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

[Oleg Nesterov <oleg@redhat.com>]

Hi Wander,

I certainly missed something ;) plus I am already sleeping. but let me try to
reply anyway.

On 05/04, Wander Lairson Costa wrote:
> On Thu, May 4, 2023 at 4:23 PM Oleg Nesterov <oleg@redhat.com> wrote:
> >
> > On 05/04, Wander Lairson Costa wrote:
> > >
> > > On Thu, May 4, 2023 at 12:23 PM Oleg Nesterov <oleg@redhat.com> wrote:
> > > >
> > > > Yes, but as Sebastian explained CONFIG_PROVE_RAW_LOCK_NESTING won't like it.
> > > >
> > > >         https://lore.kernel.org/all/Y+zFNrCjBn53%2F+Q2@linutronix.de/
> > > >
> > >
> > > 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().

the problem is that call_rcu() won't be called if !IS_ENABLED(PREEMPT_RT),
___put_task_struct() will be called.

CONFIG_PROVE_RAW_LOCK_NESTING can't know this can't happen if PREEMPT_RT
is set.

IOW. To simplify, suppose we have

	// can be called in atomic context, e.g. under
	// raw_spin_lock() so it is wrong with PREEMPT_RT
	void __put_task_struct(struct task_struct *tsk)
	{
		spin_lock(some_lock);
	}

lets "fix" the code above, lets change __put_task_struct,

	void __put_task_struct(struct task_struct *tsk)
	{
		if (!IS_ENABLED(CONFIG_PREEMPT_RT))
			return;

		spin_lock(some_lock);
	}

Now, if CONFIG_PREEMPT_RT is true then __put_task_struct() is fine
wrt lock nesting.

But, if CONFIG_PREEMPT_RT is not set, then __put_task_struct() still
does the same:

	void __put_task_struct(struct task_struct *tsk)
	{
		spin_lock(some_lock);
	}

and CONFIG_PROVE_RAW_LOCK_NESTING will complain. Because, once again,
it checks the nesting as if CONFIG_PREEMPT_RT is true, and in this case
__put_task_struct() if it is called under raw_spin_lock().

Oleg.