[PATCH 1/4] sched_ext: Make explicit scx_task_iter_relock() calls unnecessary

[Tejun Heo <tj@kernel.org>]

During tasks iteration, the locks can be dropped using
scx_task_iter_unlock() to perform e.g. sleepable allocations. Afterwards,
scx_task_iter_relock() has to be called prior to other iteration operations,
which is error-prone. This can be easily automated by tracking whether
scx_tasks_lock is held in scx_task_iter and re-acquiring when necessary. It
already tracks whether the task's rq is locked after all.

- Add scx_task_iter->list_locked which remembers whether scx_tasks_lock is
  held.

- Rename scx_task_iter->locked to scx_task_iter->locked_task to better
  distinguish it from ->list_locked.

- Replace scx_task_iter_relock() with __scx_task_iter_maybe_relock() which
  is automatically called by scx_task_iter_next() and scx_task_iter_stop().

- Drop explicit scx_task_iter_relock() calls.

The resulting behavior should be equivalent.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
 kernel/sched/ext.c | 43 +++++++++++++++++++++++--------------------
 1 file changed, 23 insertions(+), 20 deletions(-)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 7dedc9a16281..7f799345c899 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -1488,10 +1488,11 @@ struct bpf_iter_scx_dsq {
  */
 struct scx_task_iter {
 	struct sched_ext_entity		cursor;
-	struct task_struct		*locked;
+	struct task_struct		*locked_task;
 	struct rq			*rq;
 	struct rq_flags			rf;
 	u32				cnt;
+	bool				list_locked;
 };
 
 /**
@@ -1519,15 +1520,16 @@ static void scx_task_iter_start(struct scx_task_iter *iter)
 
 	iter->cursor = (struct sched_ext_entity){ .flags = SCX_TASK_CURSOR };
 	list_add(&iter->cursor.tasks_node, &scx_tasks);
-	iter->locked = NULL;
+	iter->locked_task = NULL;
 	iter->cnt = 0;
+	iter->list_locked = true;
 }
 
 static void __scx_task_iter_rq_unlock(struct scx_task_iter *iter)
 {
-	if (iter->locked) {
-		task_rq_unlock(iter->rq, iter->locked, &iter->rf);
-		iter->locked = NULL;
+	if (iter->locked_task) {
+		task_rq_unlock(iter->rq, iter->locked_task, &iter->rf);
+		iter->locked_task = NULL;
 	}
 }
 
@@ -1537,24 +1539,24 @@ static void __scx_task_iter_rq_unlock(struct scx_task_iter *iter)
  *
  * If @iter is in the middle of a locked iteration, it may be locking the rq of
  * the task currently being visited in addition to scx_tasks_lock. Unlock both.
- * This function can be safely called anytime during an iteration.
+ * This function can be safely called anytime during an iteration. The next
+ * iterator operation will automatically restore the necessary locking.
  */
 static void scx_task_iter_unlock(struct scx_task_iter *iter)
 {
 	__scx_task_iter_rq_unlock(iter);
-	spin_unlock_irq(&scx_tasks_lock);
+	if (iter->list_locked) {
+		iter->list_locked = false;
+		spin_unlock_irq(&scx_tasks_lock);
+	}
 }
 
-/**
- * scx_task_iter_relock - Lock scx_tasks_lock released by scx_task_iter_unlock()
- * @iter: iterator to re-lock
- *
- * Re-lock scx_tasks_lock unlocked by scx_task_iter_unlock(). Note that it
- * doesn't re-lock the rq lock. Must be called before other iterator operations.
- */
-static void scx_task_iter_relock(struct scx_task_iter *iter)
+static void __scx_task_iter_maybe_relock(struct scx_task_iter *iter)
 {
-	spin_lock_irq(&scx_tasks_lock);
+	if (!iter->list_locked) {
+		spin_lock_irq(&scx_tasks_lock);
+		iter->list_locked = true;
+	}
 }
 
 /**
@@ -1567,6 +1569,7 @@ static void scx_task_iter_relock(struct scx_task_iter *iter)
  */
 static void scx_task_iter_stop(struct scx_task_iter *iter)
 {
+	__scx_task_iter_maybe_relock(iter);
 	list_del_init(&iter->cursor.tasks_node);
 	scx_task_iter_unlock(iter);
 }
@@ -1584,10 +1587,12 @@ static struct task_struct *scx_task_iter_next(struct scx_task_iter *iter)
 	struct list_head *cursor = &iter->cursor.tasks_node;
 	struct sched_ext_entity *pos;
 
+	__scx_task_iter_maybe_relock(iter);
+
 	if (!(++iter->cnt % SCX_TASK_ITER_BATCH)) {
 		scx_task_iter_unlock(iter);
 		cond_resched();
-		scx_task_iter_relock(iter);
+		__scx_task_iter_maybe_relock(iter);
 	}
 
 	list_for_each_entry(pos, cursor, tasks_node) {
@@ -1650,7 +1655,7 @@ static struct task_struct *scx_task_iter_next_locked(struct scx_task_iter *iter)
 		return NULL;
 
 	iter->rq = task_rq_lock(p, &iter->rf);
-	iter->locked = p;
+	iter->locked_task = p;
 
 	return p;
 }
@@ -5713,7 +5718,6 @@ static int scx_enable(struct sched_ext_ops *ops, struct bpf_link *link)
 		ret = scx_init_task(p, task_group(p), false);
 		if (ret) {
 			put_task_struct(p);
-			scx_task_iter_relock(&sti);
 			scx_task_iter_stop(&sti);
 			scx_error(sch, "ops.init_task() failed (%d) for %s[%d]",
 				  ret, p->comm, p->pid);
@@ -5723,7 +5727,6 @@ static int scx_enable(struct sched_ext_ops *ops, struct bpf_link *link)
 		scx_set_task_state(p, SCX_TASK_READY);
 
 		put_task_struct(p);
-		scx_task_iter_relock(&sti);
 	}
 	scx_task_iter_stop(&sti);
 	scx_cgroup_unlock();
-- 
2.51.0