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From:   Valentin Schneider <valentin.schneider@arm.com>
To:     Frederic Weisbecker <frederic@kernel.org>,
        "Paul E . McKenney" <paulmck@kernel.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Uladzislau Rezki <urezki@gmail.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Neeraj Upadhyay <neeraju@codeaurora.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Joel Fernandes <joel@joelfernandes.org>, rcu@vger.kernel.org
Subject: Re: [PATCH 04/11] rcu/nocb: Make rcu_core() callbacks acceleration preempt-safe
In-Reply-To: <20210929221012.228270-5-frederic@kernel.org>
References: <20210929221012.228270-1-frederic@kernel.org> <20210929221012.228270-5-frederic@kernel.org>
Date:   Fri, 01 Oct 2021 18:50:22 +0100
Message-ID: <87bl48my75.mognet@arm.com>
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On 30/09/21 00:10, Frederic Weisbecker wrote:
> From: Thomas Gleixner <tglx@linutronix.de>
>
> While reporting a quiescent state for a given CPU, rcu_core() takes
> advantage of the freshly loaded grace period sequence number and the
> locked rnp to accelerate the callbacks whose sequence number have been
> assigned a stale value.
>
> This action is only necessary when the rdp isn't offloaded, otherwise
> the NOCB kthreads already take care of the callbacks progression.
>
> However the check for the offloaded state is volatile because it is
> performed outside the IRQs disabled section. It's possible for the
> offloading process to preempt rcu_core() at that point on PREEMPT_RT.
>
> This is dangerous because rcu_core() may end up accelerating callbacks
> concurrently with NOCB kthreads without appropriate locking.
>
> Fix this with moving the offloaded check inside the rnp locking section.
>
> Reported-by: Valentin Schneider <valentin.schneider@arm.com>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Cc: Valentin Schneider <valentin.schneider@arm.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Cc: Josh Triplett <josh@joshtriplett.org>
> Cc: Joel Fernandes <joel@joelfernandes.org>
> Cc: Boqun Feng <boqun.feng@gmail.com>
> Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
> Cc: Uladzislau Rezki <urezki@gmail.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>

Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>