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@xxxxxxxxxx> --- 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
