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Message-ID:
<TY1PR01MB1708354E1C2886F0E7989B5DCFDE2@TY1PR01MB1708.jpnprd01.prod.outlook.com>
Date: Tue, 18 Mar 2025 14:20:52 +0800
From: Amaindex <amaindex@...look.com>
To: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>,
Lance Yang <ioworker0@...il.com>
Cc: akpm@...ux-foundation.org, will@...nel.org, peterz@...radead.org,
mingo@...hat.com, longman@...hat.com, 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
Subject: Re: [PATCH RESEND v2 3/3] samples: add hung_task detector semaphore
blocking sample
Hi Masami,
On 3/18/25 9:36 AM, Masami Hiramatsu (Google) wrote:
> 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,
Thanks for your suggestion!
Yeah, I agree that keeping two separate samples is redundant. So let's
combine them into a single 'hung_task_tests.c', which can handle both
mutex and semaphore cases. This will make the sample simpler and easier to
extend.
Cheers,
Zi
>> 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
>>
>
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