linux-kernel - Re: [RFC] rcu/nocb: Fix possible bugs in rcu

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Date:   Mon, 19 Sep 2022 11:34:11 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        urezki@...il.com, neeraj.iitr10@...il.com, paulmck@...nel.org,
        rostedt@...dmis.org
Subject: Re: [RFC] rcu/nocb: Fix possible bugs in rcu_barrier()

On Sun, Sep 18, 2022 at 10:12:31PM +0000, Joel Fernandes (Google) wrote:
> When going through the lazy-rcu work, I noticed that
> rcu_barrier_entrain() does not really wake up the rcuog GP thread in any
> path after entraining. This means it is possible the GP thread is not
> awakened soon (say there were no CBs in the cblist after entraining
> time).

Right.

> 
> Further, nothing appears to be calling the rcu_barrier callback
> directly in the case the ->cblist was empty which means if the IPI gets
> delayed enough to make the ->cblist empty and it turns out to be the last
> CPU holding, then nothing calls completes rcu_state.barrier_completion.

No need for that, if the cblist is empty there is no need for a callback
to enqueue.

Thanks.