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

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Date:   Fri, 20 Aug 2021 09:11:06 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Scott Wood <swood@...hat.com>
Cc:     paulmck@...nel.org,
        Valentin Schneider <valentin.schneider@....com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        rcu@...r.kernel.org, linux-rt-users@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Steven Price <steven.price@....com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Mike Galbraith <efault@....de>
Subject: Re: [PATCH] rcutorture: Avoid problematic critical section nesting
 on RT

On 2021-08-19 23:11:12 [-0500], Scott Wood wrote:
> On Thu, 2021-08-19 at 11:20 -0700, Paul E. McKenney wrote:
> > On Thu, Aug 19, 2021 at 05:47:08PM +0200, Sebastian Andrzej Siewior wrote:
> > > On 2021-08-19 17:39:29 [+0200], To Paul E. McKenney wrote:
> > > > +	/*
> > > > +	 * Ideally these sequences would be detected in debug builds
> > > > +	 * (regardless of RT), but until then don't stop testing
> > > > +	 * them on non-RT.
> > > > +	 */
> > > > +	if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
> > > > +		/*
> > > > +		 * Can't release the outermost rcu lock in an irq disabled
> > > > +		 * section without preemption also being disabled, if irqs
> > > > +		 * had ever been enabled during this RCU critical section
> > > > +		 * (could leak a special flag and delay reporting the qs).
> > > > +		 */
> > > > +		if ((oldmask & RCUTORTURE_RDR_RCU) &&
> > > > +		    (mask & RCUTORTURE_RDR_IRQ) &&
> > > > +		    !(mask & preempts))
> > > > +			mask |= RCUTORTURE_RDR_RCU;
> > > 
> > > This piece above, I don't understand. I had it running for a while and
> > > it didn't explode. Let me try TREE01 for 30min without that piece.
> > 
> > This might be historical.  There was a time when interrupts being
> > disabled across rcu_read_unlock() meant that preemption had to have
> > been disabled across the entire RCU read-side critical section.
> > 
> > I am not seeing a purpose for it now, but I could easily be missing
> > something, especially given my tenuous grasp of RT.
> 
> Yeah, I think this was to deal with not having the irq work stuff in RT
> at the time.

Good. Thank you for the confirmation. 
I run (without the hunk above) 2x 6h of TREE01 and 4x 6h of TREE06 and
it looked good.

> -Scott

Sebastian