linux-kernel - Re: rcu/tree: Protect rcu_rdp_is

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Message-ID: <87ee9619lv.mognet@arm.com>
Date:   Thu, 30 Sep 2021 14:22:36 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>, 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>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        "Paul E. McKenney" <paulmck@...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: rcu/tree: Protect rcu_rdp_is_offloaded() invocations on RT

On 30/09/21 12:53, Frederic Weisbecker wrote:
> On Thu, Sep 30, 2021 at 10:00:39AM +0100, Valentin Schneider wrote:
>> My reasoning for adding protection in the outer functions was to prevent
>> impaired unlocks of rcu_nocb_{un}lock_irqsave(), as that too depends on the
>> offload state. Cf. Frederic's writeup:
>>
>>   http://lore.kernel.org/r/20210727230814.GC283787@lothringen
>
> I was wrong about that BTW!
> Because rcu_nocb_lock() always require IRQs to be disabled, which of course disables
> preemption, so the offloaded state can't change between
> rcu_nocb_lock[_irqsave]() and rcu_nocb_unlock[_irqrestore]() but anyway there
> were many other issues to fix :-)
>

Ooooh... Even with you pointing it out, it took me a while to see it that
way. It's tough to get out of holidays mode :-)