Re: mutter: reduce server grabs for window creation
- From: Sam Spilsbury <sam endlessm com>
- To: Owen Taylor <otaylor redhat com>
- Cc: gnome-shell-list gnome org, Daniel Drake <drake endlessm com>
- Subject: Re: mutter: reduce server grabs for window creation
- Date: Wed, 18 Dec 2013 17:03:30 -0200
On Wed, Dec 18, 2013 at 3:58 PM, Owen Taylor <otaylor redhat com> wrote:
I simply don't understand the point of doing your SwapBuffers in a thead
if you are using DRI2 and the X server is already doing the SwapBuffers
async in the background without blocking. Must have seemed like a good
idea to someone...
This is indeed one of the stranger things I have seen a driver do,
though I guess this is besides the point.
People tend to exaggerate the danger of server grabs - the main problem
with them in an application is that if you are debugging with gdb in a X
terminal, a grab can make your session lock up - but that's always the
case for the compositor. Still, certainly there are opportunities for
deadlock with them.
The danger is basically that you should never have a server grab
whilst calling into code you don't control, because it could try to
make a request on a separate X connection (this is something libraries
should not do generally, though I've seen plenty of cases where they
do it anyways).
I don't think we need to be quite that broad. If the XReparentWindow()
failed, for example, we would be fine - it's trapped in frame.c. The
only things we have to worry about are a) assumptions that the code is
making that round-trip calls to the X server are succeeding b) one-way
and round-trip calls that don't have an error trap because they were
known to be always called during the window-creation phase c) window
going away before we select for DestroyNotify.
With enough care, it might be possible to XUngrabServer() right after
selecting for DestroyNotify.
Agreed.
4. Ungrab server
5. Do anything which we don't depend on always succeeding because the window
is going to receive a DestroyNotify later, this includes setting window
properties and creating the window texture.
Does this sound like a sensible strategy or are we missing anything?
One thing that definitely should be noted is that we still have a server
grab over everything when this code is called from
meta_screen_manage_all_windows(). From a brief look, it appears to me
that this happens *after* we initialize Javascript, set up signal
connections, etc - so probably the only reason that your patch works for
you is that the driver's internal swap-buffers thread isn't doing
anything during initialisation.
Is there any need for having a server grab with a large scope here in that case?
I haven't had a chance to look too closely at the code, but perhaps we
are assuming that the results of an XQueryTree will stay constant
during initialisation. In that case, the only thing you care about is
windows being either created or destroyed before you've selected for
CreateNotify, ReparentNotify and DestroyNotify on the root window. So
really, the only thing that is necessary is to have the server grab
during XQueryTree and XSelectInput. Then you need to ostensibly
"manage" all the windows and add them to the internal stack. After
that, you can assume that any created or destroyed windows are handled
as creations or destructions that occur during the event loop and not
initialisation.
This makes me less enthusiastic about the approach - we aren't avoiding
calling application code with a server grab, we're just not doing it in
the case that was causing a problem.
Other than that, I don't have large-scale objections to the patch other
than the inherent bug-introducing potential of moving things around in
meta_window_new_with_attrs(). I would certainly wonder if we could
simply move the ungrab up earlier in the function and fix up anything
in class a) and b) described above.
If I recall correctly (now that I've refreshed my memory on this), you
only need grabs for:
1. Querying the tree and selecting for events
2. Doing the XGetWindowAttributes, XCreateWindow, XReparentWindow,
XAddToSaveSet dance.
One caveat with #2 is that if XGetWindowAttributes fails it means that
the window was destroyed - but you still need to keep it around in the
stack until you get a DestroyNotify for it because its possible that
there were already requests made relative to it.
Have you created a torture test for your app - something that repeatedly
creates a new window and after a short random delay destroys it?
I have one I used for debugging the (notorious) stacking bugs in
compiz here: https://code.launchpad.net/~smspillaz/+junk/stackingtest
It creates 50 windows at once every second.
On a side note, this wouldn't be a problem if applications didn't have
the insane behaviour of creating and destroying 20+ non-override
redirect windows on startup (I know too many examples of this...).
- Sam.
- Owen
As a point of discussion, here's a patch to work around the issue.
Make sure we call meta_workspace_add_window() with no X server grab held.
(This function emits a window-added signal which does OpenGL operations)
This is done by changing the semantics of meta_window_new_with_attrs()
so that it is never called with a grab held. It takes its own grab
for some operations, but the calls to
meta_workspace_add_window() were then moved til after the point when the
grab is released.
Index: mutter-3.10.1/src/core/screen.c
===================================================================
--- mutter-3.10.1.orig/src/core/screen.c 1999-12-31 20:15:39.695000032 -0600
+++ mutter-3.10.1/src/core/screen.c 1999-12-31 20:15:39.675000032 -0600
@@ -931,8 +931,6 @@ meta_screen_manage_all_windows (MetaScre
GList *windows;
GList *list;
- meta_display_grab (screen->display);
-
if (screen->guard_window == None)
screen->guard_window = create_guard_window (screen->display->xdisplay,
screen);
@@ -952,8 +950,6 @@ meta_screen_manage_all_windows (MetaScre
g_list_foreach (windows, (GFunc)g_free, NULL);
g_list_free (windows);
-
- meta_display_ungrab (screen->display);
}
/**
Index: mutter-3.10.1/src/core/window.c
===================================================================
--- mutter-3.10.1.orig/src/core/window.c 1999-12-31 20:15:39.695000032 -0600
+++ mutter-3.10.1/src/core/window.c 2013-12-17 14:08:38.700000160 -0600
@@ -664,46 +664,10 @@ meta_window_new (MetaDisplay *display,
Window xwindow,
gboolean must_be_viewable)
{
- XWindowAttributes attrs;
- MetaWindow *window;
-
- meta_display_grab (display);
- meta_error_trap_push (display); /* Push a trap over all of window
- * creation, to reduce XSync() calls
- */
-
- meta_error_trap_push_with_return (display);
-
- if (XGetWindowAttributes (display->xdisplay,xwindow, &attrs))
- {
- if(meta_error_trap_pop_with_return (display) != Success)
- {
- meta_verbose ("Failed to get attributes for window 0x%lx\n",
- xwindow);
- meta_error_trap_pop (display);
- meta_display_ungrab (display);
- return NULL;
- }
- window = meta_window_new_with_attrs (display, xwindow,
- must_be_viewable,
- META_COMP_EFFECT_CREATE,
- &attrs);
- }
- else
- {
- meta_error_trap_pop_with_return (display);
- meta_verbose ("Failed to get attributes for window 0x%lx\n",
- xwindow);
- meta_error_trap_pop (display);
- meta_display_ungrab (display);
- return NULL;
- }
-
-
- meta_error_trap_pop (display);
- meta_display_ungrab (display);
-
- return window;
+ return meta_window_new_with_attrs (display, xwindow,
+ must_be_viewable,
+ META_COMP_EFFECT_CREATE,
+ NULL);
}
/* The MUTTER_WM_CLASS_FILTER environment variable is designed for
@@ -822,6 +786,7 @@ meta_window_new_with_attrs (MetaDisplay
MetaCompEffect effect,
XWindowAttributes *attrs)
{
+ XWindowAttributes _attrs;
MetaWindow *window;
GSList *tmp;
MetaWorkspace *space;
@@ -830,8 +795,6 @@ meta_window_new_with_attrs (MetaDisplay
MetaMoveResizeFlags flags;
MetaScreen *screen;
- g_assert (attrs != NULL);
-
meta_verbose ("Attempting to manage 0x%lx\n", xwindow);
if (meta_display_xwindow_is_a_no_focus_window (display, xwindow))
@@ -841,6 +804,39 @@ meta_window_new_with_attrs (MetaDisplay
return NULL;
}
+ /* Grab server */
+ meta_display_grab (display);
+ meta_error_trap_push (display); /* Push a trap over all of window
+ * creation, to reduce XSync() calls
+ */
+
+ if (!attrs)
+ {
+ meta_error_trap_push_with_return (display);
+
+ if (XGetWindowAttributes (display->xdisplay, xwindow, &_attrs))
+ {
+ if(meta_error_trap_pop_with_return (display) != Success)
+ {
+ meta_verbose ("Failed to get attributes for window 0x%lx\n",
+ xwindow);
+ meta_error_trap_pop (display);
+ meta_display_ungrab (display);
+ return NULL;
+ }
+ }
+ else
+ {
+ meta_error_trap_pop_with_return (display);
+ meta_verbose ("Failed to get attributes for window 0x%lx\n",
+ xwindow);
+ meta_error_trap_pop (display);
+ meta_display_ungrab (display);
+ return NULL;
+ }
+ attrs = &_attrs;
+ }
+
screen = NULL;
for (tmp = display->screens; tmp != NULL; tmp = tmp->next)
{
@@ -873,21 +869,19 @@ meta_window_new_with_attrs (MetaDisplay
)
) {
meta_verbose ("Not managing our own windows\n");
+ meta_error_trap_pop (display);
+ meta_display_ungrab (display);
return NULL;
}
if (maybe_filter_window (display, xwindow, must_be_viewable, attrs))
{
meta_verbose ("Not managing filtered window\n");
+ meta_error_trap_pop (display);
+ meta_display_ungrab (display);
return NULL;
}
- /* Grab server */
- meta_display_grab (display);
- meta_error_trap_push (display); /* Push a trap over all of window
- * creation, to reduce XSync() calls
- */
-
meta_verbose ("must_be_viewable = %d attrs->map_state = %d (%s)\n",
must_be_viewable,
attrs->map_state,
@@ -1302,93 +1296,6 @@ meta_window_new_with_attrs (MetaDisplay
window->on_all_workspaces = should_be_on_all_workspaces (window);
- /* For the workspace, first honor hints,
- * if that fails put transients with parents,
- * otherwise put window on active space
- */
-
- if (window->initial_workspace_set)
- {
- if (window->initial_workspace == (int) 0xFFFFFFFF)
- {
- meta_topic (META_DEBUG_PLACEMENT,
- "Window %s is initially on all spaces\n",
- window->desc);
-
- /* need to set on_all_workspaces first so that it will be
- * added to all the MRU lists
- */
- window->on_all_workspaces_requested = TRUE;
- window->on_all_workspaces = TRUE;
- meta_workspace_add_window (window->screen->active_workspace, window);
- }
- else
- {
- meta_topic (META_DEBUG_PLACEMENT,
- "Window %s is initially on space %d\n",
- window->desc, window->initial_workspace);
-
- space =
- meta_screen_get_workspace_by_index (window->screen,
- window->initial_workspace);
-
- if (space)
- meta_workspace_add_window (space, window);
- }
- }
-
- /* override-redirect windows are subtly different from other windows
- * with window->on_all_workspaces == TRUE. Other windows are part of
- * some workspace (so they can return to that if the flag is turned off),
- * but appear on other workspaces. override-redirect windows are part
- * of no workspace.
- */
- if (!window->override_redirect)
- {
- if (window->workspace == NULL &&
- window->xtransient_for != None)
- {
- /* Try putting dialog on parent's workspace */
- MetaWindow *parent;
-
- parent = meta_display_lookup_x_window (window->display,
- window->xtransient_for);
-
- if (parent && parent->workspace)
- {
- meta_topic (META_DEBUG_PLACEMENT,
- "Putting window %s on same workspace as parent %s\n",
- window->desc, parent->desc);
-
- if (parent->on_all_workspaces_requested)
- {
- window->on_all_workspaces_requested = TRUE;
- window->on_all_workspaces = TRUE;
- }
-
- /* this will implicitly add to the appropriate MRU lists
- */
- meta_workspace_add_window (parent->workspace, window);
- }
- }
-
- if (window->workspace == NULL)
- {
- meta_topic (META_DEBUG_PLACEMENT,
- "Putting window %s on active workspace\n",
- window->desc);
-
- space = window->screen->active_workspace;
-
- meta_workspace_add_window (space, window);
- }
-
- /* for the various on_all_workspaces = TRUE possible above */
- meta_window_set_current_workspace_hint (window);
-
- meta_window_update_struts (window);
- }
-
g_signal_emit_by_name (window->screen, "window-entered-monitor", window->monitor->number, window);
/* Must add window to stack before doing move/resize, since the
@@ -1485,6 +1392,93 @@ meta_window_new_with_attrs (MetaDisplay
meta_error_trap_pop (display); /* pop the XSync()-reducing trap */
meta_display_ungrab (display);
+ /* For the workspace, first honor hints,
+ * if that fails put transients with parents,
+ * otherwise put window on active space
+ */
+
+ if (window->initial_workspace_set)
+ {
+ if (window->initial_workspace == (int) 0xFFFFFFFF)
+ {
+ meta_topic (META_DEBUG_PLACEMENT,
+ "Window %s is initially on all spaces\n",
+ window->desc);
+
+ /* need to set on_all_workspaces first so that it will be
+ * added to all the MRU lists
+ */
+ window->on_all_workspaces_requested = TRUE;
+ window->on_all_workspaces = TRUE;
+ meta_workspace_add_window (window->screen->active_workspace, window);
+ }
+ else
+ {
+ meta_topic (META_DEBUG_PLACEMENT,
+ "Window %s is initially on space %d\n",
+ window->desc, window->initial_workspace);
+
+ space =
+ meta_screen_get_workspace_by_index (window->screen,
+ window->initial_workspace);
+
+ if (space)
+ meta_workspace_add_window (space, window);
+ }
+ }
+
+ /* override-redirect windows are subtly different from other windows
+ * with window->on_all_workspaces == TRUE. Other windows are part of
+ * some workspace (so they can return to that if the flag is turned off),
+ * but appear on other workspaces. override-redirect windows are part
+ * of no workspace.
+ */
+ if (!window->override_redirect)
+ {
+ if (window->workspace == NULL &&
+ window->xtransient_for != None)
+ {
+ /* Try putting dialog on parent's workspace */
+ MetaWindow *parent;
+
+ parent = meta_display_lookup_x_window (window->display,
+ window->xtransient_for);
+
+ if (parent && parent->workspace)
+ {
+ meta_topic (META_DEBUG_PLACEMENT,
+ "Putting window %s on same workspace as parent %s\n",
+ window->desc, parent->desc);
+
+ if (parent->on_all_workspaces_requested)
+ {
+ window->on_all_workspaces_requested = TRUE;
+ window->on_all_workspaces = TRUE;
+ }
+
+ /* this will implicitly add to the appropriate MRU lists
+ */
+ meta_workspace_add_window (parent->workspace, window);
+ }
+ }
+
+ if (window->workspace == NULL)
+ {
+ meta_topic (META_DEBUG_PLACEMENT,
+ "Putting window %s on active workspace\n",
+ window->desc);
+
+ space = window->screen->active_workspace;
+
+ meta_workspace_add_window (space, window);
+ }
+
+ /* for the various on_all_workspaces = TRUE possible above */
+ meta_window_set_current_workspace_hint (window);
+
+ meta_window_update_struts (window);
+ }
+
window->constructing = FALSE;
meta_display_notify_window_created (display, window);
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