[Mesa-dev] [PATCH] egl/wayland: Avoid race conditions when on non-main thread

[Jonas Ådahl]

When EGL is used on some other thread than the thread that drives the
main wl_display queue, the Wayland EGL dri2 implementation is
vulnerable to a race condition related to display round trips and global
object advertisements.

The race that may happen is that after after a proxy is created, but
before the queue is set, events meant to be emitted via the yet to be
set queue may already have been queued on the wrong queue.

In order to make it possible to avoid this race, wayland 1.11
introduced new API that allows creating a proxy wrapper that may be used
as the factory proxy when creating new proxies via Wayland requests. The
queue of a proxy wrapper can be changed without effecting what queue
events emitted by the actual proxy will be queued on, while still
effecting what default queue proxies created from it will have.

By introducing a wl_display proxy wrapper and using this when performing
round trips (via wl_display_sync()) and retrieving the global objects (via
wl_display_get_registry()), the mentioned race condition is avoided.

Signed-off-by: Jonas Ådahl <jadahl at gmail.com>
---

Note that this patch adds a dependency on theh alpha release on wayland 1.11.


Jonas



 configure.ac                            |  2 +-
 src/egl/drivers/dri2/egl_dri2.h         |  1 +
 src/egl/drivers/dri2/platform_wayland.c | 30 ++++++++++++++++--------------
 3 files changed, 18 insertions(+), 15 deletions(-)

diff --git a/configure.ac b/configure.ac
index 023110e..5868989 100644
--- a/configure.ac
+++ b/configure.ac
@@ -83,7 +83,7 @@ GLPROTO_REQUIRED=1.4.14
 LIBOMXIL_BELLAGIO_REQUIRED=0.0
 LIBVA_REQUIRED=0.38.0
 VDPAU_REQUIRED=1.1
-WAYLAND_REQUIRED=1.2.0
+WAYLAND_REQUIRED=1.10.91
 XCB_REQUIRED=1.9.3
 XCBDRI2_REQUIRED=1.8
 XCBGLX_REQUIRED=1.8.1
diff --git a/src/egl/drivers/dri2/egl_dri2.h b/src/egl/drivers/dri2/egl_dri2.h
index ddb5f39..075c2a4 100644
--- a/src/egl/drivers/dri2/egl_dri2.h
+++ b/src/egl/drivers/dri2/egl_dri2.h
@@ -205,6 +205,7 @@ struct dri2_egl_display
 
 #ifdef HAVE_WAYLAND_PLATFORM
    struct wl_display        *wl_dpy;
+   struct wl_display        *wl_dpy_wrapper;
    struct wl_registry       *wl_registry;
    struct wl_drm            *wl_server_drm;
    struct wl_drm            *wl_drm;
diff --git a/src/egl/drivers/dri2/platform_wayland.c b/src/egl/drivers/dri2/platform_wayland.c
index ff0d5c8..519469e 100644
--- a/src/egl/drivers/dri2/platform_wayland.c
+++ b/src/egl/drivers/dri2/platform_wayland.c
@@ -74,9 +74,8 @@ roundtrip(struct dri2_egl_display *dri2_dpy)
    struct wl_callback *callback;
    int done = 0, ret = 0;
 
-   callback = wl_display_sync(dri2_dpy->wl_dpy);
+   callback = wl_display_sync(dri2_dpy->wl_dpy_wrapper);
    wl_callback_add_listener(callback, &sync_listener, &done);
-   wl_proxy_set_queue((struct wl_proxy *) callback, dri2_dpy->wl_queue);
    while (ret != -1 && !done)
       ret = wl_display_dispatch_queue(dri2_dpy->wl_dpy, dri2_dpy->wl_queue);
 
@@ -765,11 +764,9 @@ dri2_wl_swap_buffers_with_damage(_EGLDriver *drv,
     * handle the commit and send a release event before checking for a free
     * buffer */
    if (dri2_surf->throttle_callback == NULL) {
-      dri2_surf->throttle_callback = wl_display_sync(dri2_dpy->wl_dpy);
+      dri2_surf->throttle_callback = wl_display_sync(dri2_dpy->wl_dpy_wrapper);
       wl_callback_add_listener(dri2_surf->throttle_callback,
                                &throttle_listener, dri2_surf);
-      wl_proxy_set_queue((struct wl_proxy *) dri2_surf->throttle_callback,
-                         dri2_dpy->wl_queue);
    }
 
    wl_display_flush(dri2_dpy->wl_dpy);
@@ -1093,12 +1090,17 @@ dri2_initialize_wayland_drm(_EGLDriver *drv, _EGLDisplay *disp)
 
    dri2_dpy->wl_queue = wl_display_create_queue(dri2_dpy->wl_dpy);
 
+   dri2_dpy->wl_dpy_wrapper = wl_proxy_create_wrapper(dri2_dpy->wl_dpy);
+   if (dri2_dpy->wl_dpy_wrapper == NULL)
+      goto cleanup_dpy_wrapper;
+
+   wl_proxy_set_queue((struct wl_proxy *) dri2_dpy->wl_dpy_wrapper,
+                      dri2_dpy->wl_queue);
+
    if (dri2_dpy->own_device)
       wl_display_dispatch_pending(dri2_dpy->wl_dpy);
 
-   dri2_dpy->wl_registry = wl_display_get_registry(dri2_dpy->wl_dpy);
-   wl_proxy_set_queue((struct wl_proxy *) dri2_dpy->wl_registry,
-                      dri2_dpy->wl_queue);
+   dri2_dpy->wl_registry = wl_display_get_registry(dri2_dpy->wl_dpy_wrapper);
    wl_registry_add_listener(dri2_dpy->wl_registry,
                             &registry_listener_drm, dri2_dpy);
    if (roundtrip(dri2_dpy) < 0 || dri2_dpy->wl_drm == NULL)
@@ -1235,6 +1237,10 @@ dri2_initialize_wayland_drm(_EGLDriver *drv, _EGLDisplay *disp)
    wl_drm_destroy(dri2_dpy->wl_drm);
  cleanup_registry:
    wl_registry_destroy(dri2_dpy->wl_registry);
+   wl_proxy_wrapper_destroy(dri2_dpy->wl_dpy_wrapper);
+ cleanup_dpy_wrapper:
+   if (dri2_dpy->wl_dpy != disp->PlatformDisplay)
+      wl_display_disconnect(dri2_dpy->wl_dpy);
    wl_event_queue_destroy(dri2_dpy->wl_queue);
  cleanup_dpy:
    free(dri2_dpy);
@@ -1546,11 +1552,9 @@ dri2_wl_swrast_commit_backbuffer(struct dri2_egl_surface *dri2_surf)
     * handle the commit and send a release event before checking for a free
     * buffer */
    if (dri2_surf->throttle_callback == NULL) {
-      dri2_surf->throttle_callback = wl_display_sync(dri2_dpy->wl_dpy);
+      dri2_surf->throttle_callback = wl_display_sync(dri2_dpy->wl_dpy_wrapper);
       wl_callback_add_listener(dri2_surf->throttle_callback,
                                &throttle_listener, dri2_surf);
-      wl_proxy_set_queue((struct wl_proxy *) dri2_surf->throttle_callback,
-                         dri2_dpy->wl_queue);
    }
 
    wl_display_flush(dri2_dpy->wl_dpy);
@@ -1824,9 +1828,7 @@ dri2_initialize_wayland_swrast(_EGLDriver *drv, _EGLDisplay *disp)
    if (dri2_dpy->own_device)
       wl_display_dispatch_pending(dri2_dpy->wl_dpy);
 
-   dri2_dpy->wl_registry = wl_display_get_registry(dri2_dpy->wl_dpy);
-   wl_proxy_set_queue((struct wl_proxy *) dri2_dpy->wl_registry,
-                      dri2_dpy->wl_queue);
+   dri2_dpy->wl_registry = wl_display_get_registry(dri2_dpy->wl_dpy_wrapper);
    wl_registry_add_listener(dri2_dpy->wl_registry,
                             &registry_listener_swrast, dri2_dpy);
 
-- 
2.5.5