drm: Drop grab fpriv->fbs_lock in drm_fb_release

Paulo Zanoni reported a lockdep splat with a locking inversion between
fpriv->fbs_lock and the modeset locks. This issue was introduced in

commit f2b50c1161
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Fri Sep 12 17:07:32 2014 +0200

    drm: Fixup locking for universal cursor planes

This here is actually one of the rare cases where lockdep hits a false
positive: The deadlock only happens in drm_fb_release, which cleans up
the file private structure when all the references are gone. So the
locking is the very last one and no one else can deadlock. It also
doesn't protect anything at all, since all ioctls are guaranteed to
have returned at this point - otherwise they'd still hold a reference
on the file.

So let's just drop it and replace it with a big comment.

Cc: David Herrmann <dh.herrmann@gmail.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Paulo Zanoni <przanoni@gmail.com>
Reported-and-Tested-by: Paulo Zanoni <przanoni@gmail.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

drivers/gpu/drm/drm_crtc.c

@@ -3400,7 +3400,16 @@ void drm_fb_release(struct drm_file *priv)
 	struct drm_device *dev = priv->minor->dev;
 	struct drm_framebuffer *fb, *tfb;
 
-	mutex_lock(&priv->fbs_lock);
+	/*
+	 * When the file gets released that means no one else can access the fb
+	 * list any more, so no need to grab fpriv->fbs_lock. And we need to to
+	 * avoid upsetting lockdep since the universal cursor code adds a
+	 * framebuffer while holding mutex locks.
+	 *
+	 * Note that a real deadlock between fpriv->fbs_lock and the modeset
+	 * locks is impossible here since no one else but this function can get
+	 * at it any more.
+	 */
 	list_for_each_entry_safe(fb, tfb, &priv->fbs, filp_head) {
 
 		mutex_lock(&dev->mode_config.fb_lock);
@@ -3413,7 +3422,6 @@ void drm_fb_release(struct drm_file *priv)
 		/* This will also drop the fpriv->fbs reference. */
 		drm_framebuffer_remove(fb);
 	}
-	mutex_unlock(&priv->fbs_lock);
 }
 
 /**