Commit 790da248 authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

rcu: Make idle entry report expedited quiescent states 



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: default avatarNeeraj Upadhyay <neeraju@codeaurora.org>
Signed-off-by: default avatarPaul E. McKenney <paulmck@kernel.org>
parent 147f04b1

kernel/rcu/tree.c

+1 −1
Original line number Diff line number Diff line
@@ -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);

kernel/rcu/tree_plugin.h

+12 −1
Original line number Diff line number Diff line
@@ -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,