Re: [RFC PATCH RT 4/4] rcutorture: Avoid problematic critical section nesting

[Paul E. McKenney]

On Wed, Jun 26, 2019 at 11:49:16AM -0500, Scott Wood wrote:
> On Wed, 2019-06-26 at 11:08 -0400, Steven Rostedt wrote:
> > On Fri, 21 Jun 2019 16:59:55 -0700
> > "Paul E. McKenney" <paulmck@xxxxxxxxxxxxx> wrote:
> > 
> > > I have no objection to the outlawing of a number of these sequences in
> > > mainline, but am rather pointing out that until they really are outlawed
> > > and eliminated, rcutorture must continue to test them in mainline.
> > > Of course, an rcutorture running in -rt should avoid testing things that
> > > break -rt, including these sequences.
> > 
> > We should update lockdep to complain about these sequences. That would
> > "outlaw" them in mainline. That is, after we clean up all the current
> > sequences in the code. And we also need to get Linus's approval of this
> > as I believe he was against enforcing this in the past.
> 
> Was the opposition to prohibiting some specific sequence?  It's only certain
> misnesting scenarios that are problematic.  The rcu_read_lock/
> local_irq_disable restriction can be dropped with the IPI-to-self added in
> Paul's tree.  Are there any known instances of the other two (besides
> rcutorture)?

Given the failure scenario Sebastian Siewior reported today, there
apparently are some, at least when running threaded interrupt handlers.

							Thanx, Paul