[PATCH] drm/core: Do not preserve framebuffer on rmfb.

[Maarten Lankhorst <maarten.lankhorst@linux.intel.com>]

It turns out that preserving framebuffers after the rmfb call breaks
vmwgfx userspace. This was originally introduced because it was thought
nobody relied on the behavior, but unfortunately it seems there are
exceptions.

drm_framebuffer_remove may fail with -EINTR now, so a straight revert
is impossible. There is no way to remove the framebuffer from the lists
and active planes without introducing a race because of the different
locking requirements. Instead call drm_framebuffer_remove from a
workqueue, which is unaffected by signals.

Cc: stable@vger.kernel.org #v4.4+
Fixes: 13803132818c ("drm/core: Preserve the framebuffer after removing it.")
Testcase: kms_flip.flip-vs-rmfb-interruptible
References: https://lists.freedesktop.org/archives/dri-devel/2016-March/102876.html
Cc: Thomas Hellstrom <thellstrom@vmware.com>
Cc: David Herrmann <dh.herrmann@gmail.com>
---
 drivers/gpu/drm/drm_crtc.c | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
index e08f962288d9..b7d0b959f088 100644
--- a/drivers/gpu/drm/drm_crtc.c
+++ b/drivers/gpu/drm/drm_crtc.c
@@ -3434,6 +3434,18 @@ int drm_mode_addfb2(struct drm_device *dev,
 	return 0;
 }
 
+struct drm_mode_rmfb_work {
+	struct work_struct work;
+	struct drm_framebuffer *fb;
+};
+
+static void drm_mode_rmfb_work_fn(struct work_struct *w)
+{
+	struct drm_mode_rmfb_work *arg = container_of(w, typeof(*arg), work);
+
+	drm_framebuffer_remove(arg->fb);
+}
+
 /**
  * drm_mode_rmfb - remove an FB from the configuration
  * @dev: drm device for the ioctl
@@ -3454,6 +3466,7 @@ int drm_mode_rmfb(struct drm_device *dev,
 	struct drm_framebuffer *fbl = NULL;
 	uint32_t *id = data;
 	int found = 0;
+	struct drm_mode_rmfb_work arg;
 
 	if (!drm_core_check_feature(dev, DRIVER_MODESET))
 		return -EINVAL;
@@ -3474,7 +3487,12 @@ int drm_mode_rmfb(struct drm_device *dev,
 	mutex_unlock(&dev->mode_config.fb_lock);
 	mutex_unlock(&file_priv->fbs_lock);
 
-	drm_framebuffer_unreference(fb);
+	INIT_WORK_ONSTACK(&arg.work, drm_mode_rmfb_work_fn);
+	arg.fb = fb;
+
+	schedule_work(&arg.work);
+	flush_work(&arg.work);
+	destroy_work_on_stack(&arg.work);
 
 	return 0;
 
-- 
2.1.0

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