linux-kernel - Re: [PATCH v4 2/2] samples: Add hung_task detector mutex blocking sample

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <CAK1f24mb82Q33Bd-LyJwi7D8gcd1QNAycYgjAT00=_EC=Gxu0g@mail.gmail.com>
Date: Wed, 26 Feb 2025 19:58:53 +0800
From: Lance Yang <ioworker0@...il.com>
To: Masami Hiramatsu <mhiramat@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, 
	Will Deacon <will@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, 
	Boqun Feng <boqun.feng@...il.com>, Waiman Long <longman@...hat.com>, 
	Joel Granados <joel.granados@...nel.org>, Anna Schumaker <anna.schumaker@...cle.com>, 
	Kent Overstreet <kent.overstreet@...ux.dev>, Yongliang Gao <leonylgao@...cent.com>, 
	Steven Rostedt <rostedt@...dmis.org>, Tomasz Figa <tfiga@...omium.org>, 
	Sergey Senozhatsky <senozhatsky@...omium.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/2] samples: Add hung_task detector mutex blocking sample

On Wed, Feb 26, 2025 at 3:04 PM Masami Hiramatsu <mhiramat@...nel.org> wrote:
>
> On Wed, 26 Feb 2025 09:50:32 +0800
> Lance Yang <ioworker0@...il.com> wrote:
>
> > On Tue, Feb 25, 2025 at 3:02 PM Masami Hiramatsu (Google)
> > <mhiramat@...nel.org> wrote:
> > >
> > > From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> > >
> > > Add a hung_task detector mutex blocking test sample code.
> > >
> > > This module will create a dummy file on the debugfs. That file will
> > > cause the read process to sleep for enough long time (256 seconds)
> > > while holding a mutex. As a result, the second process will wait on
> > > the mutex for a prolonged duration and be detected by the hung_task
> > > detector.
> > >
> > > Usage is;
> > >
> > >  > cd /sys/kernel/debug/hung_task
> > >  > cat mutex & cat mutex
> > >
> > > and wait for hung_task message.
> > >
> > > Signed-off-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> > > ---
> > >  samples/Kconfig                     |    9 +++++
> > >  samples/Makefile                    |    1 +
> > >  samples/hung_task/Makefile          |    2 +
> > >  samples/hung_task/hung_task_mutex.c |   66 +++++++++++++++++++++++++++++++++++
> > >  4 files changed, 78 insertions(+)
> > >  create mode 100644 samples/hung_task/Makefile
> > >  create mode 100644 samples/hung_task/hung_task_mutex.c
> > >
> > > diff --git a/samples/Kconfig b/samples/Kconfig
> > > index 820e00b2ed68..09011be2391a 100644
> > > --- a/samples/Kconfig
> > > +++ b/samples/Kconfig
> > > @@ -300,6 +300,15 @@ config SAMPLE_CHECK_EXEC
> > >           demonstrate how they should be used with execveat(2) +
> > >           AT_EXECVE_CHECK.
> > >
> > > +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.
> > > +
> > >  source "samples/rust/Kconfig"
> >
> > I'm just wondering if we should also make the SAMPLE_HUNG_TASK
> > depend on !PREEMPT_RT for now?
>
> Yeah, I also have a question. This does not check the blocker,
> but just providing the testing interface with mutex. So there is
> no direct connection with DETECT_HUNG_TASK_BLOCKER. Thus I didn't
> add the dependency.

Yep, whether we add it or not is not a big deal ;)

Thanks,
Lance

>
> Thank you,
>
> >
> > Thanks,
> > Lance
> >
> > >
> > >  source "samples/damon/Kconfig"
> > > diff --git a/samples/Makefile b/samples/Makefile
> > > index f24cd0d72dd0..bf6e6fca5410 100644
> > > --- a/samples/Makefile
> > > +++ b/samples/Makefile
> > > @@ -42,3 +42,4 @@ obj-$(CONFIG_SAMPLE_FPROBE)           += fprobe/
> > >  obj-$(CONFIG_SAMPLES_RUST)             += rust/
> > >  obj-$(CONFIG_SAMPLE_DAMON_WSSE)                += damon/
> > >  obj-$(CONFIG_SAMPLE_DAMON_PRCL)                += damon/
> > > +obj-$(CONFIG_SAMPLE_HUNG_TASK)         += hung_task/
> > > diff --git a/samples/hung_task/Makefile b/samples/hung_task/Makefile
> > > new file mode 100644
> > > index 000000000000..fe9dde799880
> > > --- /dev/null
> > > +++ b/samples/hung_task/Makefile
> > > @@ -0,0 +1,2 @@
> > > +# SPDX-License-Identifier: GPL-2.0-only
> > > +obj-$(CONFIG_SAMPLE_HUNG_TASK) += hung_task_mutex.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
> > > new file mode 100644
> > > index 000000000000..7a29f2246d22
> > > --- /dev/null
> > > +++ b/samples/hung_task/hung_task_mutex.c
> > > @@ -0,0 +1,66 @@
> > > +// SPDX-License-Identifier: GPL-2.0-or-later
> > > +/*
> > > + * hung_task_mutex.c - Sample code which causes hung task by mutex
> > > + *
> > > + * Usage: load this module and read `<debugfs>/hung_task/mutex`
> > > + *        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/mutex.h>
> > > +
> > > +#define HUNG_TASK_DIR   "hung_task"
> > > +#define HUNG_TASK_FILE  "mutex"
> > > +#define SLEEP_SECOND 256
> > > +
> > > +static const char dummy_string[] = "This is a dummy string.";
> > > +static DEFINE_MUTEX(dummy_mutex);
> > > +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 lock, it is uninterruptible sleep. */
> > > +       guard(mutex)(&dummy_mutex);
> > > +
> > > +       /* When the first task sleep here, it is interruptible. */
> > > +       msleep_interruptible(SLEEP_SECOND * 1000);
> > > +
> > > +       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_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_remove_recursive(hung_task_dir);
> > > +}
> > > +
> > > +module_init(hung_task_sample_init);
> > > +module_exit(hung_task_sample_exit);
> > > +
> > > +MODULE_LICENSE("GPL");
> > > +MODULE_AUTHOR("Masami Hiramatsu");
> > > +MODULE_DESCRIPTION("Simple sleep under mutex file for testing hung task");
> > >
>
>
> --
> Masami Hiramatsu (Google) <mhiramat@...nel.org>