From 3f119b2fd408fb2e0f4eea07ed9618d2faa749ee Mon Sep 17 00:00:00 2001 From: Ryan Lortie Date: Tue, 11 Mar 2014 21:41:36 -0400 Subject: [PATCH] GSettingsBackend: fix a nasty race condition In the event that a GSettings object is being destroyed just as a change signal is being delivered, the destroying thread will race with the dconf worker thread for acquiring the lock on the GSettingsBackend. If the signalling thread gets there first then the destroying thread will block on the lock. The signalling thread adds a reference to the GSettings object that is being destroyed and releases the lock. The idea is that this should prevent the GSettings object from being destroyed and thus maintain its entry in the list. Unfortunately, the weak reference notify function is already running and as soon as we release the lock, the list entry is removed. The signalling thread crashes. This bug is indicative of a serious problem encountered in many situations where GObject instances are touched from multiple threads. Ideally, we will move to a place where g_object_ref() is not called at all on the GSettings object from the dconf worker thread and instead, a dispatch will be done without holding a reference (similar to how GAppInfoMonitor presently works). This would also prevent the unfortunate case of someone dropping what they assume to be the last reference on a GSettings object, only to have an already-pending signal delivered once they return to the mainloop, crashing their program. Making this change for GSettings (with multiple instances per thread, the possibility of multiple backends and each instance being interested in different events) is going to be extremely non-trivial, so it's not a change that makes sense at this point in the cycle. For now, we can do a relatively small and isolated tweak so that we never access the list except under a lock. We still perform the bad pattern of acquiring a ref in a foreign thread which means that we still risk delivering a signal to a GSettings object that the user has assumed is dead (unless they explicitly disconnect their signal handler). This is a problem that we already had, however. https://bugzilla.gnome.org/show_bug.cgi?id=710367 --- gio/gsettingsbackend.c | 44 ++++++++++++++++++++------------------------ 1 file changed, 20 insertions(+), 24 deletions(-) diff --git a/gio/gsettingsbackend.c b/gio/gsettingsbackend.c index ef31b73..f211e2c 100644 --- a/gio/gsettingsbackend.c +++ b/gio/gsettingsbackend.c @@ -136,6 +136,7 @@ struct _GSettingsBackendClosure gpointer origin_tag, gchar **names); + GMainContext *context; GObject *target; GSettingsBackend *backend; gchar *name; @@ -283,53 +284,48 @@ g_settings_backend_dispatch_signal (GSettingsBackend *backend, gpointer origin_tag, const gchar * const *names) { - GSettingsBackendWatch *suffix, *watch, *next; + GSettingsBackendWatch *watch; + GSList *closures = NULL; /* We're in a little bit of a tricky situation here. We need to hold * a lock while traversing the list, but we don't want to hold the * lock while calling back into user code. * - * Since we're not holding the lock while we call user code, we can't - * render the list immutable. We can, however, store a pointer to a - * given suffix of the list and render that suffix immutable. - * - * Adds will never modify the suffix since adds always come in the - * form of prepends. We can also prevent removes from modifying the - * suffix since removes only happen in response to the last reference - * count dropping -- so just add a reference to everything in the - * suffix. + * We work around this by creating a bunch of GSettingsBackendClosure + * objects while holding the lock and dispatching them after. We + * never touch the list without holding the lock. */ g_mutex_lock (&backend->priv->lock); - suffix = backend->priv->watches; - for (watch = suffix; watch; watch = watch->next) - g_object_ref (watch->target); - g_mutex_unlock (&backend->priv->lock); - - /* The suffix is now immutable, so this is safe. */ - for (watch = suffix; watch; watch = next) + for (watch = backend->priv->watches; watch; watch = watch->next) { GSettingsBackendClosure *closure; closure = g_slice_new (GSettingsBackendClosure); + closure->context = watch->context; closure->backend = g_object_ref (backend); - closure->target = watch->target; /* we took our ref above */ + closure->target = g_object_ref (watch->target); closure->function = G_STRUCT_MEMBER (void *, watch->vtable, function_offset); closure->name = g_strdup (name); closure->origin_tag = origin_tag; closure->names = g_strdupv ((gchar **) names); - /* we do this here because 'watch' may not live to the end of this - * iteration of the loop (since we may unref the target below). - */ - next = watch->next; + closures = g_slist_prepend (closures, closure); + } + g_mutex_unlock (&backend->priv->lock); - if (watch->context) - g_main_context_invoke (watch->context, + while (closures) + { + GSettingsBackendClosure *closure = closures->data; + + if (closure->context) + g_main_context_invoke (closure->context, g_settings_backend_invoke_closure, closure); else g_settings_backend_invoke_closure (closure); + + closures = g_slist_delete_link (closures, closures); } } -- 2.7.4