samples: enhance hung_task detector test with read-write semaphore support

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.

Link: https://lkml.kernel.org/r/20250627072924.36567-4-lance.yang@linux.dev
Signed-off-by: Zi Li <zi.li@linux.dev>
Suggested-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>

diff --git a/samples/Kconfig b/samples/Kconfig
index ffef999502066814362dfbcf8d423827d41b0cb2..a8880c62d4c88ba90ac8da0bd3a114a53a985f13 100644 (file)
--- 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 a5c09bd3a47df2de766225d9ea7b6f17ca2903c2..0360ec916890b3291a7067e370b5cb6ce9d4606d 100644 (file)
--- 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;
 }