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

[Dave Airlie]

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.

Dave.