drm: optimize drm_framebuffer_remove

Now that all framebuffer usage is properly refcounted, we are no
longer required to hold the modeset locks while dropping the last
reference. Hence implemented a fastpath which avoids the potential
stalls associated with grabbing mode_config.lock for the case where
there's no other reference around.

Explain in a big comment why it is safe. Also update kerneldocs with
the new locking rules around drm_framebuffer_remove.

Reviewed-by: Rob Clark <rob@ti.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
index 6dc75ee100793d9bacab43810b50cbfbef135ef9..9492e7ec9d9d86600da9e34124d8b0a340e7ecab 100644 (file)
--- a/drivers/gpu/drm/drm_crtc.c
+++ b/drivers/gpu/drm/drm_crtc.c
@@ -517,7 +517,11 @@ EXPORT_SYMBOL(drm_framebuffer_cleanup);
  *
  * Scans all the CRTCs and planes in @dev's mode_config.  If they're
  * using @fb, removes it, setting it to NULL. Then drops the reference to the
- * passed-in framebuffer.
+ * passed-in framebuffer. Might take the modeset locks.
+ *
+ * Note that this function optimizes the cleanup away if the caller holds the
+ * last reference to the framebuffer. It is also guaranteed to not take the
+ * modeset locks in this case.
  */
 void drm_framebuffer_remove(struct drm_framebuffer *fb)
 {
@@ -527,33 +531,51 @@ void drm_framebuffer_remove(struct drm_framebuffer *fb)
        struct drm_mode_set set;
        int ret;
 
-       WARN_ON(!drm_modeset_is_locked(dev));
        WARN_ON(!list_empty(&fb->filp_head));
 
-       /* remove from any CRTC */
-       list_for_each_entry(crtc, &dev->mode_config.crtc_list, head) {
-               if (crtc->fb == fb) {
-                       /* should turn off the crtc */
-                       memset(&set, 0, sizeof(struct drm_mode_set));
-                       set.crtc = crtc;
-                       set.fb = NULL;
-                       ret = drm_mode_set_config_internal(&set);
-                       if (ret)
-                               DRM_ERROR("failed to reset crtc %p when fb was deleted\n", crtc);
+       /*
+        * drm ABI mandates that we remove any deleted framebuffers from active
+        * useage. But since most sane clients only remove framebuffers they no
+        * longer need, try to optimize this away.
+        *
+        * Since we're holding a reference ourselves, observing a refcount of 1
+        * means that we're the last holder and can skip it. Also, the refcount
+        * can never increase from 1 again, so we don't need any barriers or
+        * locks.
+        *
+        * Note that userspace could try to race with use and instate a new
+        * usage _after_ we've cleared all current ones. End result will be an
+        * in-use fb with fb-id == 0. Userspace is allowed to shoot its own foot
+        * in this manner.
+        */
+       if (atomic_read(&fb->refcount.refcount) > 1) {
+               drm_modeset_lock_all(dev);
+               /* remove from any CRTC */
+               list_for_each_entry(crtc, &dev->mode_config.crtc_list, head) {
+                       if (crtc->fb == fb) {
+                               /* should turn off the crtc */
+                               memset(&set, 0, sizeof(struct drm_mode_set));
+                               set.crtc = crtc;
+                               set.fb = NULL;
+                               ret = drm_mode_set_config_internal(&set);
+                               if (ret)
+                                       DRM_ERROR("failed to reset crtc %p when fb was deleted\n", crtc);
+                       }
                }
-       }
 
-       list_for_each_entry(plane, &dev->mode_config.plane_list, head) {
-               if (plane->fb == fb) {
-                       /* should turn off the crtc */
-                       ret = plane->funcs->disable_plane(plane);
-                       if (ret)
-                               DRM_ERROR("failed to disable plane with busy fb\n");
-                       /* disconnect the plane from the fb and crtc: */
-                       __drm_framebuffer_unreference(plane->fb);
-                       plane->fb = NULL;
-                       plane->crtc = NULL;
+               list_for_each_entry(plane, &dev->mode_config.plane_list, head) {
+                       if (plane->fb == fb) {
+                               /* should turn off the crtc */
+                               ret = plane->funcs->disable_plane(plane);
+                               if (ret)
+                                       DRM_ERROR("failed to disable plane with busy fb\n");
+                               /* disconnect the plane from the fb and crtc: */
+                               __drm_framebuffer_unreference(plane->fb);
+                               plane->fb = NULL;
+                               plane->crtc = NULL;
+                       }
                }
+               drm_modeset_unlock_all(dev);
        }
 
        drm_framebuffer_unreference(fb);
@@ -2538,9 +2560,7 @@ int drm_mode_rmfb(struct drm_device *dev,
        mutex_unlock(&dev->mode_config.fb_lock);
        mutex_unlock(&file_priv->fbs_lock);
 
-       drm_modeset_lock_all(dev);
        drm_framebuffer_remove(fb);
-       drm_modeset_unlock_all(dev);
 
        return 0;
 
@@ -2691,9 +2711,7 @@ void drm_fb_release(struct drm_file *priv)
                list_del_init(&fb->filp_head);
 
                /* This will also drop the fpriv->fbs reference. */
-               drm_modeset_lock_all(dev);
                drm_framebuffer_remove(fb);
-               drm_modeset_unlock_all(dev);
        }
        mutex_unlock(&priv->fbs_lock);
 }