Re: [PATCH v7 2/3] sched/task: Add the put_task_struct_atomic_safe() function

[Wander Lairson Costa]

On Thu, May 4, 2023 at 4:23 PM Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
>
> On 05/04, Wander Lairson Costa wrote:
> >
> > On Thu, May 4, 2023 at 12:23 PM Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> > >
> > > Yes, but as Sebastian explained CONFIG_PROVE_RAW_LOCK_NESTING won't like it.
> > >
> > >         https://lore.kernel.org/all/Y+zFNrCjBn53%2F+Q2@xxxxxxxxxxxxx/
> > >
> >
> > I think that was my confusion in that thread. My understanding is that
> > CONFIG_PROVE_RAW_LOCK_NESTING will check lock ordering but not
> > context.
>
> Sorry, I don't understand... perhaps I missed something. But iiuc
> the problem is simple.
>
> So, this code
>
>         raw_spin_lock(one);
>         spin_lock(two);
>
> is obviously wrong if CONFIG_PREEMPT_RT.
>
> Without PREEMPT_RT this code is fine because raw_spinlock_t and spinlock_t
> are the same thing. Except they have different lockdep annotations if
> CONFIG_PROVE_RAW_LOCK_NESTING is true, LD_WAIT_SPIN and LD_WAIT_CONFIG.
>
> So if CONFIG_PROVE_RAW_LOCK_NESTING is set, lockdep will complain even
> on the !PREEMPT_RT kernel, iow it checks the nesting as if the code runs
> on with PREEMPT_RT.
>
> Cough... not sure my explanation can help ;) It looks very confusing when
> I read it.
>

Thanks for the explanation. That's my understanding too. The part I
don't get is why this would fail with a call_rcu() inside
put_task_struct().

> Oleg.
>