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

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Message-ID: <20210820065417.epdjfqsn27gmnx4x@linutronix.de>
Date:   Fri, 20 Aug 2021 08:54:17 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Scott Wood <swood@...hat.com>
Cc:     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>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        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 22:23:37 [-0500], Scott Wood wrote:
> On Tue, 2021-08-17 at 16:40 +0200, Sebastian Andrzej Siewior wrote:
> > [bigeasy: remove 'preempt_disable(); local_bh_disable(); preempt_enable();
> >  local_bh_enable();' from the examples because this works on RT now. ]
> 
> Does it actually work?  If preemption is disabled during local_bh_disable,
> softirq_ctrl.lock won't be taken.  If you then get preempted between the
> preempt_enable() and the local_bh_enable(), and another task tries to do
> local_bh_disable(), won't it successfully get softirq_ctrl.lock, add to
> softirq_ctrl.cnt, and proceed right into the critical section?
> 
> Or am I missing something?

No, I mixed it up with migrate_disable/enable. I corrected it while
redoing it yesterday.

> -Scott

Sebastian