[tip: core/rcu] rcu/tree: Add a warning if CPU being onlined did not report QS already

[tip-bot2 for Joel Fernandes (Google)]

The following commit has been merged into the core/rcu branch of tip:

Commit-ID:     9f866dac94292f93d3b6bf8dbe860a44b954e555
Gitweb:        https://git.kernel.org/tip/9f866dac94292f93d3b6bf8dbe860a44b954e555
Author:        Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
AuthorDate:    Tue, 29 Sep 2020 15:29:27 -04:00
Committer:     Paul E. McKenney <paulmck@xxxxxxxxxx>
CommitterDate: Thu, 19 Nov 2020 19:37:16 -08:00

rcu/tree: Add a warning if CPU being onlined did not report QS already

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: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx>
Suggested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 kernel/rcu/tree.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 39e14cf..e4d6d0b 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4075,7 +4075,9 @@ 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? */
+
+	/* An incoming CPU should never be blocking a grace period. */
+	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);