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

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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: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Paul E. McKenney 2018-05-15 10:14:34 -07:00
parent 928164351e
commit 5554788e1d
1 changed files with 24 additions and 24 deletions
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48
kernel/rcu/tree.c
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@ -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);
rnp = rdp->mynode;
ret = (rdp->grpmask & rcu_rnp_online_cpus(rnp)) ||
!rcu_scheduler_fully_active;
for_each_rcu_flavor(rsp) {
rdp = this_cpu_ptr(rsp->rda);
rnp = rdp->mynode;
if (rdp->grpmask & rcu_rnp_online_cpus(rnp)) {
preempt_enable();
return true;
}
}
preempt_enable();
return ret;
return false;
}
EXPORT_SYMBOL_GPL(rcu_lockdep_current_cpu_online);