schedule: zephyr_ll: user_ll: keep lock while running tasks

In user-space LL builds (CONFIG_SOF_USERSPACE_LL), the IPC user thread
cannot block interrupts while making modifications to the audio graph.

To workaround this limitation, one could either protect each pipeline
object with locks, or keep the LL level lock held while executing
LL tasks.

This patch implements support for the latter approach. If building SOF
for user LL, do not release the lock when running a task. This will help
to reduce number of syscalls during a LL iteration, while still allowing
to safely implement IPC handlers that need to modify the audio graph.

Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>

Author: kv2019i

src/schedule/zephyr_ll.c

@@ -231,7 +231,15 @@ static void zephyr_ll_run(void *data)
 		list_item_del(list);
 		list_item_append(list, &task_head);
 
+		/* in user-space LL builds, the LL lock protects LL
+		 * tasks against IPC thread, so the lock must be held
+		 * while running the tasks.
+		 * in kernel LL builds, IPC thread blocks interrupts for
+		 * critical section, so lock can be freed here.
+		 */
+#ifndef CONFIG_SOF_USERSPACE_LL
 		zephyr_ll_unlock(sch, &flags);
+#endif
 
 		/*
 		 * task's .run() should only return either
@@ -246,7 +254,9 @@ static void zephyr_ll_run(void *data)
 			state = SOF_TASK_STATE_RESCHEDULE;
 		}
 
+#ifndef CONFIG_SOF_USERSPACE_LL
 		zephyr_ll_lock(sch, &flags);
+#endif
 
 		if (pdata->freeing || state == SOF_TASK_STATE_COMPLETED) {
 			zephyr_ll_task_done(sch, task);