linux-kernel - Re: [PATCH 10/10] rcu: Comment why callbacks migration can't wait for CPUHP_RCUTREE

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Date:   Mon, 2 Oct 2023 08:48:39 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, rcu <rcu@...r.kernel.org>,
        Uladzislau Rezki <urezki@...il.com>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH 10/10] rcu: Comment why callbacks migration can't wait
 for CPUHP_RCUTREE_PREP

On Fri, Sep 08, 2023 at 10:36:03PM +0200, Frederic Weisbecker wrote:
> The callbacks migration is performed through an explicit call from
> the hotplug control CPU right after the death of the target CPU and
> before proceeding with the CPUHP_ teardown functions.
> 
> This is unusual but necessary and yet uncommented. Summarize the reason
> as explained in the changelog of:
> 
> 	a58163d8ca2c (rcu: Migrate callbacks earlier in the CPU-offline timeline)
> 
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>

Reviewed-by: Paul E. McKenney <paulmck@...nel.org>

> ---
>  kernel/cpu.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index a41a6fff3c91..b135bb481be1 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -1352,7 +1352,14 @@ static int takedown_cpu(unsigned int cpu)
>  	cpuhp_bp_sync_dead(cpu);
>  
>  	tick_cleanup_dead_cpu(cpu);
> +
> +	/*
> +	 * Callbacks must be re-integrated right away to the RCU state machine.
> +	 * Otherwise an RCU callback could block a further teardown function
> +	 * waiting for its completion.
> +	 */
>  	rcutree_migrate_callbacks(cpu);
> +
>  	return 0;
>  }
>  
> -- 
> 2.41.0
>