rcu: Fix rcu_preempt_deferred_qs_irqrestore() strict QS reporting

Suppose we have a kernel built with both CONFIG_RCU_STRICT_GRACE_PERIOD=y
and CONFIG_PREEMPT=y.  Suppose further that an RCU reader from which RCU
core needs a quiescent state ends in rcu_preempt_deferred_qs_irqrestore().
This function will then invoke rcu_report_qs_rdp() in order to immediately
report that quiescent state.  Unfortunately, it will not have cleared
that reader's CPU's rcu_data structure's ->cpu_no_qs.b.norm field.
As a result, rcu_report_qs_rdp() will take an early exit because it
will believe that this CPU has not yet encountered a quiescent state,
and there will be no reporting of the current quiescent state.

This commit therefore causes rcu_preempt_deferred_qs_irqrestore() to
clear the ->cpu_no_qs.b.norm field before invoking rcu_report_qs_rdp().

Kudos to Boqun Feng and Neeraj Upadhyay for helping with analysis of
this issue!

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 8360d86..176639c 100644 (file)
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -486,6 +486,7 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
        t->rcu_read_unlock_special.s = 0;
        if (special.b.need_qs) {
                if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD)) {
+                       rdp->cpu_no_qs.b.norm = false;
                        rcu_report_qs_rdp(rdp);
                        udelay(rcu_unlock_delay);
                } else {