cpu/cortexm: clear FPU state in cpu_switch_context_exit()

cpu/cortexm_common/thread_arch.c

@@ -282,6 +282,16 @@ void *thread_isr_stack_start(void)
 
 void NORETURN cpu_switch_context_exit(void)
 {
+#ifdef MODULE_CORTEXM_FPU
+    /* An exiting thread won't need it's FPU state anymore, so clear it here.
+     * This is important, as `sched_task_exit` clears `sched_active_thread`,
+     * which in turn causes `isr_pendsv` to skip all FPU storing/restoring.
+     * That might lead to this thread's FPU lazy stacking / FPCAR to stay active.
+     */
+    __set_FPSCR(0);
+    __set_CONTROL(__get_CONTROL() & (~(CONTROL_FPCA_Msk)));
+#endif
+
     /* enable IRQs to make sure the PENDSV interrupt is reachable */
     irq_enable();