Re: [PATCH 0/1] [RFC] DRM locking issues during early open

[Daniel Vetter <daniel@ffwll.ch>]

On Thu, Apr 19, 2012 at 05:30:03PM +0100, Dave Airlie wrote:
> On Thu, Apr 19, 2012 at 5:22 PM, Andy Whitcroft <apw@canonical.com> wrote:
> > We have been carrying a (rather poor) patch for an issue we identified in
> > the DRM driver.  This issue is triggered when a DRM device is initialising
> > and userspace attempts to open it, typically in response to the sysfs
> > device added event.  Basically we allocate the minor numbers making
> > the device available, and then call the drm load callback.  Until this
> > completes the device is really not ready and these early opens typically
> > lead to oopses.
> >
> > We have been using the following patch to avoid this by marking the minors
> > as in error until the load method has completed.  This avoids the early
> > open by simply erroring out the opens with EAGAIN.  Obviously we should
> > be delaying the open until the load method complete.
> >
> > I include the existing patch for completness (it is not really ready for
> > merging) to illustrate the issue.  I think it is logical that the wait
> > should simply be delayed until the load has completed.  I am proposing
> > to include a wait queue associated with the idr cache for the drm minors
> > which we can use to allow open callers to wait_event_interruptible() on.
> > I'll be putting together a prototype shortly and will follow up with it.
> >
> > Thoughts?
> 
> Couldn't we just delay registering things until the driver is ready to
> accept an open?

It's somewhere on my eternal&epic todo list.

> Granted the midlayer of drm doesn't make that easy,

... after fixing this one ;-)

> thanks for sending this out, it keeps falling off my radar, I don't
> think I've ever seen this reported on RHEL/Fedora, which makes me
> wonder what we are doing that makes us lucky.

I think it's just a matter of races, if you load the drm module early
enough (like fedora does already in the initrd) and ensure that nothing
pokes drm devices for a few seconds, you'll be fine. Iirc ubuntus powerd
stuff is really got at brining everything down. Also, not loading the
module with udev, but loading it with X resulted in nice fireworks last
time I've tried that (radeon ums was trying to set up the card while the
kms code was doing the same, hilarity ensued).
-Daniel
-- 
Daniel Vetter
Mail: daniel@ffwll.ch
Mobile: +41 (0)79 365 57 48