[PATCH 10/10] rcu: Comment why callbacks migration can't wait for CPUHP_RCUTREE_PREP

[Frederic Weisbecker <frederic@kernel.org>]

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@kernel.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