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Message-ID: <20c376e6-ba85-4c4a-bca6-727d3ef5e164@paulmck-laptop>
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
>
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