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SubjectRe: [PATCH 04/19] sched: Prepare for Core-wide rq->lock
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index f732642e3e09..1a81e9cc9e5d 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -290,6 +290,10 @@ static void sched_core_assert_empty(void)
> static void __sched_core_enable(void)
> {
> static_branch_enable(&__sched_core_enabled);
> + /*
> + * Ensure raw_spin_rq_*lock*() have completed before flipping.
> + */
> + synchronize_sched();

synchronize_rcu()

> __sched_core_flip(true);
> sched_core_assert_empty();
> }
> @@ -449,16 +453,22 @@ void raw_spin_rq_lock_nested(struct rq *rq, int subclass)
> {
> raw_spinlock_t *lock;
>
> + preempt_disable();
> if (sched_core_disabled()) {
> raw_spin_lock_nested(&rq->__lock, subclass);
> + /* preempt *MUST* still be disabled here */
> + preempt_enable_no_resched();
> return;
> }

This approach looks good to me. I'm guessing you went this route
instead of doing the re-check after locking in order to optimize the
disabled case?

Recommend a comment that the preempt_disable() here pairs with the
synchronize_rcu() in __sched_core_enable().

>
> for (;;) {
> lock = __rq_lockp(rq);
> raw_spin_lock_nested(lock, subclass);
> - if (likely(lock == __rq_lockp(rq)))
> + if (likely(lock == __rq_lockp(rq))) {
> + /* preempt *MUST* still be disabled here */
> + preempt_enable_no_resched();
> return;
> + }
> raw_spin_unlock(lock);
> }
> }

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