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

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Message-ID: <20190917145035.l6egzthsdzp7aipe@linutronix.de>
Date:   Tue, 17 Sep 2019 16:50:35 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Scott Wood <swood@...hat.com>
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 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. 

> > 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.

> -Scott

Sebastian