kernel: throne_tracker: offload to kthread tiann #2632

Run throne_tracker() in kthread instead of blocking the caller.
Prevents full lockup during installation and removing the manager.

By default, first run remains synchronous for compatibility purposes
(FDE, FBEv1, FBEv2)

Features:
- looks and waits for manager UID in /data/system/packages.list
- run track_throne() in a kthread after the first synchronous run
- prevent duplicate thread creation with a single-instance check
- spinlock-on-d_lock based polling adressing possible race conditions.

Race conditions adressed
- single instance kthread lock, smp_mb()
- track_throne_function, packages.list, spinlock-on-d_lock based polling
- is_manager_apk, apk, spinlock-on-d_lock based polling

This is a squash of:
https://github.com/tiann/KernelSU/pull/2632

Original skeleton based on:
`kernelsu: move throne_tracker() to kthread`
`kernelsu: check locking before accessing files and dirs during searching manager`
`kernelsu: look for manager UID in /data/system/packages.list, not /data/system/packages.list.tmp`
0b05e927...8783badd

Co-Authored-By: backslashxx <118538522+backslashxx@users.noreply.github.com>
Co-Authored-By: Yaroslav Zviezda <10716792+acroreiser@users.noreply.github.com>
Signed-off-by: backslashxx <118538522+backslashxx@users.noreply.github.com>

kernel/throne_tracker.h

@@ -7,4 +7,37 @@ void ksu_throne_tracker_exit();
 
 void track_throne();
 
+/*
+ * small helper to check if lock is held
+ * false - file is stable
+ * true - file is being deleted/renamed
+ * possibly optional
+ *
+ */
+static bool is_lock_held(const char *path) 
+{
+	struct path kpath;
+
+	// kern_path returns 0 on success
+	if (kern_path(path, 0, &kpath))
+		return true;
+
+	// just being defensive
+	if (!kpath.dentry) {
+		path_put(&kpath);
+		return true;
+	}
+
+	if (!spin_trylock(&kpath.dentry->d_lock)) {
+		pr_info("%s: lock held, bail out!\n", __func__);
+		path_put(&kpath);
+		return true;
+	}
+	// we hold it ourselves here!
+
+	spin_unlock(&kpath.dentry->d_lock);
+	path_put(&kpath);
+	return false;
+}
+
 #endif