docs/style: call out the use of GUARD macros

There use makes our code safer so we should mention them.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Message-Id: <20230424092249.58552-19-alex.bennee@linaro.org>

docs/devel/style.rst

@@ -665,6 +665,60 @@ Note that there is no need to provide typedefs for QOM structures
 since these are generated automatically by the QOM declaration macros.
 See :ref:`qom` for more details.
 
+QEMU GUARD macros
+=================
+
+QEMU provides a number of ``_GUARD`` macros intended to make the
+handling of multiple exit paths easier. For example using
+``QEMU_LOCK_GUARD`` to take a lock will ensure the lock is released on
+exit from the function.
+
+.. code-block:: c
+
+    static int my_critical_function(SomeState *s, void *data)
+    {
+        QEMU_LOCK_GUARD(&s->lock);
+        do_thing1(data);
+        if (check_state2(data)) {
+            return -1;
+        }
+        do_thing3(data);
+        return 0;
+    }
+
+will ensure s->lock is released however the function is exited. The
+equivalent code without _GUARD macro makes us to carefully put
+qemu_mutex_unlock() on all exit points:
+
+.. code-block:: c
+
+    static int my_critical_function(SomeState *s, void *data)
+    {
+        qemu_mutex_lock(&s->lock);
+        do_thing1(data);
+        if (check_state2(data)) {
+            qemu_mutex_unlock(&s->lock);
+            return -1;
+        }
+        do_thing3(data);
+        qemu_mutex_unlock(&s->lock);
+        return 0;
+    }
+
+There are often ``WITH_`` forms of macros which more easily wrap
+around a block inside a function.
+
+.. code-block:: c
+
+    WITH_RCU_READ_LOCK_GUARD() {
+        QTAILQ_FOREACH_RCU(kid, &bus->children, sibling) {
+            err = do_the_thing(kid->child);
+            if (err < 0) {
+                return err;
+            }
+        }
+    }
+
 Error handling and reporting
 ============================