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Re: kprempt, pmap_load() and copy*

On Sun, Jun 19, 2011 at 06:38:40PM +0200, Manuel Bouyer wrote:
> 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 ...

Hmmm.... lets see... thinks ...
copyin/out can fault or be interrupted, both can cause a process switch.
So there ought to be code somewhere (probably in the return from trap/intr
code) that will restore the pmap to allow the copy to continue.
This would rather indicate that the end of copyin/out ought to be clearing
a 'need process's pmap' flag.
Not looked ...


David Laight:

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