Commit 5554788e authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

rcu: Suppress false-positive offline-CPU lockdep-RCU splat 



The rcu_lockdep_current_cpu_online() function currently checks only the
RCU-sched data structures to determine whether or not RCU believes that a
given CPU is offline.  Unfortunately, there are multiple flavors of RCU,
which means that there is a short window of time during which the various
flavors disagree as to whether or not a given CPU is offline.  This can
result in false-positive lockdep-RCU splats in which some other flavor
of RCU tries to do something based on its view that the CPU is online,
only to get hit with a lockdep-RCU splat because RCU-sched instead
believes that the CPU is offline.

This commit therefore changes rcu_lockdep_current_cpu_online() to scan
all RCU flavors and to consider a given CPU to be online if any of the
RCU flavors believe it to be online, thus preventing these false-positive
splats.

Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
parent 92816435

kernel/rcu/tree.c

+24 −24
Original line number Diff line number Diff line
@@ -1030,41 +1030,41 @@ void rcu_request_urgent_qs_task(struct task_struct *t) 
#if defined(CONFIG_PROVE_RCU) && defined(CONFIG_HOTPLUG_CPU)



/*

 * Is the current CPU online?  Disable preemption to avoid false positives

 * that could otherwise happen due to the current CPU number being sampled,

 * this task being preempted, its old CPU being taken offline, resuming

 * on some other CPU, then determining that its old CPU is now offline.

 * It is OK to use RCU on an offline processor during initial boot, hence

 * the check for rcu_scheduler_fully_active.  Note also that it is OK

 * for a CPU coming online to use RCU for one jiffy prior to marking itself

 * online in the cpu_online_mask.  Similarly, it is OK for a CPU going

 * offline to continue to use RCU for one jiffy after marking itself

 * offline in the cpu_online_mask.  This leniency is necessary given the

 * non-atomic nature of the online and offline processing, for example,

 * the fact that a CPU enters the scheduler after completing the teardown

 * of the CPU.

 * Is the current CPU online as far as RCU is concerned?

 *

 * This is also why RCU internally marks CPUs online during in the

 * preparation phase and offline after the CPU has been taken down.

 * Disable preemption to avoid false positives that could otherwise

 * happen due to the current CPU number being sampled, this task being

 * preempted, its old CPU being taken offline, resuming on some other CPU,

 * then determining that its old CPU is now offline.  Because there are

 * multiple flavors of RCU, and because this function can be called in the

 * midst of updating the flavors while a given CPU coming online or going

 * offline, it is necessary to check all flavors.  If any of the flavors

 * believe that given CPU is online, it is considered to be online.

 *

 * Disable checking if in an NMI handler because we cannot safely report

 * errors from NMI handlers anyway.

 * Disable checking if in an NMI handler because we cannot safely

 * report errors from NMI handlers anyway.  In addition, it is OK to use

 * RCU on an offline processor during initial boot, hence the check for

 * rcu_scheduler_fully_active.

 */

bool rcu_lockdep_current_cpu_online(void)

{

	struct rcu_data *rdp;

	struct rcu_node *rnp;

	bool ret;

	struct rcu_state *rsp;



	if (in_nmi())

	if (in_nmi() || !rcu_scheduler_fully_active)

		return true;

	preempt_disable();

	rdp = this_cpu_ptr(&rcu_sched_data);

	for_each_rcu_flavor(rsp) {

		rdp = this_cpu_ptr(rsp->rda);

		rnp = rdp->mynode;

	ret = (rdp->grpmask & rcu_rnp_online_cpus(rnp)) ||

	      !rcu_scheduler_fully_active;

		if (rdp->grpmask & rcu_rnp_online_cpus(rnp)) {

			preempt_enable();

	return ret;

			return true;

		}

	}

	preempt_enable();

	return false;

}

EXPORT_SYMBOL_GPL(rcu_lockdep_current_cpu_online);

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