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Message-ID: <20250314144300.32542-4-ioworker0@gmail.com>
Date: Fri, 14 Mar 2025 22:43:00 +0800
From: Lance Yang <ioworker0@...il.com>
To: akpm@...ux-foundation.org
Cc: will@...nel.org,
	peterz@...radead.org,
	mingo@...hat.com,
	longman@...hat.com,
	mhiramat@...nel.org,
	anna.schumaker@...cle.com,
	boqun.feng@...il.com,
	joel.granados@...nel.org,
	kent.overstreet@...ux.dev,
	leonylgao@...cent.com,
	linux-kernel@...r.kernel.org,
	rostedt@...dmis.org,
	senozhatsky@...omium.org,
	tfiga@...omium.org,
	amaindex@...look.com,
	Lance Yang <ioworker0@...il.com>
Subject: [PATCH RESEND v2 3/3] samples: add hung_task detector semaphore blocking sample

From: Zi Li <amaindex@...look.com>

Add a hung_task detector semaphore blocking test sample code.

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

Usage is;

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

and wait for hung_task message.

Signed-off-by: Lance Yang <ioworker0@...il.com>
Signed-off-by: Zi Li <amaindex@...look.com>
---
 samples/Kconfig                         | 11 ++--
 samples/hung_task/Makefile              |  3 +-
 samples/hung_task/hung_task_mutex.c     | 20 ++++---
 samples/hung_task/hung_task_semaphore.c | 74 +++++++++++++++++++++++++
 4 files changed, 96 insertions(+), 12 deletions(-)
 create mode 100644 samples/hung_task/hung_task_semaphore.c

diff --git a/samples/Kconfig b/samples/Kconfig
index 09011be2391a..3a073d6b848b 100644
--- a/samples/Kconfig
+++ b/samples/Kconfig
@@ -304,10 +304,13 @@ 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.
+	  Build multiple modules to test the hung task detector. Each module
+	  provides a simple debugfs file corresponding to a specific
+	  synchronization primitive (e.g., mutex, semaphore, etc.). When the
+	  file is read, the module will sleep for a long time (256 seconds)
+	  while holding the respective synchronizer. If multiple processes
+	  attempt to read these files concurrently, the hung_task watchdog
+	  can detect potential hangs or deadlocks.
 
 source "samples/rust/Kconfig"
 
diff --git a/samples/hung_task/Makefile b/samples/hung_task/Makefile
index fe9dde799880..7483c2c0a0ef 100644
--- a/samples/hung_task/Makefile
+++ b/samples/hung_task/Makefile
@@ -1,2 +1,3 @@
 # SPDX-License-Identifier: GPL-2.0-only
-obj-$(CONFIG_SAMPLE_HUNG_TASK) += hung_task_mutex.o
\ No newline at end of file
+obj-$(CONFIG_SAMPLE_HUNG_TASK) += hung_task_mutex.o
+obj-$(CONFIG_SAMPLE_HUNG_TASK) += hung_task_semaphore.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
index 7a29f2246d22..e4d1d69618b8 100644
--- a/samples/hung_task/hung_task_mutex.c
+++ b/samples/hung_task/hung_task_mutex.c
@@ -22,7 +22,7 @@
 
 static const char dummy_string[] = "This is a dummy string.";
 static DEFINE_MUTEX(dummy_mutex);
-struct dentry *hung_task_dir;
+static struct dentry *hung_task_dir;
 
 static ssize_t read_dummy(struct file *file, char __user *user_buf,
 			  size_t count, loff_t *ppos)
@@ -43,19 +43,25 @@ static const struct file_operations hung_task_fops = {
 
 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);
+	hung_task_dir = debugfs_lookup(HUNG_TASK_DIR, NULL);
+	if (!hung_task_dir) {
+		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);
+	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);
+	debugfs_lookup_and_remove(HUNG_TASK_FILE, hung_task_dir);
+
+	if (simple_empty(hung_task_dir))
+		debugfs_remove(hung_task_dir);
 }
 
 module_init(hung_task_sample_init);
diff --git a/samples/hung_task/hung_task_semaphore.c b/samples/hung_task/hung_task_semaphore.c
new file mode 100644
index 000000000000..a5814971bfb8
--- /dev/null
+++ b/samples/hung_task/hung_task_semaphore.c
@@ -0,0 +1,74 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * hung_task_semaphore.c - Sample code which causes hung task by semaphore
+ *
+ * Usage: load this module and read `<debugfs>/hung_task/semaphore`
+ *        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/semaphore.h>
+
+#define HUNG_TASK_DIR   "hung_task"
+#define HUNG_TASK_FILE  "semaphore"
+#define SLEEP_SECOND 256
+
+static const char dummy_string[] = "This is a dummy string.";
+static DEFINE_SEMAPHORE(dummy_sem, 1);
+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 semaphore, it is uninterruptible sleep. */
+	down(&dummy_sem);
+
+	/* When the first task sleep here, it is interruptible. */
+	msleep_interruptible(SLEEP_SECOND * 1000);
+
+	up(&dummy_sem);
+
+	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_lookup(HUNG_TASK_DIR, NULL);
+	if (!hung_task_dir) {
+		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_lookup_and_remove(HUNG_TASK_FILE, hung_task_dir);
+
+	if (simple_empty(hung_task_dir))
+		debugfs_remove(hung_task_dir);
+}
+
+module_init(hung_task_sample_init);
+module_exit(hung_task_sample_exit);
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Zi Li");
+MODULE_DESCRIPTION("Simple sleep under semaphore file for testing hung task");
-- 
2.45.2


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