[PATCH] arm64/mm: Add memory barrier for mm_cid

[levi.yun]

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.