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

[Dave Airlie]

On Thu, Apr 19, 2012 at 5:47 PM, Dave Airlie <airlied at gmail.com> wrote:
> On Thu, Apr 19, 2012 at 5:41 PM, Andy Whitcroft <apw at 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 at 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.

Dave.