linux-kernel - Re: [PATCH 11/11] rcu/nocb: Don't invoke local rcu core on callback overload from nocb kthread

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Date:   Mon, 11 Oct 2021 14:11:22 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     "Paul E . McKenney" <paulmck@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Uladzislau Rezki <urezki@...il.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Boqun Feng <boqun.feng@...il.com>,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        Josh Triplett <josh@...htriplett.org>,
        Joel Fernandes <joel@...lfernandes.org>, rcu@...r.kernel.org
Subject: Re: [PATCH 11/11] rcu/nocb: Don't invoke local rcu core on callback
 overload from nocb kthread

Phew, the last changelog is the most careless...

On Thu, Sep 30, 2021 at 05:37:37PM +0200, Sebastian Andrzej Siewior wrote:
> On 2021-09-30 00:10:12 [+0200], Frederic Weisbecker wrote:
> > rcu_core() tries to ensure that its self-invocation in case of callbacks
> > overload only happen in softirq/rcuc mode. Indeed it doesn't make sense
> > to trigger local RCU core from nocb_cb kthread since it can execute
> > on a CPU different from the target rdp. Also in case of overload, the
> > nocb_cb kthread simply iterates a new loop of callbacks processing.
> > 
> > However the "offloaded" check that aims at preventing that is wrong.
> - that?

Rephrasing the changelog:

"However the "offloaded" check that aims at preventing misplaced
rcu_core() invocations is wrong."


> 
> > First of all that state is volatile and second: softirq/rcuc can
> > execute while the target rdp is offloaded. As a result rcu_core() can
> 
> can what?

"As a result rcu_core() can be invoked on the wrong CPU while in the
process of (de-offloading)."

Thanks!


> 
> > Fix that with moving the rcu_core() self-invocation to rcu_core() itself,
> > irrespective of the rdp offloaded state.
> 
> Sebastian