From nobody Fri Dec 19 21:34:07 2025 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 0F7A32566F6 for ; Tue, 25 Feb 2025 07:02:37 +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=1740466958; cv=none; b=JAH0hlKAkNhYoQ9w6kpZIW8iMWEbG2ElWanpQEPFZ3bywa+VQc6cUVeZMw2mLft/umdpQ7cv6iOujpaUmAyLKVJ2agSGTSYIdWD3KGynSbOGRcWQ1E8lo9zTS8Vv3m9Uqb4V0rfBAfh4jXWScbvWt77BqkLCroVq8SdCOd5csCs= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1740466958; c=relaxed/simple; bh=fR+3d4teL1VMY4KAAqHhQZ9Edcqbt0U7uMDt7545C8E=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version:Content-Type; b=XwvF5/ZepmJRTYBcek70AjzsRHSbDv4W0dyJwec90aPGeBvcBPru/SUwdqta1VffxL/25SeEwGE4PaZTlxY6iyYBsRE0546AxykPMz+j3ZjpG9s9ckiruZc/IV4qnByMSqvPq2VQXucJWAxdW1KMHqT+wggPBz9fZLLH1dwV/5g= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=H1hPwZKz; 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="H1hPwZKz" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 9C679C4CEDD; Tue, 25 Feb 2025 07:02:35 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1740466957; bh=fR+3d4teL1VMY4KAAqHhQZ9Edcqbt0U7uMDt7545C8E=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=H1hPwZKzhETAsRfMoaaI1eFyIByvtggBviWnJDfuPhGq48GW4exfVYaO1Y9X4bS0i kjNOmpPArgkteebw+/y2PeAIHD9Tqn/e28MWVWWBPSCFBCnD5oSwnrpgMWZJDyz1Ke 9tb7AXaKQAnBiafSwXqbYmzEwDOaUDVpuwSYDjjh1rIK4Ot0u0Sw3MqWZcwskw5NAb 2YmgQyebMNe58dZoqX9ZKGtUWUQ56jve5Zjn99ex+P/eS3vDV3rpm6KOcBsfyk0Ay3 19+lYXaFSpVX511nj13tp4m22YBWEkWzkFFRVo5mB3o6AKruww/fcsrHXRwCXaXMjk 7RUKpdvZbIgHA== 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 v4 1/2] hung_task: Show the blocker task if the task is hung on mutex Date: Tue, 25 Feb 2025 16:02:34 +0900 Message-ID: <174046695384.2194069.16796289525958195643.stgit@mhiramat.tok.corp.google.com> X-Mailer: git-send-email 2.48.1.658.g4767266eb4-goog In-Reply-To: <174046694331.2194069.15472952050240807469.stgit@mhiramat.tok.corp.google.com> References: <174046694331.2194069.15472952050240807469.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. Note: the owner can be changed while dumping the owner task, so this is "likely" the owner of the mutex. With this change, the hung task shows blocker task's info like below; INFO: task cat:115 blocked for more than 122 seconds. Not tainted 6.14.0-rc3-00003-ga8946be3de00 #156 "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. task:cat state:D stack:13432 pid:115 tgid:115 ppid:106 = task_flags:0x400100 flags:0x00000002 Call Trace: __schedule+0x731/0x960 ? schedule_preempt_disabled+0x54/0xa0 schedule+0xb7/0x140 ? __mutex_lock+0x51b/0xa60 ? __mutex_lock+0x51b/0xa60 schedule_preempt_disabled+0x54/0xa0 __mutex_lock+0x51b/0xa60 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:00007ffe99071828 EFLAGS: 00000246 ORIG_RAX: 0000000000000000 RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00000000004840cd RDX: 0000000000001000 RSI: 00007ffe99071870 RDI: 0000000000000003 RBP: 00007ffe99071870 R08: 0000000000000000 R09: 0000000000000000 R10: 0000000001000000 R11: 0000000000000246 R12: 0000000000001000 R13: 00000000132fd3a0 R14: 0000000000000001 R15: ffffffffffffffff INFO: task cat:115 is blocked on a mutex likely owned by task cat:114. task:cat state:S stack:13432 pid:114 tgid:114 ppid:106 = 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:00007ffe3e0147b8 EFLAGS: 00000246 ORIG_RAX: 0000000000000000 RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00000000004840cd RDX: 0000000000001000 RSI: 00007ffe3e014800 RDI: 0000000000000003 RBP: 00007ffe3e014800 R08: 0000000000000000 R09: 0000000000000000 R10: 0000000001000000 R11: 0000000000000246 R12: 0000000000001000 R13: 000000001a0a93a0 R14: 0000000000000001 R15: ffffffffffffffff Signed-off-by: Masami Hiramatsu (Google) Reviewed-by: Lance Yang Reviewed-by: Sergey Senozhatsky Reviewed-by: Waiman Long --- Changes in v4: - Make this option depends on !PREEMPT_RT because it changes mutex to rt_mutex. --- include/linux/mutex.h | 2 ++ include/linux/sched.h | 4 ++++ kernel/hung_task.c | 36 ++++++++++++++++++++++++++++++++++++ kernel/locking/mutex.c | 14 ++++++++++++++ lib/Kconfig.debug | 11 +++++++++++ 5 files changed, 67 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..ccd7217fcec1 100644 --- a/kernel/hung_task.c +++ b/kernel/hung_task.c @@ -93,6 +93,41 @@ 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; + + RCU_LOCKDEP_WARN(!rcu_read_lock_held(), "No rcu lock held"); + + lock =3D READ_ONCE(task->blocker_mutex); + if (!lock) + return; + + owner =3D mutex_get_owner(lock); + if (unlikely(!owner)) { + pr_err("INFO: task %s:%d is blocked on a mutex, but the owner is not fou= nd.\n", + task->comm, task->pid); + return; + } + + /* 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 likely owned by task %s:= %d.\n", + task->comm, task->pid, t->comm, t->pid); + sched_show_task(t); + return; + } + } +} +#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 +187,7 @@ 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..77d8c7e5ce96 100644 --- a/lib/Kconfig.debug +++ b/lib/Kconfig.debug @@ -1260,6 +1260,17 @@ 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 + depends on !PREEMPT_RT + 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 Fri Dec 19 21:34:07 2025 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 01A1C19F495 for ; 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dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="ZXUsoYSG" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 87542C4CEDD; Tue, 25 Feb 2025 07:02:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1740466966; bh=yRt/VaZXQuh0DYvzuCq9THX6LnIgF5KQbpuqcMQ4SZs=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=ZXUsoYSGHutHWA7bxOJw/lr8/GjpMaKU2/ThGC9N3iYTs2x+sgcaeIqOeq7xGfvu0 NlPFQLYfvGEEbz3og/u3BYBOA03nuv/8eBnlq/36wNPNPayEUQVHxyiffANOQQs3s/ zCFZbYUzOB6HqHs0X8pKSPgAxSr5sjI5F1JNaPfZLeO7Q6Kea5szAYGzd6XILVDASL d2SKwCAN5qqHqaBhzty+RTygCX5mVkDR5anAs3QMBjZxRDK/OATwbDomWCYwebuIu6 j6Zy5/Cqev21v/CPDN4HYjpVLHQ/7uSxBeV0yE6WrOUNkb1KtunjjVIAbOc7/mkqpq l4W9rNuBEd/xw== 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 v4 2/2] samples: Add hung_task detector mutex blocking sample Date: Tue, 25 Feb 2025 16:02:43 +0900 Message-ID: <174046696281.2194069.4567490148001547311.stgit@mhiramat.tok.corp.google.com> X-Mailer: git-send-email 2.48.1.658.g4767266eb4-goog In-Reply-To: <174046694331.2194069.15472952050240807469.stgit@mhiramat.tok.corp.google.com> References: <174046694331.2194069.15472952050240807469.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");