rcu/tree: Protect rcu_rdp_is_offloaded() invocations on RT

[Sebastian Andrzej Siewior]

On 2021-09-22 13:38:20 [+0200], Frederic Weisbecker wrote:
> > The part with rcutree.use_softirq=0 on RT does not make it any better,
> > right?
> 
> The rcuc kthread disables softirqs before calling rcu_core(), so it behaves
> pretty much the same as a softirq. Or am I missing something?

Oh, no you don't.

> > So you rely on some implicit behaviour which breaks with RT such as:
> > 
> >                               CPU 0
> >            -----------------------------------------------
> >            RANDOM TASK-A                      RANDOM TASK-B
> >            ------                             -----------
> >            int *X = &per_cpu(CPUX, 0)         int *X = &per_cpu(CPUX, 0)
> >            int A, B;                          
> > 					      spin_lock(&D);
> >            spin_lock(&C);
> > 	   				      WRITE_ONCE(*X, 0);
> >            A = READ_ONCE(*X);
> >                                               WRITE_ONCE(*X, 1);
> >            B = READ_ONCE(*X);
> > 
> > while spinlock C and D are just random locks not related to CPUX but it
> > just happens that they are held at that time. So for !RT you guarantee
> > that A == B while it is not the case on RT.
> 
> Not sure which spinlocks you are referring to here. Also most RCU spinlocks
> are raw.

I was bringing an example where you also could rely on implicit locking
provided by spin_lock() which breaks on RT.

Sebastian