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

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Message-ID: <YhqNRoEgIaoplF9b@bombadil.infradead.org>
Date:   Sat, 26 Feb 2022 12:27:50 -0800
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Aaron Tomlin <atomlin@...hat.com>, Petr Mladek <pmladek@...e.com>,
        "cl@...ux.com" <cl@...ux.com>, "mbenes@...e.cz" <mbenes@...e.cz>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "jeyu@...nel.org" <jeyu@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-modules@...r.kernel.org" <linux-modules@...r.kernel.org>,
        "void@...ifault.com" <void@...ifault.com>,
        "atomlin@...mlin.com" <atomlin@...mlin.com>,
        "allen.lkml@...il.com" <allen.lkml@...il.com>,
        "joe@...ches.com" <joe@...ches.com>,
        "msuchanek@...e.de" <msuchanek@...e.de>,
        "oleksandr@...alenko.name" <oleksandr@...alenko.name>
Subject: Re: [PATCH v8 09/13] module: Move kallsyms support into a separate
 file

On Fri, Feb 25, 2022 at 12:57:34PM +0000, Christophe Leroy wrote:
> 
> 
> 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.

I want to avoid any regressions and new complaints, fixes should be
submitted before so that if they are applicable to stable / etc they
can be sent there.

> 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.

Right, I'll just git fetch and reset to Linus' latest tree, so I'll drop
all of the stuff there now. And then the hope is to apply your new fresh new
clean v9.

Thanks for chugging on with this series!

  Luis