[PATCH] ext4: fix panic on module unload when stopping lazyinit thread

[Eric Sandeen <sandeen@redhat.com>]

This is for the latest reported module unload oops in
kernel.org bugzilla #27652

If the lazyinit thread is running, the teardown
function ext4_destroy_lazyinit_thread() has problems:

        ext4_clear_request_list();
        while (ext4_li_info->li_task) {
                wake_up(&ext4_li_info->li_wait_daemon);
                wait_event(ext4_li_info->li_wait_task,
                           ext4_li_info->li_task == NULL);
        }

clearing the request list will cause the thread to exit
and free ext4_li_info, so then we're waiting on something
which is getting freed.

Fix this up by making the thread respond to kthread_stop,
and exit, without the need to wait for that exit in some
other homegrown way.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
---

Index: linux-2.6/fs/ext4/super.c
===================================================================
--- linux-2.6.orig/fs/ext4/super.c
+++ linux-2.6/fs/ext4/super.c
@@ -77,6 +77,7 @@ static struct dentry *ext4_mount(struct 
 		       const char *dev_name, void *data);
 static void ext4_destroy_lazyinit_thread(void);
 static void ext4_unregister_li_request(struct super_block *sb);
+static void ext4_clear_request_list(void);
 
 #if !defined(CONFIG_EXT3_FS) && !defined(CONFIG_EXT3_FS_MODULE) && defined(CONFIG_EXT4_USE_FOR_EXT23)
 static struct file_system_type ext3_fs_type = {
@@ -2704,6 +2705,8 @@ static void ext4_unregister_li_request(s
 	mutex_unlock(&ext4_li_info->li_list_mtx);
 }
 
+static struct task_struct *ext4_lazyinit_task;
+
 /*
  * This is the function where ext4lazyinit thread lives. It walks
  * through the request list searching for next scheduled filesystem.
@@ -2772,6 +2775,10 @@ cont_thread:
 		if (time_before(jiffies, next_wakeup))
 			schedule();
 		finish_wait(&eli->li_wait_daemon, &wait);
+		if (kthread_should_stop()) {
+			ext4_clear_request_list();
+			goto exit_thread;
+		}
 	}
 
 exit_thread:
@@ -2796,6 +2803,7 @@ exit_thread:
 	wake_up(&eli->li_wait_task);
 
 	kfree(ext4_li_info);
+	ext4_lazyinit_task = NULL;
 	ext4_li_info = NULL;
 	mutex_unlock(&ext4_li_mtx);
 
@@ -2818,11 +2826,10 @@ static void ext4_clear_request_list(void
 
 static int ext4_run_lazyinit_thread(void)
 {
-	struct task_struct *t;
-
-	t = kthread_run(ext4_lazyinit_thread, ext4_li_info, "ext4lazyinit");
-	if (IS_ERR(t)) {
-		int err = PTR_ERR(t);
+	ext4_lazyinit_task = kthread_run(ext4_lazyinit_thread,
+					 ext4_li_info, "ext4lazyinit");
+	if (IS_ERR(ext4_lazyinit_task)) {
+		int err = PTR_ERR(ext4_lazyinit_task);
 		ext4_clear_request_list();
 		del_timer_sync(&ext4_li_info->li_timer);
 		kfree(ext4_li_info);
@@ -2973,16 +2980,10 @@ static void ext4_destroy_lazyinit_thread
 	 * If thread exited earlier
 	 * there's nothing to be done.
 	 */
-	if (!ext4_li_info)
+	if (!ext4_li_info || !ext4_lazyinit_task)
 		return;
 
-	ext4_clear_request_list();