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@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 2491766e1fd59ffe7b657a4239efa6aa4f8f7b35..3b9d5c7eb4a2a375c3e0bf66b73d12bbea081727 100644 (file)
--- 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;
 }