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Date:   Mon, 23 Mar 2020 10:17:51 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Qais Yousef <qais.yousef@arm.com>
Cc:     Davidlohr Bueso <dave@stgolabs.net>,
        Josh Triplett <josh@joshtriplett.org>,
        linux-kernel@vger.kernel.org
Subject: Re: Hit WARN_ON() in rcutorture.c:1055
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On Mon, Mar 23, 2020 at 05:06:10PM +0000, Qais Yousef wrote:
> On 03/23/20 08:57, Paul E. McKenney wrote:
> > On Mon, Mar 23, 2020 at 03:43:09PM +0000, Qais Yousef wrote:
> > > Hi
> > > 
> > > I hit the following warning while running rcutorture tests. It only happens
> > > when I try to hibernate the system (arm64 Juno-r2).
> > 
> > Hibernating the system during rcutorture tests.  Now that is gutsy!  ;-)
> 
> Hehe was just a side effect of testing the cpu hotplug stuff :-)
> 
> > 
> > > Let me know if you need additional info.
> > 
> > 1.	Do you need this to work?  If so, please tell me your use case.
> 
> Nope. It just happened while trying to stress the cpu hotplug series I just
> posted.
> 
> > 2.	What is line 1055 of your rcutorture.c?  Here is my guess:
> 
> It's 5.6-rc6, sorry should have mentioned in the report.
> 
> 		/* Cycle through nesting levels of rcu_expedite_gp() calls. */
> 		if (can_expedite &&
> 		    !(torture_random(&rand) & 0xff & (!!expediting - 1))) {
> 			WARN_ON_ONCE(expediting == 0 && rcu_gp_is_expedited());
> 			if (expediting >= 0)
> 				rcu_expedite_gp();
> 			else
> 				rcu_unexpedite_gp();
> 			if (++expediting > 3)
> 				expediting = -expediting;
> 		} else if (!can_expedite) { /* Disabled during boot, recheck. */
> 
> If it's something you don't care about, then I don't care about too. I just
> thought I'd report it in case it uncovered something worthwhile.

Well, my guess was wrong.  ;-)

This is instead rcutorture being surprised by the fact that RCU grace
periods are expedited during the hibernate process.  I could fix this
particular situation, but I bet that there are a number of others,
including my guess above.

One approach would be to halt rcutorture testing just before hibernating
and restart it just after resuming.

Thoughts?

							Thanx, Paul