sched_ext: TASK_DEAD tasks must be switched into SCX on ops_enable

During scx_ops_enable(), SCX needs to invoke the sleepable ops.init_task()
on every task. To do this, it does get_task_struct() on each iterated task,
drop the lock and then call ops.init_task().

However, a TASK_DEAD task may already have lost all its usage count and be
waiting for RCU grace period to be freed. If get_task_struct() is called on
such task, use-after-free can happen. To avoid such situations,
scx_ops_enable() skips initialization of TASK_DEAD tasks, which seems safe
as they are never going to be scheduled again.

Unfortunately, a racing sched_setscheduler(2) can grab the task before the
task is unhashed and then continue to e.g. move the task from RT to SCX
after TASK_DEAD is set and ops_enable skipped the task. As the task hasn't
gone through scx_ops_init_task(), scx_ops_enable_task() called from
switching_to_scx() triggers the following warning:

  sched_ext: Invalid task state transition 0 -> 3 for stress-ng-race-[2872]
  WARNING: CPU: 6 PID: 2367 at kernel/sched/ext.c:3327 scx_ops_enable_task+0x18f/0x1f0
  ...
  RIP: 0010:scx_ops_enable_task+0x18f/0x1f0
  ...
   switching_to_scx+0x13/0xa0
   __sched_setscheduler+0x84e/0xa50
   do_sched_setscheduler+0x104/0x1c0
   __x64_sys_sched_setscheduler+0x18/0x30
   do_syscall_64+0x7b/0x140
   entry_SYSCALL_64_after_hwframe+0x76/0x7e

As in the ops_disable path, it just doesn't seem like a good idea to leave
any task in an inconsistent state, even when the task is dead. The root
cause is ops_enable not being able to tell reliably whether a task is truly
dead (no one else is looking at it and it's about to be freed) and was
testing TASK_DEAD instead. Fix it by testing the task's usage count
directly.

- ops_init no longer ignores TASK_DEAD tasks. As now all users iterate all
  tasks, @include_dead is removed from scx_task_iter_next_locked() along
  with dead task filtering.

- tryget_task_struct() is added. Tasks are skipped iff tryget_task_struct()
  fails.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: David Vernet <void@manifault.com>
Cc: Peter Zijlstra <peterz@infradead.org>

include/linux/sched/task.h

@@ -120,6 +120,11 @@ static inline struct task_struct *get_task_struct(struct task_struct *t)
 	return t;
 }
 
+static inline struct task_struct *tryget_task_struct(struct task_struct *t)
+{
+	return refcount_inc_not_zero(&t->usage) ? t : NULL;
+}
+
 extern void __put_task_struct(struct task_struct *t);
 extern void __put_task_struct_rcu_cb(struct rcu_head *rhp);
 

kernel/sched/ext.c

@@ -1240,11 +1240,10 @@ static struct task_struct *scx_task_iter_next(struct scx_task_iter *iter)
  * whether they would like to filter out dead tasks. See scx_task_iter_init()
  * for details.
  */
-static struct task_struct *
-scx_task_iter_next_locked(struct scx_task_iter *iter, bool include_dead)
+static struct task_struct *scx_task_iter_next_locked(struct scx_task_iter *iter)
 {
 	struct task_struct *p;
-retry:
+
 	scx_task_iter_rq_unlock(iter);
 
 	while ((p = scx_task_iter_next(iter))) {
@@ -1282,16 +1281,6 @@ scx_task_iter_next_locked(struct scx_task_iter *iter, bool include_dead)
 	iter->rq = task_rq_lock(p, &iter->rf);
 	iter->locked = p;
 
-	/*
-	 * If we see %TASK_DEAD, @p already disabled preemption, is about to do
-	 * the final __schedule(), won't ever need to be scheduled again and can
-	 * thus be safely ignored. If we don't see %TASK_DEAD, @p can't enter
-	 * the final __schedle() while we're locking its rq and thus will stay
-	 * alive until the rq is unlocked.
-	 */
-	if (!include_dead && READ_ONCE(p->__state) == TASK_DEAD)
-		goto retry;
-
 	return p;
 }
 
@@ -4001,7 +3990,7 @@ static void scx_ops_disable_workfn(struct kthread_work *work)
 	 * The BPF scheduler is going away. All tasks including %TASK_DEAD ones
 	 * must be switched out and exited synchronously.
 	 */
-	while ((p = scx_task_iter_next_locked(&sti, true))) {
+	while ((p = scx_task_iter_next_locked(&sti))) {
 		const struct sched_class *old_class = p->sched_class;
 		struct sched_enq_and_set_ctx ctx;
 
@@ -4632,8 +4621,15 @@ static int scx_ops_enable(struct sched_ext_ops *ops, struct bpf_link *link)
 	spin_lock_irq(&scx_tasks_lock);
 
 	scx_task_iter_init(&sti);
-	while ((p = scx_task_iter_next_locked(&sti, false))) {
-		get_task_struct(p);
+	while ((p = scx_task_iter_next_locked(&sti))) {
+		/*
+		 * @p may already be dead, have lost all its usages counts and
+		 * be waiting for RCU grace period before being freed. @p can't
+		 * be initialized for SCX in such cases and should be ignored.
+		 */
+		if (!tryget_task_struct(p))
+			continue;
+
 		scx_task_iter_rq_unlock(&sti);
 		spin_unlock_irq(&scx_tasks_lock);
 
@@ -4686,7 +4682,7 @@ static int scx_ops_enable(struct sched_ext_ops *ops, struct bpf_link *link)
 	WRITE_ONCE(scx_switching_all, !(ops->flags & SCX_OPS_SWITCH_PARTIAL));
 
 	scx_task_iter_init(&sti);
-	while ((p = scx_task_iter_next_locked(&sti, false))) {
+	while ((p = scx_task_iter_next_locked(&sti))) {
 		const struct sched_class *old_class = p->sched_class;
 		struct sched_enq_and_set_ctx ctx;