1 // SPDX-License-Identifier: GPL-2.0-or-later
2 /*
3 * hung_task_mutex.c - Sample code which causes hung task by mutex
4 *
5 * Usage: load this module and read `<debugfs>/hung_task/mutex`
6 * by 2 or more processes.
7 *
8 * This is for testing kernel hung_task error message.
9 * Note that this will make your system freeze and maybe
10 * cause panic. So do not use this except for the test.
11 */
12
13 #include <linux/debugfs.h>
14 #include <linux/delay.h>
15 #include <linux/fs.h>
16 #include <linux/module.h>
17 #include <linux/mutex.h>
18
19 #define HUNG_TASK_DIR "hung_task"
20 #define HUNG_TASK_FILE "mutex"
21 #define SLEEP_SECOND 256
22
23 static const char dummy_string[] = "This is a dummy string.";
24 static DEFINE_MUTEX(dummy_mutex);
25 static struct dentry *hung_task_dir;
26
read_dummy(struct file * file,char __user * user_buf,size_t count,loff_t * ppos)27 static ssize_t read_dummy(struct file *file, char __user *user_buf,
28 size_t count, loff_t *ppos)
29 {
30 /* If the second task waits on the lock, it is uninterruptible sleep. */
31 guard(mutex)(&dummy_mutex);
32
33 /* When the first task sleep here, it is interruptible. */
34 msleep_interruptible(SLEEP_SECOND * 1000);
35
36 return simple_read_from_buffer(user_buf, count, ppos,
37 dummy_string, sizeof(dummy_string));
38 }
39
40 static const struct file_operations hung_task_fops = {
41 .read = read_dummy,
42 };
43
hung_task_sample_init(void)44 static int __init hung_task_sample_init(void)
45 {
46 hung_task_dir = debugfs_create_dir(HUNG_TASK_DIR, NULL);
47 if (IS_ERR(hung_task_dir))
48 return PTR_ERR(hung_task_dir);
49
50 debugfs_create_file(HUNG_TASK_FILE, 0400, hung_task_dir,
51 NULL, &hung_task_fops);
52
53 return 0;
54 }
55
hung_task_sample_exit(void)56 static void __exit hung_task_sample_exit(void)
57 {
58 debugfs_remove_recursive(hung_task_dir);
59 }
60
61 module_init(hung_task_sample_init);
62 module_exit(hung_task_sample_exit);
63
64 MODULE_LICENSE("GPL");
65 MODULE_AUTHOR("Masami Hiramatsu");
66 MODULE_DESCRIPTION("Simple sleep under mutex file for testing hung task");
67