sched_ext: Enable scx_ops_init_task() separately

scx_ops_init_task() and the follow-up scx_ops_enable_task() in the fork path
were gated by scx_enabled() test and thus __scx_ops_enabled had to be turned
on before the first scx_ops_init_task() loop in scx_ops_enable(). However,
if an external entity causes sched_class switch before the loop is complete,
tasks which are not initialized could be switched to SCX.

The following can be reproduced by running a program which keeps toggling a
process between SCHED_OTHER and SCHED_EXT using sched_setscheduler(2).

  sched_ext: Invalid task state transition 0 -> 3 for fish[1623]
  WARNING: CPU: 1 PID: 1650 at kernel/sched/ext.c:3392 scx_ops_enable_task+0x1a1/0x200
  ...
  Sched_ext: simple (enabling)
  RIP: 0010:scx_ops_enable_task+0x1a1/0x200
  ...
   switching_to_scx+0x13/0xa0
   __sched_setscheduler+0x850/0xa50
   do_sched_setscheduler+0x104/0x1c0
   __x64_sys_sched_setscheduler+0x18/0x30
   do_syscall_64+0x7b/0x140
   entry_SYSCALL_64_after_hwframe+0x76/0x7e

Fix it by gating scx_ops_init_task() separately using
scx_ops_init_task_enabled. __scx_ops_enabled is now set after all tasks are
finished with scx_ops_init_task().
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/sched/ext.c

@@ -853,6 +853,7 @@ DEFINE_STATIC_KEY_FALSE(__scx_ops_enabled);
 DEFINE_STATIC_PERCPU_RWSEM(scx_fork_rwsem);
 static atomic_t scx_ops_enable_state_var = ATOMIC_INIT(SCX_OPS_DISABLED);
 static atomic_t scx_ops_bypass_depth = ATOMIC_INIT(0);
+static bool scx_ops_init_task_enabled;
 static bool scx_switching_all;
 DEFINE_STATIC_KEY_FALSE(__scx_switched_all);
 
@@ -3565,7 +3566,7 @@ int scx_fork(struct task_struct *p)
 {
 	percpu_rwsem_assert_held(&scx_fork_rwsem);
 
-	if (scx_enabled())
+	if (scx_ops_init_task_enabled)
 		return scx_ops_init_task(p, task_group(p), true);
 	else
 		return 0;
@@ -3573,7 +3574,7 @@ int scx_fork(struct task_struct *p)
 
 void scx_post_fork(struct task_struct *p)
 {
-	if (scx_enabled()) {
+	if (scx_ops_init_task_enabled) {
 		scx_set_task_state(p, SCX_TASK_READY);
 
 		/*
@@ -4453,6 +4454,8 @@ static void scx_ops_disable_workfn(struct kthread_work *work)
 	cpus_read_lock();
 	scx_cgroup_lock();
 
+	scx_ops_init_task_enabled = false;
+
 	spin_lock_irq(&scx_tasks_lock);
 	scx_task_iter_init(&sti);
 	/*
@@ -5132,7 +5135,8 @@ static int scx_ops_enable(struct sched_ext_ops *ops, struct bpf_link *link)
 	if (ret)
 		goto err_disable_unlock_all;
 
-	static_branch_enable_cpuslocked(&__scx_ops_enabled);
+	WARN_ON_ONCE(scx_ops_init_task_enabled);
+	scx_ops_init_task_enabled = true;
 
 	/*
 	 * Enable ops for every task. Fork is excluded by scx_fork_rwsem
@@ -5175,9 +5179,11 @@ static int scx_ops_enable(struct sched_ext_ops *ops, struct bpf_link *link)
 	spin_unlock_irq(&scx_tasks_lock);
 
 	/*
-	 * All tasks are prepped but the tasks are not enabled. Switch everyone.
+	 * All tasks are READY. It's safe to turn on scx_enabled() and switch
+	 * all eligible tasks.
 	 */
 	WRITE_ONCE(scx_switching_all, !(ops->flags & SCX_OPS_SWITCH_PARTIAL));
+	static_branch_enable_cpuslocked(&__scx_ops_enabled);
 
 	/*
 	 * We're fully committed and can't fail. The task READY -> ENABLED