samples: extend hung_task detector test with semaphore support

Extend the existing hung_task detector test module to support multiple
lock types, including mutex and semaphore, with room for future additions
(e.g., spinlock, etc.).  This module creates dummy files under
<debugfs>/hung_task, such as 'mutex' and 'semaphore'.  The read process on
any of these files will sleep for enough long time (256 seconds) while
holding the respective lock.  As a result, the second process will wait on
the lock for a prolonged duration and be detected by the hung_task
detector.

This change unifies the previous mutex-only sample into a single,
extensible hung_task_tests module, reducing code duplication and improving
maintainability.

Usage is:

	> cd /sys/kernel/debug/hung_task
	> cat mutex & cat mutex          # Test mutex blocking
	> cat semaphore & cat semaphore  # Test semaphore blocking

Update the Kconfig description to reflect multiple debugfs files support.

Link: https://lkml.kernel.org/r/20250414145945.84916-4-ioworker0@gmail.com
Signed-off-by: Lance Yang <ioworker0@gmail.com>
Signed-off-by: Zi Li <amaindex@outlook.com>
Suggested-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Acked-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: John Stultz <jstultz@google.com>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: Mingzhe Yang <mingzhe.yang@ly.com>
Cc: Peter Zijlstra <peterz@infradead.org>
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

@@ -304,10 +304,11 @@ 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
+	  Build a module that provides debugfs files (e.g., mutex, semaphore,
-	  the file, it will sleep long time (256 seconds) with holding a
+	  etc.) under <debugfs>/hung_task. If user reads one of these files,
-	  mutex. Thus if there are 2 or more processes read this file, it
+	  it will sleep long time (256 seconds) with holding a lock. Thus,
-	  will be detected by the hung_task watchdog.
+	  if 2 or more processes read the same file concurrently, it will
+	  be detected by the hung_task watchdog.
 
 source "samples/rust/Kconfig"
 

samples/hung_task/Makefile

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

samples/hung_task/hung_task_mutex.c

@@ -1,66 +0,0 @@
-// 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");

samples/hung_task/hung_task_tests.c

@@ -0,0 +1,97 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * hung_task_tests.c - Sample code for testing hung tasks with mutex,
+ * semaphore, etc.
+ *
+ * Usage: Load this module and read `<debugfs>/hung_task/mutex`,
+ *        `<debugfs>/hung_task/semaphore`, etc., with 2 or more processes.
+ *
+ * This is for testing kernel hung_task error messages with various locking
+ * mechanisms (e.g., mutex, semaphore, etc.). Note that this may freeze
+ * your system or cause a panic. Use only for testing purposes.
+ */
+
+#include <linux/debugfs.h>
+#include <linux/delay.h>
+#include <linux/fs.h>
+#include <linux/module.h>
+#include <linux/mutex.h>
+#include <linux/semaphore.h>
+
+#define HUNG_TASK_DIR		"hung_task"
+#define HUNG_TASK_MUTEX_FILE	"mutex"
+#define HUNG_TASK_SEM_FILE	"semaphore"
+#define SLEEP_SECOND		256
+
+static const char dummy_string[] = "This is a dummy string.";
+static DEFINE_MUTEX(dummy_mutex);
+static DEFINE_SEMAPHORE(dummy_sem, 1);
+static struct dentry *hung_task_dir;
+
+/* Mutex-based read function */
+static ssize_t read_dummy_mutex(struct file *file, char __user *user_buf,
+				size_t count, loff_t *ppos)
+{
+	/* Second task waits on mutex, entering uninterruptible sleep */
+	guard(mutex)(&dummy_mutex);
+
+	/* First task sleeps here, interruptible */
+	msleep_interruptible(SLEEP_SECOND * 1000);
+
+	return simple_read_from_buffer(user_buf, count, ppos, dummy_string,
+				       sizeof(dummy_string));
+}
+
+/* Semaphore-based read function */
+static ssize_t read_dummy_semaphore(struct file *file, char __user *user_buf,
+				    size_t count, loff_t *ppos)
+{
+	/* Second task waits on semaphore, entering uninterruptible sleep */
+	down(&dummy_sem);
+
+	/* First task sleeps here, interruptible */
+	msleep_interruptible(SLEEP_SECOND * 1000);
+
+	up(&dummy_sem);
+
+	return simple_read_from_buffer(user_buf, count, ppos, dummy_string,
+				       sizeof(dummy_string));
+}
+
+/* File operations for mutex */
+static const struct file_operations hung_task_mutex_fops = {
+	.read = read_dummy_mutex,
+};
+
+/* File operations for semaphore */
+static const struct file_operations hung_task_sem_fops = {
+	.read = read_dummy_semaphore,
+};
+
+static int __init hung_task_tests_init(void)
+{
+	hung_task_dir = debugfs_create_dir(HUNG_TASK_DIR, NULL);
+	if (IS_ERR(hung_task_dir))
+		return PTR_ERR(hung_task_dir);
+
+	/* Create debugfs files for mutex and semaphore tests */
+	debugfs_create_file(HUNG_TASK_MUTEX_FILE, 0400, hung_task_dir, NULL,
+			    &hung_task_mutex_fops);
+	debugfs_create_file(HUNG_TASK_SEM_FILE, 0400, hung_task_dir, NULL,
+			    &hung_task_sem_fops);
+
+	return 0;
+}
+
+static void __exit hung_task_tests_exit(void)
+{
+	debugfs_remove_recursive(hung_task_dir);
+}
+
+module_init(hung_task_tests_init);
+module_exit(hung_task_tests_exit);
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Masami Hiramatsu <mhiramat@kernel.org>");
+MODULE_AUTHOR("Zi Li <amaindex@outlook.com>");
+MODULE_DESCRIPTION("Simple sleep under lock files for testing hung task");