From b3bb02fe5a2b538ae53eda1fe591dd6c81a91ad4 Mon Sep 17 00:00:00 2001 From: Frederic Weisbecker Date: Tue, 19 Oct 2021 02:08:10 +0200 Subject: [PATCH] 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 Tested-by: Sebastian Andrzej Siewior Signed-off-by: Frederic Weisbecker Cc: Valentin Schneider Cc: Peter Zijlstra Cc: Sebastian Andrzej Siewior Cc: Josh Triplett Cc: Joel Fernandes Cc: Boqun Feng Cc: Neeraj Upadhyay Cc: Uladzislau Rezki Cc: Thomas Gleixner Signed-off-by: Paul E. McKenney --- kernel/rcu/tree.c | 18 ++++++++++++++++-- 1 file changed, 16 insertions(+), 2 deletions(-) diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 5985698f3341..cb9abb80377f 100644 --- 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); + } } }