linux-kernel - Re: [PATCH 0/1] [RFC] DRM locking issues during early open

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Message-ID: <20120419164113.GA3467@shadowen.org>
Date:	Thu, 19 Apr 2012 17:41:13 +0100
From:	Andy Whitcroft <apw@...onical.com>
To:	Dave Airlie <airlied@...il.com>
Cc:	David Airlie <airlied@...ux.ie>, dri-devel@...ts.freedesktop.org,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Bryce Harrington <bryce@...onical.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/1] [RFC] DRM locking issues during early open

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@...onical.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.

-apw
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