clock: Avoid creating a weakref with every entry

Creating and destroying weakrefs takes a write lock on a global
`GRWLock`. This makes for a very contended lock when the pipeline has
many synchronizing elements.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2530>

subprojects/gstreamer/gst/gst_private.h

@@ -514,11 +514,14 @@ struct _GstDynamicTypeFactoryClass {
 /* privat flag used by GstBus / GstMessage */
 #define GST_MESSAGE_FLAG_ASYNC_DELIVERY (GST_MINI_OBJECT_FLAG_LAST << 0)
 
+/* private type used by GstClock */
+typedef struct _GstClockWeakRef GstClockWeakRef;
+
 /* private struct used by GstClock and GstSystemClock */
 struct _GstClockEntryImpl
 {
   GstClockEntry entry;
-  GWeakRef clock;
+  GstClockWeakRef *weakref;
   GDestroyNotify destroy_entry;
   gpointer padding[21];                 /* padding for allowing e.g. systemclock
                                          * to add data in lieu of overridable

subprojects/gstreamer/gst/gstclock.c

@@ -132,6 +132,60 @@ enum
 #define GST_CLOCK_SLAVE_LOCK(clock)     g_mutex_lock (&GST_CLOCK_CAST (clock)->priv->slave_lock)
 #define GST_CLOCK_SLAVE_UNLOCK(clock)   g_mutex_unlock (&GST_CLOCK_CAST (clock)->priv->slave_lock)
 
+/* An atomically ref-counted wrapper around a GWeakRef for a GstClock, created
+ * by the clock and shared with all its clock entries.
+ *
+ * This exists because g_weak_ref_ operations are quite expensive and operate
+ * with a global GRWLock. _get takes a reader lock, _init and _clear take a
+ * writer lock. We want to avoid having a GWeakRef in every clock entry.
+ *
+ * FIXME: Simplify this with g_atomic_rc_box_new (GWeakRef) once we can depend
+ *        on GLib 2.58.
+ */
+struct _GstClockWeakRef
+{
+  gint refcount;
+  GWeakRef clock;
+};
+
+static GstClockWeakRef *
+gst_clock_weak_ref_new (GstClock * clock)
+{
+  GstClockWeakRef *weakref = g_slice_new (GstClockWeakRef);
+
+  weakref->refcount = 1;
+  g_weak_ref_init (&weakref->clock, clock);
+
+  return weakref;
+}
+
+static GstClockWeakRef *
+gst_clock_weak_ref_ref (GstClockWeakRef * weakref)
+{
+  g_atomic_int_add (&weakref->refcount, 1);
+  return weakref;
+}
+
+static void
+gst_clock_weak_ref_unref (GstClockWeakRef * weakref)
+{
+  gint old_refcount;
+
+  old_refcount = g_atomic_int_add (&weakref->refcount, -1);
+  g_return_if_fail (old_refcount > 0);
+
+  if (G_UNLIKELY (old_refcount == 1)) {
+    g_weak_ref_clear (&weakref->clock);
+    g_slice_free (GstClockWeakRef, weakref);
+  }
+}
+
+static GstClock *
+gst_clock_weak_ref_get (GstClockWeakRef * weakref)
+{
+  return g_weak_ref_get (&weakref->clock);
+}
+
 struct _GstClockPrivate
 {
   GMutex slave_lock;            /* order: SLAVE_LOCK, OBJECT_LOCK */
@@ -165,11 +219,13 @@ struct _GstClockPrivate
   gint post_count;
 
   gboolean synced;
+
+  GstClockWeakRef *weakref;
 };
 
 typedef struct _GstClockEntryImpl GstClockEntryImpl;
 
-#define GST_CLOCK_ENTRY_CLOCK_WEAK_REF(entry) (&((GstClockEntryImpl *)(entry))->clock)
+#define GST_CLOCK_ENTRY_CLOCK_WEAK_REF(entry) (((GstClockEntryImpl *)(entry))->weakref)
 
 /* seqlocks */
 #define read_seqbegin(clock)                                   \
@@ -260,7 +316,8 @@ gst_clock_entry_new (GstClock * clock, GstClockTime time,
   entry->_clock = clock;
 #endif
 #endif
-  g_weak_ref_init (GST_CLOCK_ENTRY_CLOCK_WEAK_REF (entry), clock);
+  GST_CLOCK_ENTRY_CLOCK_WEAK_REF (entry) =
+      gst_clock_weak_ref_ref (clock->priv->weakref);
   entry->type = type;
   entry->time = time;
   entry->interval = interval;
@@ -374,7 +431,7 @@ _gst_clock_id_free (GstClockID id)
   if (entry_impl->destroy_entry)
     entry_impl->destroy_entry (entry_impl);
 
-  g_weak_ref_clear (GST_CLOCK_ENTRY_CLOCK_WEAK_REF (entry));
+  gst_clock_weak_ref_unref (GST_CLOCK_ENTRY_CLOCK_WEAK_REF (entry));
 
   /* FIXME: add tracer hook for struct allocations such as clock entries */
 
@@ -531,7 +588,7 @@ gst_clock_id_wait (GstClockID id, GstClockTimeDiff * jitter)
   entry = (GstClockEntry *) id;
   requested = GST_CLOCK_ENTRY_TIME (entry);
 
-  clock = g_weak_ref_get (GST_CLOCK_ENTRY_CLOCK_WEAK_REF (entry));
+  clock = gst_clock_weak_ref_get (GST_CLOCK_ENTRY_CLOCK_WEAK_REF (entry));
   if (G_UNLIKELY (clock == NULL))
     goto invalid_entry;
 
@@ -613,7 +670,7 @@ gst_clock_id_wait_async (GstClockID id,
 
   entry = (GstClockEntry *) id;
   requested = GST_CLOCK_ENTRY_TIME (entry);
-  clock = g_weak_ref_get (GST_CLOCK_ENTRY_CLOCK_WEAK_REF (entry));
+  clock = gst_clock_weak_ref_get (GST_CLOCK_ENTRY_CLOCK_WEAK_REF (entry));
   if (G_UNLIKELY (clock == NULL))
     goto invalid_entry;
 
@@ -676,7 +733,7 @@ gst_clock_id_unschedule (GstClockID id)
   g_return_if_fail (id != NULL);
 
   entry = (GstClockEntry *) id;
-  clock = g_weak_ref_get (GST_CLOCK_ENTRY_CLOCK_WEAK_REF (entry));
+  clock = gst_clock_weak_ref_get (GST_CLOCK_ENTRY_CLOCK_WEAK_REF (entry));
   if (G_UNLIKELY (clock == NULL))
     goto invalid_entry;
 
@@ -769,6 +826,7 @@ gst_clock_init (GstClock * clock)
   priv->timeout = DEFAULT_TIMEOUT;
   priv->times = g_new0 (GstClockTime, 4 * priv->window_size);
   priv->times_temp = priv->times + 2 * priv->window_size;
+  priv->weakref = gst_clock_weak_ref_new (clock);
 }
 
 static void
@@ -801,6 +859,7 @@ gst_clock_finalize (GObject * object)
   clock->priv->times_temp = NULL;
   GST_CLOCK_SLAVE_UNLOCK (clock);
 
+  gst_clock_weak_ref_unref (clock->priv->weakref);
   g_mutex_clear (&clock->priv->slave_lock);
   g_cond_clear (&clock->priv->sync_cond);
 
@@ -1374,7 +1433,7 @@ gst_clock_id_get_clock (GstClockID id)
   g_return_val_if_fail (id != NULL, NULL);
 
   entry = (GstClockEntry *) id;
-  return g_weak_ref_get (GST_CLOCK_ENTRY_CLOCK_WEAK_REF (entry));
+  return gst_clock_weak_ref_get (GST_CLOCK_ENTRY_CLOCK_WEAK_REF (entry));
 }
 
 /**
@@ -1401,7 +1460,7 @@ gst_clock_id_uses_clock (GstClockID id, GstClock * clock)
   g_return_val_if_fail (clock != NULL, FALSE);
 
   entry = (GstClockEntry *) id;
-  entry_clock = g_weak_ref_get (GST_CLOCK_ENTRY_CLOCK_WEAK_REF (entry));
+  entry_clock = gst_clock_weak_ref_get (GST_CLOCK_ENTRY_CLOCK_WEAK_REF (entry));
   if (entry_clock == clock)
     ret = TRUE;