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

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Date:   Wed, 26 Jun 2019 11:49:16 -0500
From:   Scott Wood <swood@...hat.com>
To:     Steven Rostedt <rostedt@...dmis.org>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Juri Lelli <juri.lelli@...hat.com>,
        Clark Williams <williams@...hat.com>,
        linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH RT 4/4] rcutorture: Avoid problematic critical
 section nesting

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@...ux.ibm.com> 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)?

-Scott