linux-kernel - [PATCH 16/18] rcu: Comment why callbacks migration can't wait for CPUHP_RCUTREE

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Message-Id: <20231013115902.1059735-17-frederic@kernel.org>
Date:   Fri, 13 Oct 2023 13:59:00 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Josh Triplett <josh@...htriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Neeraj Upadhyay <neeraj.upadhyay@....com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Uladzislau Rezki <urezki@...il.com>, rcu <rcu@...r.kernel.org>
Subject: [PATCH 16/18] rcu: Comment why callbacks migration can't wait for CPUHP_RCUTREE_PREP

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)

Reviewed-by: Paul E. McKenney <paulmck@...nel.org>
Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
---
 kernel/cpu.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 2491766e1fd5..3b9d5c7eb4a2 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -1372,7 +1372,14 @@ 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;
 }
 
-- 
2.34.1