linux-kernel - Re: [PATCH RT v3 5/5] rcutorture: Avoid problematic critical section nesting on RT

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Message-ID: <b6b87f7acde58fcf0c172622eb9acef43a113ec4.camel@redhat.com>
Date:   Tue, 17 Sep 2019 11:32:11 -0500
From:   Scott Wood <swood@...hat.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Paul E . McKenney" <paulmck@...ux.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Clark Williams <williams@...hat.com>
Subject: Re: [PATCH RT v3 5/5] rcutorture: Avoid problematic critical
 section nesting on RT

On Tue, 2019-09-17 at 16:50 +0200, Sebastian Andrzej Siewior wrote:
> On 2019-09-17 09:36:22 [-0500], Scott Wood wrote:
> > > On non-RT you can (but should not) use the counter part of the
> > > function
> > > in random order like:
> > > 	local_bh_disable();
> > > 	local_irq_disable();
> > > 	local_bh_enable();
> > > 	local_irq_enable();
> > 
> > Actually even non-RT will assert if you do local_bh_enable() with IRQs
> > disabled -- but the other combinations do work, and are used some places
> > via
> > spinlocks.  If they are used via direct calls to preempt_disable() or
> > local_irq_disable() (or via raw spinlocks), then that will not go away
> > on RT
> > and we'll have a problem.
> 
> lockdep_assert_irqs_enabled() is a nop with CONFIG_PROVE_LOCKING=N and
> RT breaks either way. 

Right, I meant a non-RT kernel with debug checks enabled.

> > > Since you _can_ use it in random order Paul wants to test that the
> > > random use of those function does not break RCU in any way. Since they
> > > can not be used on RT in random order it has been agreed that we keep
> > > the test for !RT but disable it on RT.
> > 
> > For now, yes.  Long term it would be good to keep track of when
> > preemption/irqs would be disabled on RT, even when running a non-RT
> > debug
> > kernel, and assert when bad things are done with it (assuming an RT-
> > capable
> > arch).  Besides detecting these fairly unusual patterns, it could also
> > detect earlier the much more common problem of nesting a non-raw
> > spinlock
> > inside a raw spinlock or other RT-atomic context.
> 
> you will be surprised but we have patches for that. We need first get
> rid of other "false positives" before plugging this in.

Nice!  Are the "false positives" real issues from components that are
currently blacklisted on RT, or something different?

-Scott