Re: [PATCH v8 09/13] module: Move kallsyms support into a separate file

Christophe Leroy
2022-02-25T12:57:42Z

Le 25/02/2022 à 13:21, Aaron Tomlin a écrit :
> On Fri 2022-02-25 10:27 +0000, Aaron Tomlin wrote:
>> On Fri 2022-02-25 11:15 +0100, Petr Mladek wrote:
>>> rcu_dereference_sched() makes sparse happy. But lockdep complains
>>> because the _rcu pointer is not accessed under:
>>>
>>> rcu_read_lock_sched();
>>> rcu_read_unlock_sched();
>>
>> Hi Petr,
>>
>>>
>>> This is not the case here. Note that module_mutex does not
>>> disable preemtion.
>>>
>>> Now, the code is safe. The RCU access makes sure that "mod"
>>> can't be freed in the meantime:
>>>
>>> + add_kallsyms() is called by the module loaded when the module
>>> is being loaded. It could not get removed in parallel
>>> by definition.
>>>
>>> + module_kallsyms_on_each_symbol() takes module_mutex.
>>> It means that the module could not get removed.
>>
>> Indeed, which is why I did not use rcu_read_lock_sched() and
>> rcu_read_unlock_sched() with rcu_dereference_sched(). That being said, I
>> should have mentioned this in the commit message.
>>
>>> IMHO, we have two possibilities here:
>>>
>>> + Make sparse and lockdep happy by using rcu_dereference_sched()
>>> and calling the code under rcu_read_lock_sched().
>>>
>>> + Cast (struct mod_kallsyms *)mod->kallsyms when accessing
>>> the value.
>>
>> I prefer the first option.
>>
>>> I do not have strong preference. I am fine with both.
>>>
>>> Anyway, such a fix should be done in a separate patch!
>>
>> Agreed.
>
> Luis,
>
> If I understand correctly, it might be cleaner to resolve the above in two
> separate patches for a v9 i.e. a) address the sparse and lockdep feedback
> and b) refactor the code, before the latest version [1] is merged into
> module-next. I assume the previous iteration will be reverted first?
>
> Please let me know your thoughts
>
> [1]: https://lore.kernel.org/all/20220222141303.1392190-1-atomlin@redhat.com/
>

I would do it the other way: first move the code into a separate file,
and then handle the sparse __rcu feedback as a followup patch to the series.

Regarding module-next, AFAICS at the moment we still have only the 10
first patches of v6 in the tree. I guess the way forward will be to
rebase module-next and drop those patches and commit v9 instead.

Christophe