vrend: fix fence polling without sync thread

This was regressed by commit 3a2a537c (vrend: hook up per-context
fencing internally), bisected by Gert.

When there is no sync thread, vrend_renderer_check_fences checks fences
on vrend_state.fence_list for signaled ones and call ctx->fence_retire
on them.  Because fences belonging to the same context are ordered, as
an optimization, we ideally want to call ctx->fence_retire only on the
last fence of those belonging to the same context. (Note we are close to
that but not quite because we want to avoid complex changes until virgl
starts using per-context fencing, if ever)

The issue is in need_fence_retire_signal_locked.  It has this check

    if (fence->fences.next == &vrend_state.fence_list)
       return true;

which says that, if the fence is the last one in vrend_state.fence_list,
call ctx->fence_retire (because it must be the last fence of some
context).  The check is incorrect because not all fences on the list
have signaled when the sync thread is not used.  It will fail when there
are also unsignaled fences on the list.

To fix the issue, we contruct a list of signaled fences first before
calling need_fence_retire_signal_locked.  We could merge the paths with
and without sync thread further because the non-sync-thread path just
needs this extra step to construct a list of signaled fences.  But we
will save that for future and focus on fixing the performance
regression.

Signed-off-by: Chia-I Wu <olvaffe@gmail.com>
Reviewed-by: Gert Wollny <gert.wollny@collabora.com>

src/vrend_renderer.c

@@ -9255,12 +9255,13 @@ int vrend_renderer_create_fence(struct vrend_context *ctx,
 
 static void vrend_renderer_check_queries(void);
 
-static bool need_fence_retire_signal_locked(struct vrend_fence *fence)
+static bool need_fence_retire_signal_locked(struct vrend_fence *fence,
+                                            const struct list_head *signaled_list)
 {
    struct vrend_fence *next;
 
    /* last fence */
-   if (fence->fences.next == &vrend_state.fence_list)
+   if (fence->fences.next == signaled_list)
       return true;
 
    /* next fence belongs to a different context */
@@ -9294,7 +9295,7 @@ void vrend_renderer_check_fences(void)
             continue;
          }
 
-         if (need_fence_retire_signal_locked(fence)) {
+         if (need_fence_retire_signal_locked(fence, &vrend_state.fence_list)) {
             list_del(&fence->fences);
             list_addtail(&fence->fences, &retired_fences);
          } else {
@@ -9307,17 +9308,18 @@ void vrend_renderer_check_fences(void)
 
       LIST_FOR_EACH_ENTRY_SAFE(fence, stor, &vrend_state.fence_list, fences) {
          if (do_wait(fence, /* can_block */ false)) {
-            if (need_fence_retire_signal_locked(fence)) {
-               list_del(&fence->fences);
-               list_addtail(&fence->fences, &retired_fences);
-            } else {
-               free_fence_locked(fence);
-            }
+            list_del(&fence->fences);
+            list_addtail(&fence->fences, &retired_fences);
          } else {
             /* don't bother checking any subsequent ones */
             break;
          }
       }
+
+      LIST_FOR_EACH_ENTRY_SAFE(fence, stor, &retired_fences, fences) {
+         if (!need_fence_retire_signal_locked(fence, &retired_fences))
+            free_fence_locked(fence);
+      }
    }
 
    if (LIST_IS_EMPTY(&retired_fences))