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Message-ID: <20b9051fe47b7068ed3496bd7f5d417b1af69e3a.camel@redhat.com>
Subject: Re: [PATCH] rcutorture: Avoid problematic critical section nesting
 on RT
From:   Scott Wood <swood@redhat.com>
To:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Valentin Schneider <valentin.schneider@arm.com>
Cc:     linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        rcu@vger.kernel.org, linux-rt-users@vger.kernel.org,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>, Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Steven Rostedt <rostedt@goodmis.org>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        Steven Price <steven.price@arm.com>,
        Ard Biesheuvel <ardb@kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Mike Galbraith <efault@gmx.de>
Date:   Thu, 19 Aug 2021 22:23:37 -0500
In-Reply-To: <20210817144018.nqssoq475vitrqlv@linutronix.de>
References: <20210811201354.1976839-1-valentin.schneider@arm.com>
         <20210811201354.1976839-2-valentin.schneider@arm.com>
         <20210817121345.5iyj5epemczn3a52@linutronix.de>
         <20210817131741.evduh4fw7vyv2dzt@linutronix.de>
         <20210817144018.nqssoq475vitrqlv@linutronix.de>
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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?

-Scott