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);
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