[PATCH 1/2] rcu/tree: Add a warning if CPU being onlined did not report QS already

Joel Fernandes (Google)
2020-09-29T19:29:47Z

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@kernel.org>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Suggested-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
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 55d3700dd1e7..5efe0a98ea45 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4119,7 +4119,9 @@ 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);
--
2.28.0.709.gb0816b6eb0-goog