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 <[email protected]>
---
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..e4f7d08bde64 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,
+ "synchronize_rcu_tasks called too soon"))
+ 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
On Wed, Nov 30, 2022 at 09:24:45AM +0800, Zqiang wrote:
> 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 <[email protected]>
> ---
> 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..e4f7d08bde64 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,
> + "synchronize_rcu_tasks called too soon"))
Much better, thank you!
But as long as you are touching this line of code, could you please also
fix it to print rtp->name instead of "synchronize_rcu_tasks"?
Thanx, Paul
> + 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
>