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.
Signed-off-by: Daniel Vetter <daniel.vetter at ffwll.ch>
---
drivers/gpu/drm/drm_crtc.c | 72 +++++++++++++++++++++++++++-----------------
1 file changed, 45 insertions(+), 27 deletions(-)
diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
index 6562eba..6dd441c 100644
--- a/drivers/gpu/drm/drm_crtc.c
+++ b/drivers/gpu/drm/drm_crtc.c
@@ -516,7 +516,11 @@ 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)
{
@@ -526,33 +530,51 @@ 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);
@@ -2537,9 +2559,7 @@ 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;
@@ -2687,9 +2707,7 @@ 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);
}
--
1.7.10.4
