drm/core: Do not preserve framebuffer on rmfb, v4.

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.

Changes since v1:
- Add comment.
Changes since v2:
- Add fastpath for refcount = 1. (danvet)
Changes since v3:
- Rebased.
- Restore lastclose framebuffer removal too.

Cc: stable@vger.kernel.org #v4.4+
Fixes: 13803132818c ("drm/core: Preserve the framebuffer after removing it.")
Testcase: kms_rmfb_basic
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>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Tested-by: Thomas Hellstrom <thellstrom@vmware.com> #v3
Tested-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/6c63ca37-0e7e-ac7f-a6d2-c7822e3d611f@linux.intel.com

drivers/gpu/drm/drm_crtc.c

@@ -3462,6 +3462,24 @@ int drm_mode_addfb2(struct drm_device *dev,
 	return 0;
 }
 
+struct drm_mode_rmfb_work {
+	struct work_struct work;
+	struct list_head fbs;
+};
+
+static void drm_mode_rmfb_work_fn(struct work_struct *w)
+{
+	struct drm_mode_rmfb_work *arg = container_of(w, typeof(*arg), work);
+
+	while (!list_empty(&arg->fbs)) {
+		struct drm_framebuffer *fb =
+			list_first_entry(&arg->fbs, typeof(*fb), filp_head);
+
+		list_del_init(&fb->filp_head);
+		drm_framebuffer_remove(fb);
+	}
+}
+
 /**
  * drm_mode_rmfb - remove an FB from the configuration
  * @dev: drm device for the ioctl
@@ -3502,12 +3520,29 @@ int drm_mode_rmfb(struct drm_device *dev,
 	list_del_init(&fb->filp_head);
 	mutex_unlock(&file_priv->fbs_lock);
 
-	/* we now own the reference that was stored in the fbs list */
-	drm_framebuffer_unreference(fb);
-
 	/* drop the reference we picked up in framebuffer lookup */
 	drm_framebuffer_unreference(fb);
 
+	/*
+	 * we now own the reference that was stored in the fbs list
+	 *
+	 * drm_framebuffer_remove may fail with -EINTR on pending signals,
+	 * so run this in a separate stack as there's no way to correctly
+	 * handle this after the fb is already removed from the lookup table.
+	 */
+	if (drm_framebuffer_read_refcount(fb) > 1) {
+		struct drm_mode_rmfb_work arg;
+
+		INIT_WORK_ONSTACK(&arg.work, drm_mode_rmfb_work_fn);
+		INIT_LIST_HEAD(&arg.fbs);
+		list_add_tail(&fb->filp_head, &arg.fbs);
+
+		schedule_work(&arg.work);
+		flush_work(&arg.work);
+		destroy_work_on_stack(&arg.work);
+	} else
+		drm_framebuffer_unreference(fb);
+
 	return 0;
 
 fail_unref:
@@ -3657,7 +3692,6 @@ out_err1:
 	return ret;
 }
 
-
 /**
  * drm_fb_release - remove and free the FBs on this file
  * @priv: drm file for the ioctl
@@ -3672,6 +3706,9 @@ out_err1:
 void drm_fb_release(struct drm_file *priv)
 {
 	struct drm_framebuffer *fb, *tfb;
+	struct drm_mode_rmfb_work arg;
+
+	INIT_LIST_HEAD(&arg.fbs);
 
 	/*
 	 * When the file gets released that means no one else can access the fb
@@ -3684,10 +3721,22 @@ void drm_fb_release(struct drm_file *priv)
 	 * at it any more.
 	 */
 	list_for_each_entry_safe(fb, tfb, &priv->fbs, filp_head) {
-		list_del_init(&fb->filp_head);
+		if (drm_framebuffer_read_refcount(fb) > 1) {
+			list_move_tail(&fb->filp_head, &arg.fbs);
+		} else {
+			list_del_init(&fb->filp_head);
 
-		/* This drops the fpriv->fbs reference. */
-		drm_framebuffer_unreference(fb);
+			/* This drops the fpriv->fbs reference. */
+			drm_framebuffer_unreference(fb);
+		}
+	}
+
+	if (!list_empty(&arg.fbs)) {
+		INIT_WORK_ONSTACK(&arg.work, drm_mode_rmfb_work_fn);
+
+		schedule_work(&arg.work);
+		flush_work(&arg.work);
+		destroy_work_on_stack(&arg.work);
 	}
 }