diff --git a/src/mesa/state_tracker/st_cb_syncobj.c b/src/mesa/state_tracker/st_cb_syncobj.c
index 123925a..31b25f1 100644
--- a/src/mesa/state_tracker/st_cb_syncobj.c
+++ b/src/mesa/state_tracker/st_cb_syncobj.c
@@ -81,17 +81,26 @@ static void st_check_sync(struct gl_context *ctx, struct gl_sync_object *obj)
    struct pipe_context *pipe = st_context(ctx)->pipe;
    struct pipe_screen *screen = pipe->screen;
    struct st_sync_object *so = (struct st_sync_object*)obj;
+   struct pipe_fence_handle *fence = NULL;
+
+   mtx_lock(&ctx->Shared->Mutex);
+   screen->fence_reference(screen, &fence, so->fence);
+   mtx_unlock(&ctx->Shared->Mutex);
 
    /* If the fence doesn't exist, assume it's signalled. */
-   if (!so->fence) {
+   if (!fence) {
       so->b.StatusFlag = GL_TRUE;
       return;
    }
 
-   if (screen->fence_finish(screen, pipe, so->fence, 0)) {
+   if (screen->fence_finish(screen, pipe, fence, 0)) {
+      mtx_lock(&ctx->Shared->Mutex);
       screen->fence_reference(screen, &so->fence, NULL);
+      mtx_unlock(&ctx->Shared->Mutex);
       so->b.StatusFlag = GL_TRUE;
    }
+
+   screen->fence_reference(screen, &fence, NULL);
 }
 
 static void st_client_wait_sync(struct gl_context *ctx,
@@ -101,9 +110,14 @@ static void st_client_wait_sync(struct gl_context *ctx,
    struct pipe_context *pipe = st_context(ctx)->pipe;
    struct pipe_screen *screen = pipe->screen;
    struct st_sync_object *so = (struct st_sync_object*)obj;
+   struct pipe_fence_handle *fence = NULL;
+
+   mtx_lock(&ctx->Shared->Mutex);
+   screen->fence_reference(screen, &fence, so->fence);
+   mtx_unlock(&ctx->Shared->Mutex);
 
    /* If the fence doesn't exist, assume it's signalled. */
-   if (!so->fence) {
+   if (!fence) {
       so->b.StatusFlag = GL_TRUE;
       return;
    }
@@ -120,11 +134,14 @@ static void st_client_wait_sync(struct gl_context *ctx,
     * Assume GL_SYNC_FLUSH_COMMANDS_BIT is always set, because applications
     * forget to set it.
     */
-   if (so->fence &&
-       screen->fence_finish(screen, pipe, so->fence, timeout)) {
+   if (screen->fence_finish(screen, pipe, fence, timeout)) {
+      mtx_lock(&ctx->Shared->Mutex);
       screen->fence_reference(screen, &so->fence, NULL);
+      mtx_unlock(&ctx->Shared->Mutex);
       so->b.StatusFlag = GL_TRUE;
    }
+
+   screen->fence_reference(screen, &fence, NULL);
 }
 
 static void st_server_wait_sync(struct gl_context *ctx,