block/graph-lock: Make WITH_GRAPH_RDLOCK_GUARD() fully checked

Upstream clang 18 (and backports to clang 17 in Fedora and RHEL)
implemented support for __attribute__((cleanup())) in its Thread Safety
Analysis, so we can now actually have a proper implementation of
WITH_GRAPH_RDLOCK_GUARD() that understands when we acquire and when we
release the lock.

-Wthread-safety is now only enabled if the compiler is new enough to
understand this pattern. In theory, we could have used some #ifdefs to
keep the existing basic checks on old compilers, but as long as someone
runs a newer compiler (and our CI does), we will catch locking problems,
so it's probably not worth keeping multiple implementations for this.

The implementation can't use g_autoptr any more because the glib macros
define wrapper functions that don't have the right TSA attributes, so
the compiler would complain about them. Just use the cleanup attribute
directly instead.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20240627181245.281403-3-kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

include/block/graph-lock.h

@@ -209,31 +209,38 @@ typedef struct GraphLockable { } GraphLockable;
  * unlocked. TSA_ASSERT_SHARED() makes sure that the following calls know that
  * we hold the lock while unlocking is left unchecked.
  */
-static inline GraphLockable * TSA_ASSERT_SHARED(graph_lock) TSA_NO_TSA coroutine_fn
+static inline GraphLockable * TSA_ACQUIRE_SHARED(graph_lock) coroutine_fn
 graph_lockable_auto_lock(GraphLockable *x)
 {
     bdrv_graph_co_rdlock();
     return x;
 }
 
-static inline void TSA_NO_TSA coroutine_fn
-graph_lockable_auto_unlock(GraphLockable *x)
+static inline void TSA_RELEASE_SHARED(graph_lock) coroutine_fn
+graph_lockable_auto_unlock(GraphLockable **x)
 {
     bdrv_graph_co_rdunlock();
 }
 
-G_DEFINE_AUTOPTR_CLEANUP_FUNC(GraphLockable, graph_lockable_auto_unlock)
+#define GRAPH_AUTO_UNLOCK __attribute__((cleanup(graph_lockable_auto_unlock)))
 
+/*
+ * @var is only used to break the loop after the first iteration.
+ * @unlock_var can't be unlocked and then set to NULL because TSA wants the lock
+ * to be held at the start of every iteration of the loop.
+ */
 #define WITH_GRAPH_RDLOCK_GUARD_(var)                                         \
-    for (g_autoptr(GraphLockable) var = graph_lockable_auto_lock(GML_OBJ_()); \
+    for (GraphLockable *unlock_var GRAPH_AUTO_UNLOCK =                        \
+            graph_lockable_auto_lock(GML_OBJ_()),                             \
+            *var = unlock_var;                                                \
          var;                                                                 \
-         graph_lockable_auto_unlock(var), var = NULL)
+         var = NULL)
 
 #define WITH_GRAPH_RDLOCK_GUARD() \
     WITH_GRAPH_RDLOCK_GUARD_(glue(graph_lockable_auto, __COUNTER__))
 
 #define GRAPH_RDLOCK_GUARD(x)                                       \
-    g_autoptr(GraphLockable)                                        \
+    GraphLockable * GRAPH_AUTO_UNLOCK                               \
     glue(graph_lockable_auto, __COUNTER__) G_GNUC_UNUSED =          \
             graph_lockable_auto_lock(GML_OBJ_())
 

meson.build

@@ -649,7 +649,19 @@ warn_flags = [
 ]
 
 if host_os != 'darwin'
-  warn_flags += ['-Wthread-safety']
+  tsa_has_cleanup = cc.compiles('''
+    struct __attribute__((capability("mutex"))) mutex {};
+    void lock(struct mutex *m) __attribute__((acquire_capability(m)));
+    void unlock(struct mutex *m) __attribute__((release_capability(m)));
+
+    void test(void) {
+      struct mutex __attribute__((cleanup(unlock))) m;
+      lock(&m);
+    }
+  ''', args: ['-Wthread-safety', '-Werror'])
+  if tsa_has_cleanup
+    warn_flags += ['-Wthread-safety']
+  endif
 endif
 
 # Set up C++ compiler flags