kernel: sched: assert sys_workq is not unreadied while busy

The system workqueue should never be unreadied from a work item
handler (while busy). This assert will help avoid deadlocking the
system work queue, which is an exceptionally hard bug to hunt down.

Signed-off-by: Bjarki Arge Andreasen <[email protected]>

Author: bjarki-andreasen

doc/kernel/services/threads/workqueue.rst

@@ -109,6 +109,9 @@ its queue until the handler function finishes executing.
    potentially blocking, as there is no guarantee that work items submitted to
    it do not depend on subsequent work items in the queue to unblock them.
 
+   Attempting to perform a blocking call will fail an assert if
+   :kconfig:option:`CONFIG_ASSERT` is enabled.
+
 The single argument that is passed to a handler function can be ignored if it
 is not required. If the handler function requires additional information about
 the work it is to perform, the work item can be embedded in a larger data

kernel/sched.c

@@ -523,6 +523,7 @@ static inline void z_vrfy_k_thread_resume(k_tid_t thread)
 
 static void unready_thread(struct k_thread *thread)
 {
+	__ASSERT_NO_MSG(!k_is_in_sys_work());
 	if (z_is_thread_queued(thread)) {
 		dequeue_thread(thread);
 	}