kernel/sched: Fix preemption logic

The should_preempt() code was catching some of the "unrunnable" cases
but not all of them, opening the possibility of failing to preempt a
just-pended thread and thus waking it up synchronously.  There are
reports of this causing spin loops over k_poll() in the network stack
work queues (see #8049).

Note that the previous _is_dummy() call is folded into (the somewhat
verbosely named) _is_thread_prevented_from_running(), and that the
order of tests has been changed/optimized to hopefully catch common
cases earlier.

Suggested-by: Michael Scott <michael@opensourcefoundries.com>
Signed-off-by: Andy Ross <andrew.j.ross@intel.com>

kernel/sched.c

@@ -114,23 +114,15 @@ int _is_t1_higher_prio_than_t2(struct k_thread *t1, struct k_thread *t2)
 
 static int should_preempt(struct k_thread *th, int preempt_ok)
 {
-	/* System initialization states can have dummy threads, never
-	 * refuse to swap those
+	/* Preemption is OK if it's being explicitly allowed by
+	 * software state (e.g. the thread called k_yield())
 	 */
-	if (!_current || _is_thread_dummy(_current)) {
-		return 1;
-	}
-
-	/* The idle threads can look "cooperative" if there are no
-	 * preemptible priorities (this is sort of an API glitch).
-	 * They must always be preemptible.
-	 */
-	if (_is_idle(_current)) {
+	if (preempt_ok) {
 		return 1;
 	}
 
-	/* Preemption is OK if it's being explicitly allowed */
-	if (preempt_ok) {
+	/* Or if we're pended/suspended/dummy (duh) */
+	if (!_current || _is_thread_prevented_from_running(_current)) {
 		return 1;
 	}
 
@@ -141,6 +133,14 @@ static int should_preempt(struct k_thread *th, int preempt_ok)
 		return 1;
 	}
 
+	/* The idle threads can look "cooperative" if there are no
+	 * preemptible priorities (this is sort of an API glitch).
+	 * They must always be preemptible.
+	 */
+	if (_is_idle(_current)) {
+		return 1;
+	}
+
 	return 0;
 }