linux-kernel - Re: scheduler problems in -next (was: Re: [PATCH 6.4 000/227] 6.4.7-rc1 review)

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Date:   Wed, 02 Aug 2023 16:31:12 +0100
From:   Roy Hopkins <rhopkins@...e.de>
To:     paulmck@...nel.org
Cc:     Guenter Roeck <linux@...ck-us.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Pavel Machek <pavel@...x.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, patches@...ts.linux.dev,
        linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
        akpm@...ux-foundation.org, shuah@...nel.org, patches@...nelci.org,
        lkft-triage@...ts.linaro.org, jonathanh@...dia.com,
        f.fainelli@...il.com, sudipm.mukherjee@...il.com,
        srw@...dewatkins.net, rwarsow@....de, conor@...nel.org,
        rcu@...r.kernel.org, Ingo Molnar <mingo@...nel.org>
Subject: Re: scheduler problems in -next (was: Re: [PATCH 6.4 000/227]
 6.4.7-rc1 review)

On Wed, 2023-08-02 at 08:05 -0700, Paul E. McKenney wrote:
> On Wed, Aug 02, 2023 at 02:57:56PM +0100, Roy Hopkins wrote:
> > On Tue, 2023-08-01 at 12:11 -0700, Paul E. McKenney wrote:
> > > On Tue, Aug 01, 2023 at 10:32:45AM -0700, Guenter Roeck wrote:
> > > 
> > > 
> > > Please see below for my preferred fix.  Does this work for you guys?
> > > 
> > > Back to figuring out why recent kernels occasionally to blow up all
> > > rcutorture guest OSes...
> > > 
> > >                                                         Thanx, Paul
> > > 
> > > ------------------------------------------------------------------------
> > > 
> > > diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
> > > index 7294be62727b..2d5b8385c357 100644
> > > --- a/kernel/rcu/tasks.h
> > > +++ b/kernel/rcu/tasks.h
> > > @@ -570,10 +570,12 @@ static void rcu_tasks_one_gp(struct rcu_tasks *rtp,
> > > bool midboot)
> > >         if (unlikely(midboot)) {
> > >                 needgpcb = 0x2;
> > >         } else {
> > > +               mutex_unlock(&rtp->tasks_gp_mutex);
> > >                 set_tasks_gp_state(rtp, RTGS_WAIT_CBS);
> > >                 rcuwait_wait_event(&rtp->cbs_wait,
> > >                                    (needgpcb = rcu_tasks_need_gpcb(rtp)),
> > >                                    TASK_IDLE);
> > > +               mutex_lock(&rtp->tasks_gp_mutex);
> > >         }
> > >  
> > >         if (needgpcb & 0x2) {
> > 
> > Your preferred fix looks good to me.
> > 
> > With the original code I can quite easily reproduce the problem on my 
> > system every 10 reboots or so. With your fix in place the problem no
> > longer occurs.
> 
> Very good, thank you!  May I add your Tested-by?
> 
>                                                         Thanx, Paul
Yes, please do.