Re: [PATCH sched_ext/for-7.3 19/32] sched_ext: Add sub_ecaps_updated() effective-cap change notifier

[Tejun Heo <tj@kernel.org>]

> [High] Early clearing of `ecaps_to_sync_node` in
> `scx_process_sync_ecaps()` causes a Use-After-Free of `pcpu` and `sch`
> during sub-scheduler teardown.

pcpu is freed only by the post-unlink queue_rcu_work(), and this CPU
stays non-quiescent through balance_one() (the rq-lock handoff keeps
IRQs disabled), so the grace period can't complete before the write.

> [High] Concurrent freeing of a sub-scheduler can permanently discard
> ecaps syncs for all active sub-schedulers on a CPU that is coming
> online.

v2 will change the discard predicate to !cpu_active(cpu). The online
reseed is queued before SCX_RQ_ONLINE is set, so gating on the flag can
discard a live sched's reseed; cpu_active() brackets the reseed
correctly and matches scx_process_sync_ecaps().

> [High] scx_discard_ecaps_to_sync() can hang indefinitely in a worker
> thread if the target CPU is continuously executing higher-priority
> tasks, locking up the system RCU workqueue.

The ext deadline server guarantees ext_sched_class bandwidth, so
balance_one() runs and drains the sync. Disabling the DL server and
saturating the CPU with higher classes is accepted starvation.

Thanks.