From nobody Mon Feb 9 02:42:28 2026 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 9EF7F20FA99 for ; Fri, 21 Feb 2025 14:30:05 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1740148205; cv=none; b=n9ptxQGv5ykE8JtZ08c+8QoUdkfaiuRfIxVDtuD1vXFYy4YRkzaQ4NWOBpQZXDUMSD8dqQlCVwVz0TQUniqL6nENJEjfumgH1AeRSmOfz+c8N0xJA/eSaP0BxlXDM2KTs+xxWy4LQNcmKwVRCU5eySuET1U46rvft0wpacKCjjc= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1740148205; c=relaxed/simple; bh=fYdqXlx+xtNh07wwI7HeC9dZi2obJjt0rHcBHY8Bbus=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version:Content-Type; b=MqWFsBZKI93oLIjahFX4nAJyjvUzd5hEGSuwH6L9hiOYOTZMBMg4ZXj4tOhn3jEnEJ42wFn/QNdaaZhwwFCBSJI6OQA2nXzJQRAIZXfHGk6cbYx+iR7qAfYfiiCSSUSXXSkAXZhSi+6dqclCWEPh1A2mhLN7AblS40QLSnJbtbA= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=P3T8daPO; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="P3T8daPO" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 206BEC4CED6; Fri, 21 Feb 2025 14:30:03 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1740148205; bh=fYdqXlx+xtNh07wwI7HeC9dZi2obJjt0rHcBHY8Bbus=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=P3T8daPOHbnPO5rIlITBNNQNus7ylWRkKJCFQ8VnHPKgszE4yw3c5SwIj6BZi969Y 77VRam7i2vZZK9mhc8qb4Lfs8wMLqz5JH3uyBM9K6+UYoBuPm+/Zm2UccUzH1biW48 Uui6zxAYNn+w/awekrBZnOtHmameqdaeUOlr7slXpFGB5+7aXp95fOM+wTg4c1OWIO E1VeAFMXoYkEY7ucx3NWp9JBBSZgUjIluUFCKad1Ft3D6DPzWs6mZJ3yoS2iINuYK7 UGh4i7fPLovnmLEhE5ETUn9qgN3nQqMNo4rrC6a/nahxuZ2xSXLYyxZ1QdF5BuIgAz 7wtT03eMOIznw== From: "Masami Hiramatsu (Google)" To: Peter Zijlstra , Ingo Molnar , Will Deacon , Andrew Morton Cc: Boqun Feng , Waiman Long , Joel Granados , Masami Hiramatsu , Anna Schumaker , Lance Yang , Kent Overstreet , Yongliang Gao , Steven Rostedt , Tomasz Figa , Sergey Senozhatsky , linux-kernel@vger.kernel.org Subject: [PATCH v2 1/2] hung_task: Show the blocker task if the task is hung on mutex Date: Fri, 21 Feb 2025 23:30:01 +0900 Message-ID: <174014820133.967666.5855819828684207404.stgit@mhiramat.tok.corp.google.com> X-Mailer: git-send-email 2.48.1.658.g4767266eb4-goog In-Reply-To: <174014819072.967666.10146255401631551816.stgit@mhiramat.tok.corp.google.com> References: <174014819072.967666.10146255401631551816.stgit@mhiramat.tok.corp.google.com> User-Agent: StGit/0.19 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable From: Masami Hiramatsu (Google) The "hung_task" shows a long-time uninterruptible slept task, but most often, it's blocked on a mutex acquired by another task. Without dumping such a task, investigating the root cause of the hung task problem is very difficult. This introduce task_struct::blocker_mutex to point the mutex lock which this task is waiting for. Since the mutex has "owner" information, we can find the owner task and dump it with hung tasks. With this change, the hung task shows blocker task's info like below; INFO: task cat:113 blocked for more than 122 seconds. Not tainted 6.14.0-rc3-00002-g6afe972e1b9b #152 "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. task:cat state:D stack:13432 pid:113 tgid:113 ppid:103 = task_flags:0x400100 flags:0x00000002 Call Trace: __schedule+0x731/0x960 ? schedule_preempt_disabled+0x54/0xa0 schedule+0xb7/0x140 ? __mutex_lock+0x51d/0xa50 ? __mutex_lock+0x51d/0xa50 schedule_preempt_disabled+0x54/0xa0 __mutex_lock+0x51d/0xa50 ? current_time+0x3a/0x120 read_dummy+0x23/0x70 full_proxy_read+0x6a/0xc0 vfs_read+0xc2/0x340 ? __pfx_direct_file_splice_eof+0x10/0x10 ? do_sendfile+0x1bd/0x2e0 ksys_read+0x76/0xe0 do_syscall_64+0xe3/0x1c0 ? exc_page_fault+0xa9/0x1d0 entry_SYSCALL_64_after_hwframe+0x77/0x7f RIP: 0033:0x4840cd RSP: 002b:00007ffe632b76c8 EFLAGS: 00000246 ORIG_RAX: 0000000000000000 RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00000000004840cd RDX: 0000000000001000 RSI: 00007ffe632b7710 RDI: 0000000000000003 RBP: 00007ffe632b7710 R08: 0000000000000000 R09: 0000000000000000 R10: 0000000001000000 R11: 0000000000000246 R12: 0000000000001000 R13: 000000003a8b63a0 R14: 0000000000000001 R15: ffffffffffffffff INFO: task cat:113 is blocked on a mutex owned by task cat:112. task:cat state:S stack:13432 pid:112 tgid:112 ppid:103 = task_flags:0x400100 flags:0x00000002 Call Trace: __schedule+0x731/0x960 ? schedule_timeout+0xa8/0x120 schedule+0xb7/0x140 schedule_timeout+0xa8/0x120 ? __pfx_process_timeout+0x10/0x10 msleep_interruptible+0x3e/0x60 read_dummy+0x2d/0x70 full_proxy_read+0x6a/0xc0 vfs_read+0xc2/0x340 ? __pfx_direct_file_splice_eof+0x10/0x10 ? do_sendfile+0x1bd/0x2e0 ksys_read+0x76/0xe0 do_syscall_64+0xe3/0x1c0 ? exc_page_fault+0xa9/0x1d0 entry_SYSCALL_64_after_hwframe+0x77/0x7f RIP: 0033:0x4840cd RSP: 002b:00007ffd69513748 EFLAGS: 00000246 ORIG_RAX: 0000000000000000 RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00000000004840cd RDX: 0000000000001000 RSI: 00007ffd69513790 RDI: 0000000000000003 RBP: 00007ffd69513790 R08: 0000000000000000 R09: 0000000000000000 R10: 0000000001000000 R11: 0000000000000246 R12: 0000000000001000 R13: 0000000029d8d3a0 R14: 0000000000000001 R15: ffffffffffffffff Signed-off-by: Masami Hiramatsu (Google) --- Changes in v2: - Introduce CONFIG_DETECT_HUNG_TASK_BLOCKER for this feature. - Introduce task_struct::blocker_mutex to point the mutex. - Rename debug_mutex_get_owner() to mutex_get_owner(). - Remove unneeded mutex_waiter::mutex. --- include/linux/mutex.h | 2 ++ include/linux/sched.h | 4 ++++ kernel/hung_task.c | 31 +++++++++++++++++++++++++++++++ kernel/locking/mutex.c | 14 ++++++++++++++ lib/Kconfig.debug | 10 ++++++++++ 5 files changed, 61 insertions(+) diff --git a/include/linux/mutex.h b/include/linux/mutex.h index 2bf91b57591b..2143d05116be 100644 --- a/include/linux/mutex.h +++ b/include/linux/mutex.h @@ -202,4 +202,6 @@ DEFINE_GUARD(mutex, struct mutex *, mutex_lock(_T), mut= ex_unlock(_T)) DEFINE_GUARD_COND(mutex, _try, mutex_trylock(_T)) DEFINE_GUARD_COND(mutex, _intr, mutex_lock_interruptible(_T) =3D=3D 0) =20 +extern unsigned long mutex_get_owner(struct mutex *lock); + #endif /* __LINUX_MUTEX_H */ diff --git a/include/linux/sched.h b/include/linux/sched.h index 9632e3318e0d..0cebdd736d44 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -1217,6 +1217,10 @@ struct task_struct { struct mutex_waiter *blocked_on; #endif =20 +#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER + struct mutex *blocker_mutex; +#endif + #ifdef CONFIG_DEBUG_ATOMIC_SLEEP int non_block_count; #endif diff --git a/kernel/hung_task.c b/kernel/hung_task.c index 04efa7a6e69b..aa5bc7fe09c2 100644 --- a/kernel/hung_task.c +++ b/kernel/hung_task.c @@ -93,6 +93,36 @@ static struct notifier_block panic_block =3D { .notifier_call =3D hung_task_panic, }; =20 + +#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER +static void debug_show_blocker(struct task_struct *task) +{ + struct task_struct *g, *t; + unsigned long owner; + struct mutex *lock; + + lock =3D READ_ONCE(task->blocker_mutex); + if (!lock) + return; + + owner =3D mutex_get_owner(lock); + if (likely(owner)) { + /* Ensure the owner information is correct. */ + for_each_process_thread(g, t) + if ((unsigned long)t =3D=3D owner) { + pr_err("INFO: task %s:%d is blocked on a mutex owned by task %s:%d.\n", + task->comm, task->pid, t->comm, t->pid); + sched_show_task(t); + return; + } + } + pr_err("INFO: task %s:%d is blocked on a mutex, but the owner is not foun= d.\n", + task->comm, task->pid); +} +#else +#define debug_show_blocker(t) do {} while (0) +#endif + static void check_hung_task(struct task_struct *t, unsigned long timeout) { unsigned long switch_count =3D t->nvcsw + t->nivcsw; @@ -152,6 +182,7 @@ static void check_hung_task(struct task_struct *t, unsi= gned long timeout) pr_err("\"echo 0 > /proc/sys/kernel/hung_task_timeout_secs\"" " disables this message.\n"); sched_show_task(t); + debug_show_blocker(t); hung_task_show_lock =3D true; =20 if (sysctl_hung_task_all_cpu_backtrace) diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c index b36f23de48f1..6a543c204a14 100644 --- a/kernel/locking/mutex.c +++ b/kernel/locking/mutex.c @@ -72,6 +72,14 @@ static inline unsigned long __owner_flags(unsigned long = owner) return owner & MUTEX_FLAGS; } =20 +/* Do not use the return value as a pointer directly. */ +unsigned long mutex_get_owner(struct mutex *lock) +{ + unsigned long owner =3D atomic_long_read(&lock->owner); + + return (unsigned long)__owner_task(owner); +} + /* * Returns: __mutex_owner(lock) on failure or NULL on success. */ @@ -180,6 +188,9 @@ static void __mutex_add_waiter(struct mutex *lock, struct mutex_waiter *waiter, struct list_head *list) { +#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER + WRITE_ONCE(current->blocker_mutex, lock); +#endif debug_mutex_add_waiter(lock, waiter, current); =20 list_add_tail(&waiter->list, list); @@ -195,6 +206,9 @@ __mutex_remove_waiter(struct mutex *lock, struct mutex_= waiter *waiter) __mutex_clear_flag(lock, MUTEX_FLAGS); =20 debug_mutex_remove_waiter(lock, waiter, current); +#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER + WRITE_ONCE(current->blocker_mutex, NULL); +#endif } =20 /* diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug index 1af972a92d06..91dfd8c10fbb 100644 --- a/lib/Kconfig.debug +++ b/lib/Kconfig.debug @@ -1260,6 +1260,16 @@ config BOOTPARAM_HUNG_TASK_PANIC =20 Say N if unsure. =20 +config DETECT_HUNG_TASK_BLOCKER + bool "Dump Hung Tasks Blocker" + depends on DETECT_HUNG_TASK + default y + help + Say Y here to show the blocker task's stacktrace who acquires + the mutex lock which "hung tasks" are waiting. + This will add overhead a bit but shows suspicious tasks and + call trace if it comes from waiting a mutex. + config WQ_WATCHDOG bool "Detect Workqueue Stalls" depends on DEBUG_KERNEL From nobody Mon Feb 9 02:42:28 2026 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 836A420B80A for ; 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dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="ksk3S1tg" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 116EDC4CED6; Fri, 21 Feb 2025 14:30:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1740148214; bh=yRt/VaZXQuh0DYvzuCq9THX6LnIgF5KQbpuqcMQ4SZs=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=ksk3S1tgYuROL7UgfHUrXCnpirZOHp1GEEzJx56LQVwdwK0q8OD3m2ByYUp+IyJbG 1L8g58dUPlnuRJYwb7AKBg4Vj+JpSNa7rKh7t9y0xWTUdSUuuS3iMDi+iGdbWoK0D/ bjvXkfDMLX6WW3yyqWV6lPSRSo6GMs5TviI7d6mlVG07179/bugan8Vt1hnOO87RLx ly7CSZ0unrVCVk+ZfKwtmiBthadPLPficgKRAEIVaXmhRcksYH7CZxjKa3iFeDJvPo OUQ7oOpF06OFEHXEzyw2MoGRHUP0e9C2p8rTPeSQwiwJPjnN++tEAXndA6IBiJFI1/ 9trgp4Cd0+wSw== From: "Masami Hiramatsu (Google)" To: Peter Zijlstra , Ingo Molnar , Will Deacon , Andrew Morton Cc: Boqun Feng , Waiman Long , Joel Granados , Masami Hiramatsu , Anna Schumaker , Lance Yang , Kent Overstreet , Yongliang Gao , Steven Rostedt , Tomasz Figa , Sergey Senozhatsky , linux-kernel@vger.kernel.org Subject: [PATCH v2 2/2] samples: Add hung_task detector mutex blocking sample Date: Fri, 21 Feb 2025 23:30:10 +0900 Message-ID: <174014821034.967666.8904118579315019613.stgit@mhiramat.tok.corp.google.com> X-Mailer: git-send-email 2.48.1.658.g4767266eb4-goog In-Reply-To: <174014819072.967666.10146255401631551816.stgit@mhiramat.tok.corp.google.com> References: <174014819072.967666.10146255401631551816.stgit@mhiramat.tok.corp.google.com> User-Agent: StGit/0.19 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable From: Masami Hiramatsu (Google) 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) --- 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. =20 +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" =20 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-$(CONFIG_SAMPLE_FPROBE) +=3D fprobe/ obj-$(CONFIG_SAMPLES_RUST) +=3D rust/ obj-$(CONFIG_SAMPLE_DAMON_WSSE) +=3D damon/ obj-$(CONFIG_SAMPLE_DAMON_PRCL) +=3D damon/ +obj-$(CONFIG_SAMPLE_HUNG_TASK) +=3D hung_task/ diff --git a/samples/hung_task/Makefile b/samples/hung_task/Makefile new file mode 100644 index 000000000000..fe9dde799880 --- /dev/null +++ b/samples/hung_task/Makefile @@ -0,0 +1,2 @@ +# SPDX-License-Identifier: GPL-2.0-only +obj-$(CONFIG_SAMPLE_HUNG_TASK) +=3D 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_t= ask_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 `/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 +#include +#include +#include +#include + +#define HUNG_TASK_DIR "hung_task" +#define HUNG_TASK_FILE "mutex" +#define SLEEP_SECOND 256 + +static const char dummy_string[] =3D "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 =3D { + .read =3D read_dummy, +}; + +static int __init hung_task_sample_init(void) +{ + hung_task_dir =3D 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");