st/mesa: disallow deferred flush if there are multiple contexts

u_threaded can hang in these situation, with one context waiting on a
deferred fence from the other context.
But the other context isn't flushing its pending work (because it's waiting
for more work to pushed) so everything is stuck.

Fixes: d17b35e671a ("gallium: add PIPE_FLUSH_DEFERRED")
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/1430
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4213>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4213>

diff --git a/src/mesa/state_tracker/st_cb_syncobj.c b/src/mesa/state_tracker/st_cb_syncobj.c
index 48e7647b602d804b250229a46a765755da6f74f8..2cbbc580c8cc26386156687d9e151e71e47dc7a5 100644 (file)
--- a/src/mesa/state_tracker/st_cb_syncobj.c
+++ b/src/mesa/state_tracker/st_cb_syncobj.c
@@ -74,7 +74,8 @@ static void st_fence_sync(struct gl_context *ctx, struct gl_sync_object *obj,
    assert(condition == GL_SYNC_GPU_COMMANDS_COMPLETE && flags == 0);
    assert(so->fence == NULL);
 
-   pipe->flush(pipe, &so->fence, PIPE_FLUSH_DEFERRED);
+   /* Deferred flush are only allowed when there's a single context. See issue 1430 */
+   pipe->flush(pipe, &so->fence, ctx->Shared->RefCount == 1 ? PIPE_FLUSH_DEFERRED : 0);
 }
 
 static void st_client_wait_sync(struct gl_context *ctx,