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rcu: Fix rcu_preempt_deferred_qs_irqrestore() strict QS reporting

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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>
This commit is contained in:
Paul E. McKenney 2022-02-16 09:54:56 -08:00
parent d22959aa93
commit 90d2efe7bd
1 changed files with 1 additions and 0 deletions
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kernel/rcu/tree_plugin.h
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@ -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 {