[PATCH v4 1/5] rcu/tree: Add a warning if CPU being onlined did not report QS already

[Joel Fernandes (Google)]

Currently, rcu_cpu_starting() checks to see if the RCU core expects a
quiescent state from the incoming CPU.  However, the current interaction
between RCU quiescent-state reporting and CPU-hotplug operations should
mean that the incoming CPU never needs to report a quiescent state.
First, the outgoing CPU reports a quiescent state if needed.  Second,
the race where the CPU is leaving just as RCU is initializing a new
grace period is handled by an explicit check for this condition.  Third,
the CPU's leaf rcu_node structure's ->lock serializes these checks.

This means that if rcu_cpu_starting() ever feels the need to report
a quiescent state, then there is a bug somewhere in the CPU hotplug
code or the RCU grace-period handling code.  This commit therefore
adds a WARN_ON_ONCE() to bring that bug to everyone's attention.

Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx>
Suggested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
---
 kernel/rcu/tree.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 65e1b5e92319..a49fa3b60faa 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3996,7 +3996,14 @@ void rcu_cpu_starting(unsigned int cpu)
 	rcu_gpnum_ovf(rnp, rdp); /* Offline-induced counter wrap? */
 	rdp->rcu_onl_gp_seq = READ_ONCE(rcu_state.gp_seq);
 	rdp->rcu_onl_gp_flags = READ_ONCE(rcu_state.gp_flags);
-	if (rnp->qsmask & mask) { /* RCU waiting on incoming CPU? */
+
+	/*
+	 * XXX: The following rcu_report_qs_rnp() is redundant. If the below
+	 * warning does not fire, consider replacing it with the "else" block,
+	 * by June 2021 or so (while keeping the warning). Refer to RCU's
+	 * Requirements documentation for the rationale.
+	 */
+	if (WARN_ON_ONCE(rnp->qsmask & mask)) { /* RCU waiting on incoming CPU? */
 		rcu_disable_urgency_upon_qs(rdp);
 		/* Report QS -after- changing ->qsmaskinitnext! */
 		rcu_report_qs_rnp(mask, rnp, rnp->gp_seq, flags);
-- 
2.28.0.236.gb10cc79966-goog