If a task is on the rcu_node_entry list, its blocked flag should
already be set (it's set before adding to any list in
rcu_note_context_switch()). The current code silently re-sets it,
which could mask bugs.

Add a WARN_ON_ONCE to detect this invariant violation. If this
warning ever fires, it indicates a bug where a task was added to
a blocked list without properly setting the blocked flag first.

Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
---
 kernel/rcu/tree_plugin.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index dbe2d02be824..73ba5f4a968d 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -846,6 +846,7 @@ void exit_rcu(void)
 	if (unlikely(!list_empty(&current->rcu_node_entry))) {
 		rcu_preempt_depth_set(1);
 		barrier();
+		WARN_ON_ONCE(!t->rcu_read_unlock_special.b.blocked);
 		WRITE_ONCE(t->rcu_read_unlock_special.b.blocked, true);
 	} else if (unlikely(rcu_preempt_depth())) {
 		rcu_preempt_depth_set(1);
-- 
2.34.1

On Fri,  2 Jan 2026 19:23:30 -0500
Joel Fernandes <joelagnelf@nvidia.com> wrote:

> Add a WARN_ON_ONCE to detect this invariant violation. If this
> warning ever fires, it indicates a bug where a task was added to
> a blocked list without properly setting the blocked flag first.
> 
> Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
> ---
>  kernel/rcu/tree_plugin.h | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index dbe2d02be824..73ba5f4a968d 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -846,6 +846,7 @@ void exit_rcu(void)
>  	if (unlikely(!list_empty(&current->rcu_node_entry))) {
>  		rcu_preempt_depth_set(1);
>  		barrier();
> +		WARN_ON_ONCE(!t->rcu_read_unlock_special.b.blocked);
>  		WRITE_ONCE(t->rcu_read_unlock_special.b.blocked, true);

If we warn when it is not set, could we just remove setting it?
Or do:

		if (WARN_ON_ONCE(!t->rcu_read_unlock_special.b.blocked))
	 		WRITE_ONCE(t->rcu_read_unlock_special.b.blocked, true);

-- Steve


>  	} else if (unlikely(rcu_preempt_depth())) {
>  		rcu_preempt_depth_set(1);
> --

On 1/5/2026 10:31 AM, Steven Rostedt wrote:
> On Fri,  2 Jan 2026 19:23:30 -0500
> Joel Fernandes <joelagnelf@nvidia.com> wrote:
> 
>> Add a WARN_ON_ONCE to detect this invariant violation. If this
>> warning ever fires, it indicates a bug where a task was added to
>> a blocked list without properly setting the blocked flag first.
>>
>> Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
>> ---
>>  kernel/rcu/tree_plugin.h | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
>> index dbe2d02be824..73ba5f4a968d 100644
>> --- a/kernel/rcu/tree_plugin.h
>> +++ b/kernel/rcu/tree_plugin.h
>> @@ -846,6 +846,7 @@ void exit_rcu(void)
>>  	if (unlikely(!list_empty(&current->rcu_node_entry))) {
>>  		rcu_preempt_depth_set(1);
>>  		barrier();
>> +		WARN_ON_ONCE(!t->rcu_read_unlock_special.b.blocked);
>>  		WRITE_ONCE(t->rcu_read_unlock_special.b.blocked, true);

Right, so it is ever not set then we are trying to "right" a "wrong", so..

> 
> If we warn when it is not set, could we just remove setting it?
> Or do:
> 
> 		if (WARN_ON_ONCE(!t->rcu_read_unlock_special.b.blocked))
> 	 		WRITE_ONCE(t->rcu_read_unlock_special.b.blocked, true);

.. we could just do this. I'll apply the suggestion, thanks!

 - Joel