sched: schedule() no longer switches threads when preempt_count > 0

sched/core.c

@@ -70,6 +70,8 @@ void context_switch(struct thread *old, struct thread *new)
 
 void __schedule(enum sched_mode mode)
 {
+	ml_int_disable();
+
 	struct cpu_data *this_cpu = get_this_cpu();
 	struct runqueue *rq = &this_cpu->c_rq;
 
@@ -78,6 +80,10 @@ void __schedule(enum sched_mode mode)
 	unsigned long flags;
 	rq_lock(rq, &flags);
 
+	/* subtrace one to compensate for the fact that get_this_cpu()
+	   increases preempt_count */
+	int preempt = READ_ONCE(this_cpu->c_preempt_count) - 1;
+
 	put_cpu(this_cpu);
 
 	struct thread *prev = rq->rq_cur;
@@ -86,6 +92,11 @@ void __schedule(enum sched_mode mode)
 		prev->tr_quantum_cycles = 0;
 	}
 
+	if (preempt > 0) {
+		rq_unlock(rq, flags);
+		return;
+	}
+
 	enum thread_state prev_state = READ_ONCE(prev->tr_state);
 
 	if ((mode == SCHED_IRQ || prev_state == THREAD_READY) && prev != rq->rq_idle) {
@@ -108,6 +119,8 @@ void __schedule(enum sched_mode mode)
 	if (prev != next) {
 		context_switch(prev, next);
 	}
+
+	ml_int_enable();
 }
 
 void schedule(enum sched_mode mode)