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rcu: Make idle entry report expedited quiescent states

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In non-preemptible kernels, an unfortunately timed expedited grace period
can result in the rcu_exp_handler() IPI handler setting the rcu_data
structure's cpu_no_qs.b.exp field just as the target CPU enters idle.
There are situations in which this field will not be checked until after
that CPU exits idle.  The resulting grace-period latency does not qualify
as "expedited".

This commit therefore checks this field upon non-preemptible idle entry in
the rcu_preempt_deferred_qs() function.  It also qualifies the rcu_core()
preempt_count() check with IS_ENABLED(CONFIG_PREEMPT_COUNT) to prevent
false-positive quiescent states from count-free kernels.

Reported-by: Neeraj Upadhyay <neeraju@codeaurora.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Paul E. McKenney 2021-09-29 11:09:34 -07:00
parent 147f04b14a
commit 790da24897
2 changed files with 13 additions and 2 deletions
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2
kernel/rcu/tree.c
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@ -2714,7 +2714,7 @@ static __latent_entropy void rcu_core(void)
WARN_ON_ONCE(!rdp->beenonline);
/* Report any deferred quiescent states if preemption enabled. */
if (!(preempt_count() & PREEMPT_MASK)) {
if (IS_ENABLED(CONFIG_PREEMPT_COUNT) && (!(preempt_count() & PREEMPT_MASK))) {
rcu_preempt_deferred_qs(current);
} else if (rcu_preempt_need_deferred_qs(current)) {
set_tsk_need_resched(current);

13
kernel/rcu/tree_plugin.h
View file

@ -927,7 +927,18 @@ static bool rcu_preempt_need_deferred_qs(struct task_struct *t)
{
return false;
}
static void rcu_preempt_deferred_qs(struct task_struct *t) { }
// Except that we do need to respond to a request by an expedited grace
// period for a quiescent state from this CPU. Note that requests from
// tasks are handled when removing the task from the blocked-tasks list
// below.
static void rcu_preempt_deferred_qs(struct task_struct *t)
{
struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
if (rdp->cpu_no_qs.b.exp)
rcu_report_exp_rdp(rdp);
}
/*
* Because there is no preemptible RCU, there can be no readers blocked,