rcu/nocb: Make rcu_core() callbacks acceleration (de-)offloading safe

When callbacks are offloaded, the NOCB kthreads handle the callbacks
progression on behalf of rcu_core().

However during the (de-)offloading process, the kthread may not be
entirely up to the task. As a result some callbacks grace period
sequence number may remain stale for a while because rcu_core() won't
take care of them either.

Fix this with forcing callbacks acceleration from rcu_core() as long
as the offloading process isn't complete.

Reported-and-tested-by: Valentin Schneider <valentin.schneider@arm.com>
Tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Valentin Schneider <valentin.schneider@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: Uladzislau Rezki <urezki@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 5985698..cb9abb8 100644 (file)
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2278,6 +2278,7 @@ rcu_report_qs_rdp(struct rcu_data *rdp)
        unsigned long flags;
        unsigned long mask;
        bool needwake = false;
+       bool needacc = false;
        struct rcu_node *rnp;
 
        WARN_ON_ONCE(rdp->cpu != smp_processor_id());
@@ -2305,16 +2306,29 @@ rcu_report_qs_rdp(struct rcu_data *rdp)
                 * This GP can't end until cpu checks in, so all of our
                 * callbacks can be processed during the next GP.
                 *
-                * NOCB kthreads have their own way to deal with that.
+                * NOCB kthreads have their own way to deal with that...
                 */
-               if (!rcu_rdp_is_offloaded(rdp))
+               if (!rcu_rdp_is_offloaded(rdp)) {
                        needwake = rcu_accelerate_cbs(rnp, rdp);
+               } else if (!rcu_segcblist_completely_offloaded(&rdp->cblist)) {
+                       /*
+                        * ...but NOCB kthreads may miss or delay callbacks acceleration
+                        * if in the middle of a (de-)offloading process.
+                        */
+                       needacc = true;
+               }
 
                rcu_disable_urgency_upon_qs(rdp);
                rcu_report_qs_rnp(mask, rnp, rnp->gp_seq, flags);
                /* ^^^ Released rnp->lock */
                if (needwake)
                        rcu_gp_kthread_wake();
+
+               if (needacc) {
+                       rcu_nocb_lock_irqsave(rdp, flags);
+                       rcu_accelerate_cbs_unlocked(rnp, rdp);
+                       rcu_nocb_unlock_irqrestore(rdp, flags);
+               }
        }
 }