Re: rcu/tree: Protect rcu_rdp_is_offloaded() invocations on RT

Frederic Weisbecker
2021-09-23T10:02:41Z

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!