[vlc-devel] [PATCH v2] wayland: xdg-shell: use vlc_cond_t instead of dispatch

Alexandre Janniaux ajanni at videolabs.io
Tue Apr 7 10:20:01 CEST 2020

When using wl_display_dispatch, there is a race condition with the main
loop, in which the condition `(!sys->wm.configured)` would have been
checked as true, but the event dispatched from the main thread after
that so wl_display_dispatch would block indefinitely.

Said differently, wl_display_dispatch should not be called from a
different thread than the main thread (in wayland terms).

By using a condition variable instead, we let the main thread unblock
the vout_window_Enable and prevent this deadlock when the callback for
the configured event is called. In addition, it adds the correct lock
around the updates and reads of the wm.configured boolean.

Fixes #24586
 modules/video_output/wayland/xdg-shell.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/modules/video_output/wayland/xdg-shell.c b/modules/video_output/wayland/xdg-shell.c
index 765ffb2a2f..0b0eba0be0 100644
--- a/modules/video_output/wayland/xdg-shell.c
+++ b/modules/video_output/wayland/xdg-shell.c
@@ -106,6 +106,7 @@ typedef struct
     struct wl_surface *cursor_surface;
     vlc_mutex_t lock;
+    vlc_cond_t cond_configured;
     vlc_thread_t thread;
 } vout_window_sys_t;
@@ -262,8 +263,10 @@ static int Enable(vout_window_t *wnd, const vout_window_cfg_t *restrict cfg)
 #ifdef XDG_SHELL
+    vlc_mutex_lock(&sys->lock);
     while (!sys->wm.configured)
-        wl_display_dispatch(display);
+        vlc_cond_wait(&sys->cond_configured, &sys->lock);
+    vlc_mutex_unlock(&sys->lock);
     return VLC_SUCCESS;
@@ -339,7 +342,11 @@ static void xdg_surface_configure_cb(void *data, struct xdg_surface *surface,
     xdg_surface_ack_configure(surface, serial);
+    vlc_mutex_lock(&sys->lock);
     sys->wm.configured = true;
+    vlc_cond_signal(&sys->cond_configured);
+    vlc_mutex_unlock(&sys->lock);
 static const struct xdg_surface_listener xdg_surface_cbs =
@@ -617,6 +624,7 @@ static int Open(vout_window_t *wnd)
     sys->cursor_theme = NULL;
     sys->cursor_surface = NULL;
+    vlc_cond_init(&sys->cond_configured);
     wnd->sys = sys;
     wnd->handle.wl = NULL;

More information about the vlc-devel mailing list