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Message-Id: <20250318103615.bce4a3291786c2bd525f0f02@kernel.org>
Date: Tue, 18 Mar 2025 10:36:15 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Lance Yang <ioworker0@...il.com>
Cc: akpm@...ux-foundation.org, 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
Subject: Re: [PATCH RESEND v2 3/3] samples: add hung_task detector semaphore
blocking sample
On Fri, 14 Mar 2025 22:43:00 +0800
Lance Yang <ioworker0@...il.com> wrote:
> 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.
Thanks for updating the sample code. BTW, do we need to have almost same
2 samples? I think we can unify it (one module provides both "mutex" and
"semaphre" test files.) to "hung_task_tests.c"
That will help us to extend it for other types easier (and less code).
Thank you,
>
> 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
>
--
Masami Hiramatsu (Google) <mhiramat@...nel.org>
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