SIGBUS on device disappearance (Re: Warnings in DRM code when removing/unbinding a driver)

[Pekka Paalanen <ppaalanen@gmail.com>]

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On Thu, 19 Dec 2019 13:42:33 +0100
Daniel Vetter <daniel@ffwll.ch> wrote:

> On Thu, Dec 19, 2019 at 12:32 PM Gerd Hoffmann <kraxel@redhat.com> wrote:
> >
> > While being at it:  How would a driver cleanup properly cleanup gem
> > objects created by userspace on hotunbind?  Specifically a gem object
> > pinned to vram?  
> 
> Two things:
> - the mmap needs to be torn down and replaced by something which will
> sigbus. Probably should have that as a helper (plus vram fault code
> should use drm_dev_enter/exit to plug races).

Hi,

I assume SIGBUS is the traditional way to say "oops, the memory you
mmapped and tried to access no longer exists". Is there nothing
else for this?

I'm asking, because SIGBUS is really hard to handle right in
userspace. It can be caused by any number of wildly different
reasons, yet being a signal means that a userspace process can only
have a single global handler for it. That makes it almost
impossible to use safely in libraries, because you would want to
register independent handlers from multiple libraries in the same
process. Some libraries may also be using threads.

How to handle a SIGBUS completely depends on what triggered it.
Almost always userspace wants it to be a non-fatal error. A Wayland
compositor can hit SIGBUS on accessing wl_shm-based client buffers
(regular mmapped files), and then it just wants to continue with
garbage data as if nothing happened and possibly send a protocol
error to the client provoking it.

I would also imagine that Mesa, when it starts looking into
supporting GPU hotunplug, needs to handle vanished mmaps. I don't
think Mesa can ever install signal handlers, because that would
mess with the applications that may already be using SIGBUS for
handling disappearing mmapped files. It needs to start returning
errors via API calls. I cannot imagine a way to reliably prevent
such SIGBUS either by e.g. ensuring Mesa gets notified of removal
before it actually starts failing.

For now, I'm just looking for a simple "yes" or "no" here for the
something else. If it's "no" like I expect, creating something else
is probably in the order of years to get into a usable state. Does
anyone already have plans towards that?


Thanks,
pq

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