Messages in this thread | | | Date | Thu, 19 Apr 2012 09:55:16 -0700 | From | Jesse Barnes <> | Subject | Re: [PATCH 0/1] [RFC] DRM locking issues during early open |
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On Thu, 19 Apr 2012 17:52:39 +0100 Dave Airlie <airlied@gmail.com> wrote:
> On Thu, Apr 19, 2012 at 5:47 PM, Dave Airlie <airlied@gmail.com> wrote: > > On Thu, Apr 19, 2012 at 5:41 PM, Andy Whitcroft <apw@canonical.com> 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@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? > >>> > >>> 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.
Since you can still get at things with a static node, it seems like locking is the real issue here? Is there no mutex we can take across init to block any openers until we're done?
-- Jesse Barnes, Intel Open Source Technology Center -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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