Re: [PATCH v7 1/6] sched: Annotate rq->rd with __rcu and update lockless readers

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Aug 26, 2026 at 06:42:33PM -0400, Aaron Tomlin wrote:
> The root_domain pointer rd field in struct rq is updated dynamically
> using RCU, and its memory reclamation is deferred via call_rcu() in
> rq_attach_root(). However, struct rq's rd field was missing the __rcu
> compiler annotation, and several lockless readers across the scheduler
> subsystem accessed rq->rd directly without using RCU dereference
> primitives.
> 
> Add the __rcu annotation to struct rq's rd field in kernel/sched/sched.h.
> Update lockless readers across kernel/sched/ to use rcu_dereference(),
> rcu_dereference_sched() or rcu_access_pointer() appropriately. This
> ensures proper data-dependency barriers on all architectures, enables
> Sparse static analysis validation, and documents RCU read-side ownership
> contracts.
> 
> Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
> ---
>  kernel/sched/core.c     | 24 ++++++++++-----
>  kernel/sched/deadline.c | 67 +++++++++++++++++++++++------------------
>  kernel/sched/fair.c     | 29 +++++++++---------
>  kernel/sched/rt.c       | 64 ++++++++++++++++++++++-----------------
>  kernel/sched/sched.h    |  2 +-
>  kernel/sched/topology.c | 11 ++++---
>  6 files changed, 113 insertions(+), 84 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 2e7cde033a31..86de58f5825b 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -8547,8 +8547,10 @@ void set_rq_online(struct rq *rq)
>  {
>  	if (!rq->online) {
>  		const struct sched_class *class;
> +		struct root_domain *rd;
>  
> -		cpumask_set_cpu(rq->cpu, rq->rd->online);
> +		rd = rcu_dereference_protected(rq->rd, lockdep_is_held(&rq->__lock));

This seems wrong; rcu_dereference_protected() is only supposed to be
used during the update. And rq->lock is very much not the update side
lock of the topology.

Also, &rq->__lock is wrong.

And this is far too verbose to endlessly repeat.