Hi will.
We already have a stronger barrier than smp_mb() (dsb ish) in __switch_to(). Is that not sufficient?
IIUC, It's not sufficient with smp_mb() in __switch_to().
Because, it can be broken in sched_mm_cid_remote_clear()
CPU0 in __schedule() CPU1 in sched_mm_cid_remote_clear() rq->curr = new_task; <no barrier> mm_get_cid remote_clear - check valid cid and use it. Invalidate CID. <barrier> rq->curr (not observed). unset the cid (<<BUG).
If change of rq->curr couldn't be observed in sched_mm_cid_remote_clear(), It could unset actively used cid. Note that __switch_to()'s smp_mb() is called AFTER switch_mm_cid(). That means before __switch_to(), there's possibility that sched_mm_cid_remote_clear() couldn't observe new active task, after it sets lazy_put on active cid used by new active task.