rcu/tree: Protect rcu_rdp_is_offloaded() invocations on RT

[Frederic Weisbecker]

On Wed, Sep 22, 2021 at 03:02:32PM +0200, Sebastian Andrzej Siewior wrote:
> On 2021-09-22 13:38:20 [+0200], Frederic Weisbecker wrote:
> > > 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.

Good point!