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gobject: drop OPTIONAL_BIT_LOCK_WEAK_REFS bit lock

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With the previous changes, all accesses to the WeakRefStack go through
g_datalist_id_update_atomic() and are guarded by the GData's bit lock.
At this point, the OPTIONAL_BIT_LOCK_WEAK_REFS lock is unnecessary and
can be dropped.

A minor benefit is that g_object_weak_{ref,unref}() needs now one lock
less.

Also note that this rework fixes a potential race for weak refs. Note
that we have two calls

  g_datalist_id_set_data (&object->qdata, quark_weak_notifies, NULL);

that don't take OPTIONAL_BIT_LOCK_WEAK_REFS lock. One of these calls is
right before finalize(). At that point, no other thread can hold a
reference to the object to race and we are good. However, the other call
is from g_object_real_dispose(). At that point, theoretically the object
could have been resurrected and a pointer could have been passed to
another thread. Calling then g_object_weak_ref()/g_object_weak_unref()
will race. We would have required a OPTIONAL_BIT_LOCK_WEAK_REFS lock
around those g_datalist_id_set_data(,,NULL) calls.

Instead, this is now also fixed, because every update to the
WeakRefStack happens while holding the GData lock. So if you call
g_datalist_id_set_data(,,NULL), the WeakRefStack is removed from the
GData (and later freed by weak_refs_notify() and can no longer
concurrently updated by g_object_weak_{ref,unref}().
This commit is contained in:
Thomas Haller 2024-01-29 15:41:40 +01:00 committed by Philip Withnall
parent 5da7ed2bc9
commit 93b5b8a051
1 changed files with 2 additions and 7 deletions
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9
gobject/gobject.c
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@ -124,9 +124,8 @@ enum {
* parallel as possible. The alternative would be to add individual locking
* integers to GObjectPrivate. But increasing memory usage for more parallelism
* (per-object!) is not worth it. */
#define OPTIONAL_BIT_LOCK_WEAK_REFS 1
#define OPTIONAL_BIT_LOCK_NOTIFY 2
#define OPTIONAL_BIT_LOCK_TOGGLE_REFS 3
#define OPTIONAL_BIT_LOCK_NOTIFY 1
#define OPTIONAL_BIT_LOCK_TOGGLE_REFS 2
#if SIZEOF_INT == 4 && GLIB_SIZEOF_VOID_P >= 8
#define HAVE_OPTIONAL_FLAGS_IN_GOBJECT 1
@ -3732,12 +3731,10 @@ g_object_weak_ref (GObject *object,
g_return_if_fail (notify != NULL);
g_return_if_fail (g_atomic_int_get (&object->ref_count) >= 1);
object_bit_lock (object, OPTIONAL_BIT_LOCK_WEAK_REFS);
_g_datalist_id_update_atomic (&object->qdata,
quark_weak_notifies,
g_object_weak_ref_cb,
((gpointer[]){ object, notify, data }));
object_bit_unlock (object, OPTIONAL_BIT_LOCK_WEAK_REFS);
}
static gpointer
@ -3795,12 +3792,10 @@ g_object_weak_unref (GObject *object,
g_return_if_fail (G_IS_OBJECT (object));
g_return_if_fail (notify != NULL);
object_bit_lock (object, OPTIONAL_BIT_LOCK_WEAK_REFS);
_g_datalist_id_update_atomic (&object->qdata,
quark_weak_notifies,
g_object_weak_unref_cb,
((gpointer[]){ notify, data }));
object_bit_unlock (object, OPTIONAL_BIT_LOCK_WEAK_REFS);
}
/**