Re: [2.6.39 regression] hard lock when GNOME starts

From: Andrew Lutomirski <luto@mit.edu>
To: intel-gfx@lists.freedesktop.org, Chris Wilson <chris@chris-wilson.co.uk>
Subject: Re: [2.6.39 regression] hard lock when GNOME starts
Date: Fri, 13 May 2011 12:07:59 -0400	[thread overview]
Message-ID: <BANLkTikjUQWgveWjoR=uPO5UQXkTTysxng@mail.gmail.com> (raw)
In-Reply-To: <BANLkTi=HDaOeoz7zQ82oog1yTw2zGF18Yw@mail.gmail.com>

On Fri, May 13, 2011 at 1:38 AM, Andrew Lutomirski <luto@mit.edu> wrote:
> Hi-
>
> Something in the range ^8aa7500 40c7f2112ce18fa5eb ^b04d0a90908c
> causes by Q67 Sandy Bridge box to lock hard about one second after I
> start GNOME.  It locks so hard that the reset button doesn't work and
> netconsole doesn't say anything.
>
> I'm about to have trouble finishing the bisection because I've had
> trouble running revisions that are older than 2.6.38 and the next bit
> of bisection has merge conflicts if I merge in 2.6.38.  I can power
> through it, but maybe one of you will recognize the bug from the fact
> that there aren't that many revisions in there and there can't be many
> ways to make the reset button stop working.

It's semaphores.  I have:

00:02.0 VGA compatible controller: Intel Corporation Sandy Bridge
Integrated Graphics Controller (rev 09)

which is on a Q67/i7-2600 box.

The git history for semaphores is worthless.

AFAICT they were disabled around 2.6.38-rc7 by a merge that pulled in
a1656b9090f7008d2941c314f5a64724bea2ae37.

They were then re-enabled by this commit:

commit 47ae63e0c2e5fdb582d471dc906eb29be94c732f
Merge: c59a333 467cffb
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Mon Mar 7 12:32:44 2011 +0000

    Merge branch 'drm-intel-fixes' into drm-intel-next

    Apply the trivial conflicting regression fixes, but keep GPU semaphores
    enabled.

    Conflicts:
        drivers/gpu/drm/i915/i915_drv.h
        drivers/gpu/drm/i915/i915_gem_execbuffer.c

This pulled in the disable-semaphores patch.  So the merge with
trivial regression fixes undid the patch that disabled semaphores.

This is really annoying for two reasons.  One is that the commit
message sucks.  Two is much worse: the semaphore disabling patch made
it into 2.6.38 but this merge went in for 2.6.39.  That means that the
patch that made my system unusable on 2.6.39 versions *does not exist
in the set v2.6.38..v2.6.39-rc7* except as a "trivial" merge.  Which
means that the two fscking days I spent bisecting this were completely
wasted because there was no way that the bisection could have found
the bug.

In the future, if you change something, that might cause severe
breakage, please make it an actual commit.

--Andy

>
> --Andy
>