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Message-ID: <20250627072924.36567-4-lance.yang@linux.dev>
Date: Fri, 27 Jun 2025 15:29:24 +0800
From: Lance Yang <ioworker0@...il.com>
To: akpm@...ux-foundation.org
Cc: zi.li@...ux.dev,
anna.schumaker@...cle.com,
boqun.feng@...il.com,
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peterz@...radead.org,
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will@...nel.org
Subject: [PATCH 3/3] samples: enhance hung_task detector test with read-write semaphore support
From: Zi Li <zi.li@...ux.dev>
Extend the hung_task detector test module to include read-write semaphore
support alongside existing mutex and semaphore tests. This module now
creates additional debugfs files under <debugfs>/hung_task, namely
'rw_semaphore_read' and 'rw_semaphore_write', in addition to 'mutex' and
'semaphore'. Reading these files with multiple processes triggers a
prolonged sleep (256 seconds) while holding the respective lock, enabling
hung_task detector testing for various locking mechanisms.
This change builds on the extensible hung_task_tests module, adding
read-write semaphore functionality to improve test coverage for kernel
locking primitives. The implementation ensures proper lock handling and
includes checks to prevent redundant data reads.
Usage is:
> cd /sys/kernel/debug/hung_task
> cat mutex & cat mutex # Test mutex blocking
> cat semaphore & cat semaphore # Test semaphore blocking
> cat rw_semaphore_write \
& cat rw_semaphore_read # Test rwsem blocking
> cat rw_semaphore_write \
& cat rw_semaphore_write # Test rwsem blocking
Update the Kconfig description to reflect the addition of read-write
semaphore debugfs files.
Suggested-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
Signed-off-by: Zi Li <zi.li@...ux.dev>
---
samples/Kconfig | 7 ++-
samples/hung_task/hung_task_tests.c | 81 ++++++++++++++++++++++++++---
2 files changed, 77 insertions(+), 11 deletions(-)
diff --git a/samples/Kconfig b/samples/Kconfig
index ffef99950206..a8880c62d4c8 100644
--- a/samples/Kconfig
+++ b/samples/Kconfig
@@ -316,10 +316,9 @@ config SAMPLE_HUNG_TASK
depends on DETECT_HUNG_TASK && DEBUG_FS
help
Build a module that provides debugfs files (e.g., mutex, semaphore,
- etc.) under <debugfs>/hung_task. If user reads one of these files,
- it will sleep long time (256 seconds) with holding a lock. Thus,
- if 2 or more processes read the same file concurrently, it will
- be detected by the hung_task watchdog.
+ rw_semaphore_read, rw_semaphore_write) under <debugfs>/hung_task.
+ Reading these files with multiple processes triggers hung task
+ detection by holding locks for a long time (256 seconds).
source "samples/rust/Kconfig"
diff --git a/samples/hung_task/hung_task_tests.c b/samples/hung_task/hung_task_tests.c
index a5c09bd3a47d..0360ec916890 100644
--- a/samples/hung_task/hung_task_tests.c
+++ b/samples/hung_task/hung_task_tests.c
@@ -4,11 +4,12 @@
* semaphore, etc.
*
* Usage: Load this module and read `<debugfs>/hung_task/mutex`,
- * `<debugfs>/hung_task/semaphore`, etc., with 2 or more processes.
+ * `<debugfs>/hung_task/semaphore`, `<debugfs>/hung_task/rw_semaphore_read`,
+ * `<debugfs>/hung_task/rw_semaphore_write`, 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.
+ * mechanisms (e.g., mutex, semaphore, rw_semaphore_read, rw_semaphore_write, etc.).
+ * Note that this may freeze your system or cause a panic. Use only for testing purposes.
*/
#include <linux/debugfs.h>
@@ -17,21 +18,29 @@
#include <linux/module.h>
#include <linux/mutex.h>
#include <linux/semaphore.h>
+#include <linux/rwsem.h>
-#define HUNG_TASK_DIR "hung_task"
-#define HUNG_TASK_MUTEX_FILE "mutex"
-#define HUNG_TASK_SEM_FILE "semaphore"
-#define SLEEP_SECOND 256
+#define HUNG_TASK_DIR "hung_task"
+#define HUNG_TASK_MUTEX_FILE "mutex"
+#define HUNG_TASK_SEM_FILE "semaphore"
+#define HUNG_TASK_RWSEM_READ_FILE "rw_semaphore_read"
+#define HUNG_TASK_RWSEM_WRITE_FILE "rw_semaphore_write"
+#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 DECLARE_RWSEM(dummy_rwsem);
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)
{
+ /* Check if data is already read */
+ if (*ppos >= sizeof(dummy_string))
+ return 0;
+
/* Second task waits on mutex, entering uninterruptible sleep */
guard(mutex)(&dummy_mutex);
@@ -46,6 +55,10 @@ static ssize_t read_dummy_mutex(struct file *file, char __user *user_buf,
static ssize_t read_dummy_semaphore(struct file *file, char __user *user_buf,
size_t count, loff_t *ppos)
{
+ /* Check if data is already read */
+ if (*ppos >= sizeof(dummy_string))
+ return 0;
+
/* Second task waits on semaphore, entering uninterruptible sleep */
down(&dummy_sem);
@@ -58,6 +71,46 @@ static ssize_t read_dummy_semaphore(struct file *file, char __user *user_buf,
sizeof(dummy_string));
}
+/* Read-write semaphore read function */
+static ssize_t read_dummy_rwsem_read(struct file *file, char __user *user_buf,
+ size_t count, loff_t *ppos)
+{
+ /* Check if data is already read */
+ if (*ppos >= sizeof(dummy_string))
+ return 0;
+
+ /* Acquires read lock, allowing concurrent readers but blocks if write lock is held */
+ down_read(&dummy_rwsem);
+
+ /* Sleeps here, potentially triggering hung task detection if lock is held too long */
+ msleep_interruptible(SLEEP_SECOND * 1000);
+
+ up_read(&dummy_rwsem);
+
+ return simple_read_from_buffer(user_buf, count, ppos, dummy_string,
+ sizeof(dummy_string));
+}
+
+/* Read-write semaphore write function */
+static ssize_t read_dummy_rwsem_write(struct file *file, char __user *user_buf,
+ size_t count, loff_t *ppos)
+{
+ /* Check if data is already read */
+ if (*ppos >= sizeof(dummy_string))
+ return 0;
+
+ /* Acquires exclusive write lock, blocking all other readers and writers */
+ down_write(&dummy_rwsem);
+
+ /* Sleeps here, potentially triggering hung task detection if lock is held too long */
+ msleep_interruptible(SLEEP_SECOND * 1000);
+
+ up_write(&dummy_rwsem);
+
+ 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,
@@ -68,6 +121,16 @@ static const struct file_operations hung_task_sem_fops = {
.read = read_dummy_semaphore,
};
+/* File operations for rw_semaphore read */
+static const struct file_operations hung_task_rwsem_read_fops = {
+ .read = read_dummy_rwsem_read,
+};
+
+/* File operations for rw_semaphore write */
+static const struct file_operations hung_task_rwsem_write_fops = {
+ .read = read_dummy_rwsem_write,
+};
+
static int __init hung_task_tests_init(void)
{
hung_task_dir = debugfs_create_dir(HUNG_TASK_DIR, NULL);
@@ -79,6 +142,10 @@ static int __init hung_task_tests_init(void)
&hung_task_mutex_fops);
debugfs_create_file(HUNG_TASK_SEM_FILE, 0400, hung_task_dir, NULL,
&hung_task_sem_fops);
+ debugfs_create_file(HUNG_TASK_RWSEM_READ_FILE, 0400, hung_task_dir, NULL,
+ &hung_task_rwsem_read_fops);
+ debugfs_create_file(HUNG_TASK_RWSEM_WRITE_FILE, 0400, hung_task_dir, NULL,
+ &hung_task_rwsem_write_fops);
return 0;
}
--
2.49.0
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