samples: add hung_task detector mutex blocking sample

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.

[akpm@linux-foundation.org: make `hung_task_dir' static]
  Closes: https://lore.kernel.org/oe-kbuild-all/202503180827.4StpuFrD-lkp@intel.com/
Link: https://lkml.kernel.org/r/174046696281.2194069.4567490148001547311.stgit@mhiramat.tok.corp.google.com
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Cc: Anna Schumaker <anna.schumaker@oracle.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Joel Granados <joel.granados@kernel.org>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: Lance Yang <ioworker0@gmail.com>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Tomasz Figa <tfiga@chromium.org>
Cc: Waiman Long <longman@redhat.com>
Cc: Will Deacon <will@kernel.org>
Cc: Yongliang Gao <leonylgao@tencent.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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"

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/

samples/hung_task/Makefile

@@ -0,0 +1,2 @@
+# SPDX-License-Identifier: GPL-2.0-only
+obj-$(CONFIG_SAMPLE_HUNG_TASK) += hung_task_mutex.o

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);
+static 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");