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 14:57:56 +0100
From:   Roy Hopkins <rhopkins@...e.de>
To:     paulmck@...nel.org, Guenter Roeck <linux@...ck-us.net>
Cc:     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 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.