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https://gitlab.gnome.org/GNOME/glib/-/merge_requests/4575
From d6798089d447977ef4416d124a83344241aab14b Mon Sep 17 00:00:00 2001
From: Sam James <sam@gentoo.org>
Date: Thu, 3 Apr 2025 19:03:27 +0100
Subject: [PATCH] gclosure: fix ATOMIC_CHANGE_FIELD to read vint atomically
Depending on luck, g_closure_ref may access closure->vint and observe
different values between reads. This manifests as a test failure in
signals-refcount{2,4}, properties-refcount1, and closure-refcount depending
on timing and re-runs.
Jakub Jelinek analysed this on GCC bug PR119607 after I'd reported it
over there as a possible GCC regression.
The critical part being g_closure_ref -> ATOMIC_INC_ASSIGN -> ATOMIC_CHANGE_FIELD
where closure->vint gets re-read repeatedly, both outside and inside the retry
loop. To fix that:
1. Atomically fetch it the first time;
2. Use the cached read, not a fresh read, of vint in the loop;
3. Use g_atomic_int_compare_and_exchange_full in the loop so we get a freshly
cached vint if it changed in another thread.
Bug: https://gcc.gnu.org/PR119607
Fixes: 834ddd19 ('turned all modifications to the first 32 integer bits in a closure into')
Co-authored-by: Jakub Jelinek <jakub@redhat.com>
---
gobject/gclosure.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)
diff --git a/gobject/gclosure.c b/gobject/gclosure.c
index 2552946e3b..e6e9769e46 100644
--- a/gobject/gclosure.c
+++ b/gobject/gclosure.c
@@ -110,15 +110,17 @@ typedef union {
G_STMT_START { \
ClosureInt *cunion = (ClosureInt*) _closure; \
gint new_int, old_int, success; \
+ old_int = g_atomic_int_get (&cunion->vint); \
do \
{ \
ClosureInt tmp; \
- tmp.vint = old_int = cunion->vint; \
+ tmp.vint = old_int; \
_SET_OLD tmp.closure._field; \
tmp.closure._field _OP _value; \
_SET_NEW tmp.closure._field; \
new_int = tmp.vint; \
- success = g_atomic_int_compare_and_exchange (&cunion->vint, old_int, new_int); \
+ success = g_atomic_int_compare_and_exchange_full (&cunion->vint, old_int, new_int,\
+ &old_int); \
} \
while (!success && _must_set); \
} G_STMT_END
--
GitLab
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