Re: [PATCH RFC 01/14] rcu: Add WARN_ON_ONCE for blocked flag invariant in exit_rcu()

[Steven Rostedt]

On Fri,  2 Jan 2026 19:23:30 -0500
Joel Fernandes <joelagn...@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 <joelagn...@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);
> --