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

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Message-ID: <20210922063208.ltf7sdou4tr5yrnc@linutronix.de>
Date:   Wed, 22 Sep 2021 08:32:08 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        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>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        "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 2021-09-22 01:45:18 [+0200], Frederic Weisbecker wrote:
> 
> Also while at it, I'm asking again: traditionally softirqs could assume that
> manipulating a local state was safe against !irq_count() code fiddling with
> the same state on the same CPU.
> 
> Now with preemptible softirqs, that assumption can be broken anytime. RCU was
> fortunate enough to have a warning for that. But who knows how many issues like
> this are lurking?

If "local state" is modified then it is safe as long as it is modified
within a local_bh_disable() section. And we are in this section while
invoking a forced-threaded interrupt. The special part about RCU is
that it is used in_irq() as part of core-code.

> Thanks.

Sebastian