On Thu, Apr 19, 2012 at 5:41 PM, Andy Whitcroft <apw@xxxxxxxxxxxxx> wrote: > 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@xxxxxxxxxxxxx> 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? >> >> Granted the midlayer of drm doesn't make that easy, > > It seems that we need the dri minor allocated before we hit the load > function as things are done right now. > >> 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. > > We never hit it until we started doing things earlier and quicker. I first > found it in the prettification of boot so we were keen to get plymouth > running as soon as possible. That lead to random panics and me finding > this bug. The window is tiny as far as I know and it tends to be specific > machines and specific package combinations which trigger it reliably. > > I suspect that a proper fix would allow delaying the registration as you > suggest but in the interim a wait would at least avoid the issues we are > seeing. I will see how awful it looks. Just to confirm its the drm_sysfs_device_add that causes the race we care about. it needs to happen after the driver is happy. Since it calls device_register and that is what triggers udev magic to load the userspace. If you have a userspace app banging on a static device node that might need another set of fun fixes. Dave. _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/dri-devel
