How to handle Hotplug with UIO userspace driver

[Mandeep Sandhu]

>
> > While the kernel driver has no problem with hot-removal, I see that if I
> > "unregister" our UIO driver in response to the removal, then the mapping
> for
> > the hardware's physical memory would become "invalid" (done by UIO on
> > unregsiter). This could possibly cause the user-space process to crash
> due to
> > illegal memory access.
>
> You should just get 0xff on the memory reads, right?  You can catch the
> signal for invalid memory accesses.
>

Wouldn't I get an illegal memory access, as UIO would've removed the
mappings that it setup when I registered the UIO driver? I saw the
userspace process segfault after removal.


>
> > Is there a "standard" way to handle this scenario, i.e for hotpluggable
> > hardware using UIO?
>
> Your kernel driver knows when the device is removed, so have it tell
> userspace this.  Or, just tie into libudev and have your userspace
> program be notified when the device goes away.
>

Right. This is what I'm trying to do using a udev "remove" rule. Although
the question still remains, what will happen if the device is being
actively accessed _before_ the signal reaches the process. Is it safe to do
"uio_unregister_device" while a userspace process is accessing the device?

I saw a patch for this very situation (UIO & hotplug) being discussed on
LKML almost 4 yrs back, although I don't see it in my kernel version -
3.16.0 (Ubuntu 3.16.0-24-generic).
The LKML link:

https://lkml.org/lkml/2010/9/20/21

Wouldn't this solve the issue? I wonder why it didn't make it into mainline?


>
> Do you have a pointer to the source of your uio kernel driver anywhere?
>

Not yet, as this is still under active development (we don't have the h/w
to test yet :/ ). Maybe once things stabilize a little, we'll probably
open-source it along with the h/w too! :)

Thanks,
-mandeep



>
> thanks,
>
> greg k-h
>
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