1 // SPDX-License-Identifier: GPL-2.0-or-later
2 /*
3 * hung_task_tests.c - Sample code for testing hung tasks with mutex,
4 * semaphore, etc.
5 *
6 * Usage: Load this module and read `<debugfs>/hung_task/mutex`,
7 * `<debugfs>/hung_task/semaphore`, `<debugfs>/hung_task/rw_semaphore_read`,
8 * `<debugfs>/hung_task/rw_semaphore_write`, etc., with 2 or more processes.
9 *
10 * This is for testing kernel hung_task error messages with various locking
11 * mechanisms (e.g., mutex, semaphore, rw_semaphore_read, rw_semaphore_write, etc.).
12 * Note that this may freeze your system or cause a panic. Use only for testing purposes.
13 */
14
15 #include <linux/debugfs.h>
16 #include <linux/delay.h>
17 #include <linux/fs.h>
18 #include <linux/module.h>
19 #include <linux/mutex.h>
20 #include <linux/semaphore.h>
21 #include <linux/rwsem.h>
22
23 #define HUNG_TASK_DIR "hung_task"
24 #define HUNG_TASK_MUTEX_FILE "mutex"
25 #define HUNG_TASK_SEM_FILE "semaphore"
26 #define HUNG_TASK_RWSEM_READ_FILE "rw_semaphore_read"
27 #define HUNG_TASK_RWSEM_WRITE_FILE "rw_semaphore_write"
28 #define SLEEP_SECOND 256
29
30 static const char dummy_string[] = "This is a dummy string.";
31 static DEFINE_MUTEX(dummy_mutex);
32 static DEFINE_SEMAPHORE(dummy_sem, 1);
33 static DECLARE_RWSEM(dummy_rwsem);
34 static struct dentry *hung_task_dir;
35
36 /* Mutex-based read function */
read_dummy_mutex(struct file * file,char __user * user_buf,size_t count,loff_t * ppos)37 static ssize_t read_dummy_mutex(struct file *file, char __user *user_buf,
38 size_t count, loff_t *ppos)
39 {
40 /* Check if data is already read */
41 if (*ppos >= sizeof(dummy_string))
42 return 0;
43
44 /* Second task waits on mutex, entering uninterruptible sleep */
45 guard(mutex)(&dummy_mutex);
46
47 /* First task sleeps here, interruptible */
48 msleep_interruptible(SLEEP_SECOND * 1000);
49
50 return simple_read_from_buffer(user_buf, count, ppos, dummy_string,
51 sizeof(dummy_string));
52 }
53
54 /* Semaphore-based read function */
read_dummy_semaphore(struct file * file,char __user * user_buf,size_t count,loff_t * ppos)55 static ssize_t read_dummy_semaphore(struct file *file, char __user *user_buf,
56 size_t count, loff_t *ppos)
57 {
58 /* Check if data is already read */
59 if (*ppos >= sizeof(dummy_string))
60 return 0;
61
62 /* Second task waits on semaphore, entering uninterruptible sleep */
63 down(&dummy_sem);
64
65 /* First task sleeps here, interruptible */
66 msleep_interruptible(SLEEP_SECOND * 1000);
67
68 up(&dummy_sem);
69
70 return simple_read_from_buffer(user_buf, count, ppos, dummy_string,
71 sizeof(dummy_string));
72 }
73
74 /* Read-write semaphore read function */
read_dummy_rwsem_read(struct file * file,char __user * user_buf,size_t count,loff_t * ppos)75 static ssize_t read_dummy_rwsem_read(struct file *file, char __user *user_buf,
76 size_t count, loff_t *ppos)
77 {
78 /* Check if data is already read */
79 if (*ppos >= sizeof(dummy_string))
80 return 0;
81
82 /* Acquires read lock, allowing concurrent readers but blocks if write lock is held */
83 down_read(&dummy_rwsem);
84
85 /* Sleeps here, potentially triggering hung task detection if lock is held too long */
86 msleep_interruptible(SLEEP_SECOND * 1000);
87
88 up_read(&dummy_rwsem);
89
90 return simple_read_from_buffer(user_buf, count, ppos, dummy_string,
91 sizeof(dummy_string));
92 }
93
94 /* Read-write semaphore write function */
read_dummy_rwsem_write(struct file * file,char __user * user_buf,size_t count,loff_t * ppos)95 static ssize_t read_dummy_rwsem_write(struct file *file, char __user *user_buf,
96 size_t count, loff_t *ppos)
97 {
98 /* Check if data is already read */
99 if (*ppos >= sizeof(dummy_string))
100 return 0;
101
102 /* Acquires exclusive write lock, blocking all other readers and writers */
103 down_write(&dummy_rwsem);
104
105 /* Sleeps here, potentially triggering hung task detection if lock is held too long */
106 msleep_interruptible(SLEEP_SECOND * 1000);
107
108 up_write(&dummy_rwsem);
109
110 return simple_read_from_buffer(user_buf, count, ppos, dummy_string,
111 sizeof(dummy_string));
112 }
113
114 /* File operations for mutex */
115 static const struct file_operations hung_task_mutex_fops = {
116 .read = read_dummy_mutex,
117 };
118
119 /* File operations for semaphore */
120 static const struct file_operations hung_task_sem_fops = {
121 .read = read_dummy_semaphore,
122 };
123
124 /* File operations for rw_semaphore read */
125 static const struct file_operations hung_task_rwsem_read_fops = {
126 .read = read_dummy_rwsem_read,
127 };
128
129 /* File operations for rw_semaphore write */
130 static const struct file_operations hung_task_rwsem_write_fops = {
131 .read = read_dummy_rwsem_write,
132 };
133
hung_task_tests_init(void)134 static int __init hung_task_tests_init(void)
135 {
136 hung_task_dir = debugfs_create_dir(HUNG_TASK_DIR, NULL);
137 if (IS_ERR(hung_task_dir))
138 return PTR_ERR(hung_task_dir);
139
140 /* Create debugfs files for mutex and semaphore tests */
141 debugfs_create_file(HUNG_TASK_MUTEX_FILE, 0400, hung_task_dir, NULL,
142 &hung_task_mutex_fops);
143 debugfs_create_file(HUNG_TASK_SEM_FILE, 0400, hung_task_dir, NULL,
144 &hung_task_sem_fops);
145 debugfs_create_file(HUNG_TASK_RWSEM_READ_FILE, 0400, hung_task_dir, NULL,
146 &hung_task_rwsem_read_fops);
147 debugfs_create_file(HUNG_TASK_RWSEM_WRITE_FILE, 0400, hung_task_dir, NULL,
148 &hung_task_rwsem_write_fops);
149
150 return 0;
151 }
152
hung_task_tests_exit(void)153 static void __exit hung_task_tests_exit(void)
154 {
155 debugfs_remove_recursive(hung_task_dir);
156 }
157
158 module_init(hung_task_tests_init);
159 module_exit(hung_task_tests_exit);
160
161 MODULE_LICENSE("GPL");
162 MODULE_AUTHOR("Masami Hiramatsu <mhiramat@kernel.org>");
163 MODULE_AUTHOR("Zi Li <amaindex@outlook.com>");
164 MODULE_DESCRIPTION("Simple sleep under lock files for testing hung task");
165