[PATCH 05/11] rcu/nocb: Make rcu_core() callbacks acceleration (de-)offloading safe

Frederic Weisbecker
2021-10-11T14:52:18Z

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>
---
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 4869a6856bf1..a43924244000 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2288,6 +2288,7 @@ 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());
@@ -2315,16 +2316,29 @@ 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);
+ }
}
}

--
2.25.1