Commit 4efec6c0 authored by Zi Li's avatar Zi Li Committed by Andrew Morton
Browse files

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: default avatarZi Li <zi.li@linux.dev>
Suggested-by: default avatarMasami 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: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 77da18de

samples/Kconfig

+3 −4
Original line number Diff line number Diff line
@@ -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"

samples/hung_task/hung_task_tests.c

+74 −7
Original line number Diff line number Diff line
@@ -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 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;

}