the kernel is booted
3) SYSCTLS's in the GENERIC kernel.

I'm not 100% sure, but I think 1) and 2) are treated differently. 
Correct me if I'm wrong.

--HPS