Commit b3bb02fe authored Oct 19, 2021 by Frederic Weisbecker Committed by Paul E. McKenney Dec 07, 2021

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>

parent 24ee940d

kernel/rcu/tree.c

+16 −2

Original line number	Diff line number	Diff line
		@@ -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);
		}
		}
		}