On Thu, Apr 19, 2012 at 5:47 PM, Dave Airlie <airlied@xxxxxxxxx> wrote: > 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. Okay the sysfs add and the idr_replace are the things we need to delay. Dave. _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/dri-devel
