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

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Message-ID: <CAPM=9tzfYtjxjM6tkj3p2mo=teALApyFxU3QAUkCBpBFnp3DSQ@mail.gmail.com>
Date:	Thu, 19 Apr 2012 18:00:48 +0100
From:	Dave Airlie <airlied@...il.com>
To:	Jesse Barnes <jbarnes@...tuousgeek.org>
Cc:	Andy Whitcroft <apw@...onical.com>,
	David Airlie <airlied@...ux.ie>,
	dri-devel@...ts.freedesktop.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 5:47 PM, Dave Airlie <airlied@...il.com> wrote:
>>> > On Thu, Apr 19, 2012 at 5:41 PM, Andy Whitcroft <apw@...onical.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@...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.
>>> >
>>> > 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?
>
> well the idr replace should be the thing that matters, since before
> that openers get -ENODEV, after it they end up success.
> we may need a lock around that once we fix the logic.\

Here's my predinner hack, contains random rtl change as well, plz ignore.

now for dinner.

Dave.

Download attachment "myhack" of type "application/octet-stream" (2821 bytes)