From 89dac0c95125ec71c62443ac315c05713c162c0e Mon Sep 17 00:00:00 2001
From: Max Wash <mbswash@gmail.com>
Date: Sat, 14 Mar 2026 22:22:05 +0000
Subject: [PATCH] sched: add a thread flag to indicate when a thread is
 scheduled on a runqueue

this prevents runqueue corruption that can occur if rq_enqueue is called on
a thread that's already on a runqueue.
---
 include/kernel/thread.h |  5 ++++-
 sched/runqueue.c        | 15 +++++++++++++++
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/include/kernel/thread.h b/include/kernel/thread.h
index 619fe17..7608c62 100644
--- a/include/kernel/thread.h
+++ b/include/kernel/thread.h
@@ -14,8 +14,11 @@ enum thread_state {
 };
 
 enum thread_flags {
+	/* this thread has exhausted its quantum and is due to be re-scheduled.
+	 */
 	THREAD_F_NEED_RESCHED = 0x01u,
-	THREAD_F_NO_PREEMPT = 0x02u,
+	/* this thread is currently scheduled (i.e. is present on a runqueue) */
+	THREAD_F_SCHEDULED = 0x04u,
 };
 
 struct thread {
diff --git a/sched/runqueue.c b/sched/runqueue.c
index ca076b3..38c6da9 100644
--- a/sched/runqueue.c
+++ b/sched/runqueue.c
@@ -1,6 +1,7 @@
 #include <kernel/cpu.h>
 #include <kernel/percpu.h>
 #include <kernel/sched.h>
+#include <kernel/task.h>
 #include <kernel/thread.h>
 
 #define PRIO_MASK(p)  (((uint32_t)1) << (p))
@@ -20,6 +21,7 @@ struct thread *rq_dequeue(struct runqueue *rq)
 	}
 
 	struct queue *q = &rq->rq_queues[prio];
+
 	struct queue_entry *qe = queue_pop_front(q);
 	if (!qe) {
 		rq->rq_readybits &= ~PRIO_MASK(prio);
@@ -27,6 +29,7 @@ struct thread *rq_dequeue(struct runqueue *rq)
 	}
 
 	struct thread *thr = QUEUE_CONTAINER(struct thread, tr_rqentry, qe);
+	thr->tr_flags &= ~THREAD_F_SCHEDULED;
 
 	if (rq->rq_nthreads > 0) {
 		rq->rq_nthreads--;
@@ -41,17 +44,24 @@ struct thread *rq_dequeue(struct runqueue *rq)
 
 void rq_enqueue(struct runqueue *rq, struct thread *thr)
 {
+	if (thr->tr_flags & THREAD_F_SCHEDULED) {
+		return;
+	}
+
 	int prio = thread_priority(thr);
 	if (prio < 0 || prio > PRIO_MAX) {
 		return;
 	}
 
 	struct queue *q = &rq->rq_queues[prio];
+
 	queue_push_back(q, &thr->tr_rqentry);
+
 	rq->rq_nthreads++;
 
 	rq->rq_readybits |= PRIO_MASK(thread_priority(thr));
 	thr->tr_rq = rq;
+	thr->tr_flags |= THREAD_F_SCHEDULED;
 }
 
 struct runqueue *cpu_rq(unsigned int cpu)
@@ -65,6 +75,10 @@ struct runqueue *cpu_rq(unsigned int cpu)
 
 void rq_remove_thread(struct runqueue *rq, struct thread *thr)
 {
+	if (!(thr->tr_flags & THREAD_F_SCHEDULED)) {
+		return;
+	}
+
 	int prio = thread_priority(thr);
 	if (prio < 0 || prio > PRIO_MAX) {
 		return;
@@ -72,6 +86,7 @@ void rq_remove_thread(struct runqueue *rq, struct thread *thr)
 
 	struct queue *q = &rq->rq_queues[prio];
 	queue_delete(q, &thr->tr_rqentry);
+	thr->tr_flags &= ~THREAD_F_SCHEDULED;
 
 	if (rq->rq_nthreads > 0) {
 		rq->rq_nthreads--;