Currently, invoke rcu_tasks_rude_wait_gp() to wait one rude
RCU-tasks grace period, if __num_online_cpus == 1, will return
directly, indicates the end of the rude RCU-task grace period.
suppose the system has two cpus, consider the following scenario:
CPU0 CPU1 (going offline)
migration/1 task:
cpu_stopper_thread
-> take_cpu_down
-> _cpu_disable
(dec __num_online_cpus)
->cpuhp_invoke_callback
preempt_disable
access old_data0
task1
del old_data0 .....
synchronize_rcu_tasks_rude()
task1 schedule out
....
task2 schedule in
rcu_tasks_rude_wait_gp()
->__num_online_cpus == 1
->return
....
task1 schedule in
->free old_data0
preempt_enable
when CPU1 dec __num_online_cpus and __num_online_cpus is equal one,
the CPU1 has not finished offline, stop_machine task(migration/1)
still running on CPU1, maybe still accessing 'old_data0', but the
'old_data0' has freed on CPU0.
In order to prevent the above scenario from happening, this commit
remove check for __num_online_cpus == 0 and add handling of calling
synchronize_rcu_tasks_generic() during early boot(when the
rcu_scheduler_active variable is RCU_SCHEDULER_INACTIVE, the scheduler
not yet initialized and only one boot-CPU online).
Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
---
kernel/rcu/tasks.h | 8 +++-----
1 file changed, 3 insertions(+), 5 deletions(-)
diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 4dda8e6e5707..9f0c439c73d2 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -560,8 +560,9 @@ static int __noreturn rcu_tasks_kthread(void *arg)
static void synchronize_rcu_tasks_generic(struct rcu_tasks *rtp)
{
/* Complain if the scheduler has not started. */
- WARN_ONCE(rcu_scheduler_active == RCU_SCHEDULER_INACTIVE,
- "synchronize_rcu_tasks called too soon");
+ if (WARN_ONCE(rcu_scheduler_active == RCU_SCHEDULER_INACTIVE,
+ "%s called too soon", rtp->name))
+ return;
// If the grace-period kthread is running, use it.
if (READ_ONCE(rtp->kthread_ptr)) {
@@ -1064,9 +1065,6 @@ static void rcu_tasks_be_rude(struct work_struct *work)
// Wait for one rude RCU-tasks grace period.
static void rcu_tasks_rude_wait_gp(struct rcu_tasks *rtp)
{
- if (num_online_cpus() <= 1)
- return; // Fastpath for only one CPU.
-
rtp->n_ipis += cpumask_weight(cpu_online_mask);
schedule_on_each_cpu(rcu_tasks_be_rude);
}
--
2.25.1
