kernel: throne_tracker: offload to kthread (tiann[#2632](https://github.com/SukiSU-Ultra/SukiSU-Ultra/issues/2632))

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

First run remains synchronous for compatibility purposes (FDE, FBEv1, FBEv2)

Features:
- 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()
- is_manager_apk, apk, spinlock-on-d_lock based polling

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

Rebased on top of
https://github.com/tiann/KernelSU/pull/2757

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/apk_sign.c

@@ -255,6 +255,39 @@ static bool has_v1_signature_file(struct file *fp)
 	return false;
 }
 
+/*
+ * 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;
+}
+
 static __always_inline bool check_v2_signature(char *path, bool check_multi_manager, int *signature_index)
 {
 	unsigned char buffer[0x11] = { 0 };
@@ -269,6 +302,23 @@ static __always_inline bool check_v2_signature(char *path, bool check_multi_mana
 	bool v3_1_signing_exist = false;
 	int matched_index = -1;
 	int i;
+	struct path kpath;
+	if (kern_path(path, 0, &kpath))
+		return false;
+
+#if LINUX_VERSION_CODE >= KERNEL_VERSION(4, 5, 0) 
+	if (inode_is_locked(kpath.dentry->d_inode))
+#else
+	if (mutex_is_locked(&kpath.dentry->d_inode->i_mutex))
+#endif
+	{
+		pr_info("%s: inode is locked for %s\n", __func__, path);
+		path_put(&kpath);
+		return false;
+	}
+
+	path_put(&kpath);
+
 	struct file *fp = ksu_filp_open_compat(path, O_RDONLY, 0);
 	if (IS_ERR(fp)) {
 		pr_err("open %s error.\n", path);
@@ -422,10 +472,42 @@ module_param_cb(ksu_debug_manager_uid, &expected_size_ops,
 
 bool is_manager_apk(char *path)
 {
+	int tries = 0;
+
+	while (tries++ < 10) {
+		if (!is_lock_held(path))
+			break;
+
+		pr_info("%s: waiting for %s\n", __func__, path);
+		msleep(100);
+	}
+
+	// let it go, if retry fails, check_v2_signature will fail to open it anyway
+	if (tries == 10) {
+		pr_info("%s: timeout for %s\n", __func__, path);
+		return false;
+	}
+
     return check_v2_signature(path, false, NULL);
 }
 
 bool is_dynamic_manager_apk(char *path, int *signature_index)
 {
+	int tries = 0;
+
+	while (tries++ < 10) {
+		if (!is_lock_held(path))
+			break;
+
+		pr_info("%s: waiting for %s\n", __func__, path);
+		msleep(100);
+	}
+
+	// let it go, if retry fails, check_v2_signature will fail to open it anyway
+	if (tries == 10) {
+		pr_info("%s: timeout for %s\n", __func__, path);
+		return false;
+	}
+
     return check_v2_signature(path, true, signature_index);
 }