Re: [RFC PATCH bpf-next v2 1/9] cgroup: Make operations on the cgroup root_list RCU safe

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On Fri, Oct 20, 2023 at 05:36:57PM +0800, Yafang Shao wrote:
> On Fri, Oct 20, 2023 at 3:43 AM Waiman Long <longman@xxxxxxxxxx> wrote:
> >
> > On 10/19/23 15:08, Tejun Heo wrote:
> > > On Thu, Oct 19, 2023 at 02:38:52PM +0800, Yafang Shao wrote:
> > >>>> -     BUG_ON(!res_cgroup);
> > >>>> +     WARN_ON_ONCE(!res_cgroup && lockdep_is_held(&cgroup_mutex));
> > >>> This doesn't work. lockdep_is_held() is always true if !PROVE_LOCKING.
> > >> will use mutex_is_locked() instead.
> > > But then, someone else can hold the lock and trigger the condition
> > > spuriously. The kernel doesn't track who's holding the lock unless lockdep
> > > is enabled.
> >
> > It is actually possible to detect if the current process is the owner of
> > a mutex since there is a owner field in the mutex structure. However,
> > the owner field also contains additional information which need to be
> > masked off before comparing with "current". If such a functionality is
> > really needed, we will have to add a helper function mutex_is_held(),
> > for example, to kernel/locking/mutex.c.
>
> Agreed. We should first introduce mutex_is_held(). Thanks for your suggestion.

I'm not sure this is the right occassion to add such thing. It's just a
warn_on, we can either pass in the necessary condition from the callers or
just drop the warning.

Thanks.

-- 
tejun




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