[PATCH 2/2] samples: Add hung_task detector mutex blocking sample

[Masami Hiramatsu (Google)]

From: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>

Add a hung_task detector mutex blocking test sample code.

This module will create a dummy file on the debugfs. That file will
cause the read process to sleep for enough long time (256 seconds)
while holding a mutex. As a result, the second process will wait on
the mutex for a prolonged duration and be detected by the hung_task
detector.

Usage is;

 > cd /sys/kernel/debug/hung_task
 > cat mutex & cat mutex

and wait for hung_task message.

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
---
 samples/Kconfig                     |    9 +++++
 samples/Makefile                    |    1 +
 samples/hung_task/Makefile          |    2 +
 samples/hung_task/hung_task_mutex.c |   66 +++++++++++++++++++++++++++++++++++
 4 files changed, 78 insertions(+)
 create mode 100644 samples/hung_task/Makefile
 create mode 100644 samples/hung_task/hung_task_mutex.c

diff --git a/samples/Kconfig b/samples/Kconfig
index 820e00b2ed68..09011be2391a 100644
--- a/samples/Kconfig
+++ b/samples/Kconfig
@@ -300,6 +300,15 @@ config SAMPLE_CHECK_EXEC
 	  demonstrate how they should be used with execveat(2) +
 	  AT_EXECVE_CHECK.
 
+config SAMPLE_HUNG_TASK
+	tristate "Hung task detector test code"
+	depends on DETECT_HUNG_TASK && DEBUG_FS
+	help
+	  Build a module which provide a simple debugfs file. If user reads
+	  the file, it will sleep long time (256 seconds) with holding a
+	  mutex. Thus if there are 2 or more processes read this file, it
+	  will be detected by the hung_task watchdog.
+
 source "samples/rust/Kconfig"
 
 source "samples/damon/Kconfig"
diff --git a/samples/Makefile b/samples/Makefile
index f24cd0d72dd0..bf6e6fca5410 100644
--- a/samples/Makefile
+++ b/samples/Makefile
@@ -42,3 +42,4 @@ obj-$(CONFIG_SAMPLE_FPROBE)		+= fprobe/
 obj-$(CONFIG_SAMPLES_RUST)		+= rust/
 obj-$(CONFIG_SAMPLE_DAMON_WSSE)		+= damon/
 obj-$(CONFIG_SAMPLE_DAMON_PRCL)		+= damon/
+obj-$(CONFIG_SAMPLE_HUNG_TASK)		+= hung_task/
diff --git a/samples/hung_task/Makefile b/samples/hung_task/Makefile
new file mode 100644
index 000000000000..fe9dde799880
--- /dev/null
+++ b/samples/hung_task/Makefile
@@ -0,0 +1,2 @@
+# SPDX-License-Identifier: GPL-2.0-only
+obj-$(CONFIG_SAMPLE_HUNG_TASK) += hung_task_mutex.o
\ No newline at end of file
diff --git a/samples/hung_task/hung_task_mutex.c b/samples/hung_task/hung_task_mutex.c
new file mode 100644
index 000000000000..7a29f2246d22
--- /dev/null
+++ b/samples/hung_task/hung_task_mutex.c
@@ -0,0 +1,66 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * hung_task_mutex.c - Sample code which causes hung task by mutex
+ *
+ * Usage: load this module and read `<debugfs>/hung_task/mutex`
+ *        by 2 or more processes.
+ *
+ * This is for testing kernel hung_task error message.
+ * Note that this will make your system freeze and maybe
+ * cause panic. So do not use this except for the test.
+ */
+
+#include <linux/debugfs.h>
+#include <linux/delay.h>
+#include <linux/fs.h>
+#include <linux/module.h>
+#include <linux/mutex.h>
+
+#define HUNG_TASK_DIR   "hung_task"
+#define HUNG_TASK_FILE  "mutex"
+#define SLEEP_SECOND 256
+
+static const char dummy_string[] = "This is a dummy string.";
+static DEFINE_MUTEX(dummy_mutex);
+struct dentry *hung_task_dir;
+
+static ssize_t read_dummy(struct file *file, char __user *user_buf,
+			  size_t count, loff_t *ppos)
+{
+	/* If the second task waits on the lock, it is uninterruptible sleep. */
+	guard(mutex)(&dummy_mutex);
+
+	/* When the first task sleep here, it is interruptible. */
+	msleep_interruptible(SLEEP_SECOND * 1000);
+
+	return simple_read_from_buffer(user_buf, count, ppos,
+				dummy_string, sizeof(dummy_string));
+}
+
+static const struct file_operations hung_task_fops = {
+	.read = read_dummy,
+};
+
+static int __init hung_task_sample_init(void)
+{
+	hung_task_dir = debugfs_create_dir(HUNG_TASK_DIR, NULL);
+	if (IS_ERR(hung_task_dir))
+		return PTR_ERR(hung_task_dir);
+
+	debugfs_create_file(HUNG_TASK_FILE, 0400, hung_task_dir,
+			    NULL, &hung_task_fops);
+
+	return 0;
+}
+
+static void __exit hung_task_sample_exit(void)
+{
+	debugfs_remove_recursive(hung_task_dir);
+}
+
+module_init(hung_task_sample_init);
+module_exit(hung_task_sample_exit);
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Masami Hiramatsu");
+MODULE_DESCRIPTION("Simple sleep under mutex file for testing hung task");