rcu/nocb: Don't deoffload an offline CPU with pending work

Offloaded CPUs do not migrate their callbacks, instead relying on
their rcuo kthread to invoke them.  But if the CPU is offline, it
will be running neither its RCU_SOFTIRQ handler nor its rcuc kthread.
This means that de-offloading an offline CPU that still has pending
callbacks will strand those callbacks.  This commit therefore refuses
to toggle offline CPUs having pending callbacks.

Cc: Josh Triplett <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Cc: Lai Jiangshan <[email protected]>
Cc: Joel Fernandes <[email protected]>
Cc: Neeraj Upadhyay <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Suggested-by: Paul E. McKenney <[email protected]>
Tested-by: Boqun Feng <[email protected]>
Signed-off-by: Frederic Weisbecker <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>

1 files changed, 9 insertions, 0 deletions

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 1b870d0d2445..b70cc91a7831 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2227,6 +2227,15 @@ static int __rcu_nocb_rdp_deoffload(struct rcu_data *rdp)
 	printk("De-offloading %d\n", rdp->cpu);
 
 	rcu_nocb_lock_irqsave(rdp, flags);
+	/*
+	 * If there are still pending work offloaded, the offline
+	 * CPU won't help much handling them.
+	 */
+	if (cpu_is_offline(rdp->cpu) && !rcu_segcblist_empty(&rdp->cblist)) {
+		rcu_nocb_unlock_irqrestore(rdp, flags);
+		return -EBUSY;
+	}
+
 	rcu_segcblist_offload(cblist, false);
 
 	if (rdp->nocb_cb_sleep) {