kernel: sys_workq: add option to allow blocking sys_workq

Add option which allows calling blocking APIs from the system
workqueue. This option is inherently unsafe, but users may want to
do it anyway.

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

Author: bjarki-andreasen

doc/kernel/services/threads/workqueue.rst

@@ -110,7 +110,9 @@ its queue until the handler function finishes executing.
    it do not depend on subsequent work items in the queue to unblock it.
 
    Attempting to perform a blocking call will fail an assert if
-   :kconfig:option:`CONFIG_ASSERT` is enabled.
+   :kconfig:option:`CONFIG_ASSERT` is enabled, unless
+   :kconfig:option:`CONFIG_SYS_WORKQUEUE_BLOCKING` is selected. Use at your
+   own risk.
 
 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

kernel/Kconfig

@@ -600,6 +600,15 @@ config SYSTEM_WORKQUEUE_NO_YIELD
 	  cooperative and a sequence of work items is expected to complete
 	  without yielding.
 
+config SYSTEM_WORKQUEUE_BLOCKING
+	bool "Select whether system work queue allows blocking calls"
+	help
+	  By default, the system work queue disallows blocking calls, to
+	  prevent deadlocking the system work queue. This is ensured by
+	  asserting that no work item handler unreadyes the system work
+	  queue thread. Selecting this removes the assert. Note that this
+	  option only has an effect if CONFIG_ASSERT=y.
+
 endmenu
 
 menu "Barrier Operations"

kernel/sched.c

@@ -523,7 +523,9 @@ static inline void z_vrfy_k_thread_resume(k_tid_t thread)
 
 static void unready_thread(struct k_thread *thread)
 {
+#ifndef CONFIG_SYSTEM_WORKQUEUE_BLOCKING
 	__ASSERT_NO_MSG(!k_is_in_sys_work());
+#endif /* CONFIG_SYSTEM_WORKQUEUE_BLOCKING */
 	if (z_is_thread_queued(thread)) {
 		dequeue_thread(thread);
 	}

tests/kernel/workq/sys_workq_blocking/CMakeLists.txt

@@ -0,0 +1,7 @@
+# SPDX-License-Identifier: Apache-2.0
+
+cmake_minimum_required(VERSION 3.20.0)
+find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
+project(sys_workq_blocking)
+
+target_sources(app PRIVATE src/main.c)

tests/kernel/workq/sys_workq_blocking/prj.conf

@@ -0,0 +1 @@
+CONFIG_ASSERT=y

tests/kernel/workq/sys_workq_blocking/src/main.c

@@ -0,0 +1,21 @@
+/*
+ * Copyright (c) 2025 Nordic Semiconductor ASA
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <zephyr/kernel.h>
+
+void blocking_work_handler(struct k_work *work)
+{
+	ARG_UNUSED(work);
+
+	k_msleep(1);
+}
+
+K_WORK_DEFINE(blocking_work, blocking_work_handler);
+
+int main(void)
+{
+	k_work_submit(&blocking_work);
+}

tests/kernel/workq/sys_workq_blocking/testcase.yaml

@@ -0,0 +1,10 @@
+tests:
+  common:
+    tags:
+      - kernel
+      - workqueue
+    min_flash: 34
+  kernel.workq.sys_workq_blocking.disallow:
+  kernel.workq.sys_workq_blocking.allow:
+    extra_configs:
+      - CONFIG_SYSTEM_WORKQUEUE_BLOCKING=y