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) <j...@joelfernandes.org>
AuthorDate: Tue, 29 Sep 2020 15:29:27 -04:00
Committer: Paul E. McKenney <paul...@kernel.org>
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 <neer...@codeaurora.org>
Suggested-by: Paul E. McKenney <paul...@kernel.org>
Signed-off-by: Joel Fernandes (Google) <j...@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paul...@kernel.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 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);
