[3.5.y.z extended stable] Patch "drm/i915: don't update the dri1 breadcrumb with modesetting" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    drm/i915: don't update the dri1 breadcrumb with modesetting

to the linux-3.5.y-queue branch of the 3.5.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.5.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.5.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From 17e2077d9f5f0ea10afada4af51005d2ca05f9af Mon Sep 17 00:00:00 2001
From: Daniel Vetter <daniel.vetter at ffwll.ch>
Date: Tue, 10 Dec 2013 13:20:59 +0100
Subject: drm/i915: don't update the dri1 breadcrumb with modesetting

commit 6c719faca2aceca72f1bf5b1645c1734ed3e9447 upstream.

The update is horribly racy since it doesn't protect at all against
concurrent closing of the master fd. And it can't really since that
requires us to grab a mutex.

Instead of jumping through hoops and offloading this to a worker
thread just block this bit of code for the modesetting driver.

Note that the race is fairly easy to hit since we call the breadcrumb
function for any interrupt. So the vblank interrupt (which usually
keeps going for a bit) is enough. But even if we'd block this and only
update the breadcrumb for user interrupts from the CS we could hit
this race with kms/gem userspace: If a non-master is waiting somewhere
(and hence has interrupts enabled) and the master closes its fd
(probably due to crashing).

v2: Add a code comment to explain why fixing this for real isn't
really worth it. Also improve the commit message a bit.

v3: Fix the spelling in the comment.

Reported-by: Eugene Shatokhin <eugene.shatokhin at rosalab.ru>
Cc: Eugene Shatokhin <eugene.shatokhin at rosalab.ru>
Acked-by: Chris Wilson <chris at chris-wilson.co.uk>
Tested-by: Eugene Shatokhin <eugene.shatokhin at rosalab.ru>
Signed-off-by: Daniel Vetter <daniel.vetter at ffwll.ch>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 drivers/gpu/drm/i915/i915_dma.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/gpu/drm/i915/i915_dma.c b/drivers/gpu/drm/i915/i915_dma.c
index 36822b9..e676408 100644
--- a/drivers/gpu/drm/i915/i915_dma.c
+++ b/drivers/gpu/drm/i915/i915_dma.c
@@ -85,6 +85,14 @@ void i915_update_dri1_breadcrumb(struct drm_device *dev)
 	drm_i915_private_t *dev_priv = dev->dev_private;
 	struct drm_i915_master_private *master_priv;

+	/*
+	 * The dri breadcrumb update races against the drm master disappearing.
+	 * Instead of trying to fix this (this is by far not the only ums issue)
+	 * just don't do the update in kms mode.
+	 */
+	if (drm_core_check_feature(dev, DRIVER_MODESET))
+		return;
+
 	if (dev->primary->master) {
 		master_priv = dev->primary->master->driver_priv;
 		if (master_priv->sarea_priv)
--
1.8.3.2