kprempt, pmap_load() and copy*

[Manuel Bouyer <bouyer%antioche.eu.org@localhost>]

Hello,
I have a question about the kernel copy* function vs lazy pmap switching
and kernel preemption.
on amd64, lazy pmap switching is used: pmap_activate() just sets a per-cpu
variable ci_want_pmapload to 1; the pmap is really loaded on the cpu
just in time (i.e. when returning to userland, or something in the
kernel needs it).
The copyin/copyout & friend checks ci_want_pmapload and call do_pmap_load()
before doing the work. do_pmap_load() will disable kernel preemtion
before calling pmap_load() reenable it after and let kernel preemtion
occur if needed. Before returning, do_pmap_load() checks ci_want_pmapload
again and loops back to the beggining.

Now, what happens if preemtion and pmap switching occurs after that, while
the copy* functions are working ? what is making sure that the right
pmap is loaded again before returning to the interrupted copy* function ?
Either the check before return in do_pmap_load() is not needed, or
we can potentially copy data to/from the wrong user process here ...

-- 
Manuel Bouyer <bouyer%antioche.eu.org@localhost>
     NetBSD: 26 ans d'experience feront toujours la difference
--