* [PATCH v4 1/3] hung_task: replace blocker_mutex with encoded blocker
2025-03-20 6:49 [PATCH v4 0/3] hung_task: extend blocking task stacktrace dump to semaphore Lance Yang
@ 2025-03-20 6:49 ` Lance Yang
2025-04-07 20:08 ` Andrew Morton
2025-03-20 6:49 ` [PATCH v4 2/3] hung_task: show the blocker task if the task is hung on semaphore Lance Yang
` (2 subsequent siblings)
3 siblings, 1 reply; 10+ messages in thread
From: Lance Yang @ 2025-03-20 6:49 UTC (permalink / raw)
To: akpm
Cc: will, peterz, mingo, longman, mhiramat, anna.schumaker,
boqun.feng, joel.granados, kent.overstreet, leonylgao,
linux-kernel, rostedt, senozhatsky, tfiga, amaindex, jstultz,
Lance Yang, Mingzhe Yang
This patch replaces 'struct mutex *blocker_mutex' with 'unsigned long
blocker', as only one blocker is active at a time.
The blocker filed can store both the lock addrees and the lock type, with
LSB used to encode the type as Masami suggested, making it easier to extend
the feature to cover other types of locks.
Also, once the lock type is determined, we can directly extract the address
and cast it to a lock pointer ;)
Suggested-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Mingzhe Yang <mingzhe.yang@ly.com>
Signed-off-by: Lance Yang <ioworker0@gmail.com>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
---
include/linux/hung_task.h | 99 +++++++++++++++++++++++++++++++++++++++
include/linux/sched.h | 2 +-
kernel/hung_task.c | 13 +++--
kernel/locking/mutex.c | 5 +-
4 files changed, 111 insertions(+), 8 deletions(-)
create mode 100644 include/linux/hung_task.h
diff --git a/include/linux/hung_task.h b/include/linux/hung_task.h
new file mode 100644
index 000000000000..a5414d7b402d
--- /dev/null
+++ b/include/linux/hung_task.h
@@ -0,0 +1,99 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Detect Hung Task: detecting tasks stuck in D state
+ *
+ * Copyright (C) 2025 Tongcheng Travel (www.ly.com)
+ * Author: Lance Yang <mingzhe.yang@ly.com>
+ */
+#ifndef __LINUX_HUNG_TASK_H
+#define __LINUX_HUNG_TASK_H
+
+#include <linux/bug.h>
+#include <linux/sched.h>
+#include <linux/compiler.h>
+
+/*
+ * @blocker: Combines lock address and blocking type.
+ *
+ * Since lock pointers are at least 4-byte aligned(32-bit) or 8-byte
+ * aligned(64-bit). This leaves the 2 least bits (LSBs) of the pointer
+ * always zero. So we can use these bits to encode the specific blocking
+ * type.
+ *
+ * Type encoding:
+ * 00 - Blocked on mutex (BLOCKER_TYPE_MUTEX)
+ * 01 - Blocked on semaphore (BLOCKER_TYPE_SEM)
+ * 10 - Blocked on rt-mutex (BLOCKER_TYPE_RTMUTEX)
+ * 11 - Blocked on rw-semaphore (BLOCKER_TYPE_RWSEM)
+ */
+#define BLOCKER_TYPE_MUTEX 0x00UL
+#define BLOCKER_TYPE_SEM 0x01UL
+#define BLOCKER_TYPE_RTMUTEX 0x02UL
+#define BLOCKER_TYPE_RWSEM 0x03UL
+
+#define BLOCKER_TYPE_MASK 0x03UL
+
+#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
+static inline void hung_task_set_blocker(void *lock, unsigned long type)
+{
+ unsigned long lock_ptr = (unsigned long)lock;
+
+ WARN_ON_ONCE(!lock_ptr);
+ WARN_ON_ONCE(READ_ONCE(current->blocker));
+
+ /*
+ * If the lock pointer matches the BLOCKER_TYPE_MASK, return
+ * without writing anything.
+ */
+ if (WARN_ON_ONCE(lock_ptr & BLOCKER_TYPE_MASK))
+ return;
+
+ WRITE_ONCE(current->blocker, lock_ptr | type);
+}
+
+static inline void hung_task_clear_blocker(void)
+{
+ WARN_ON_ONCE(!READ_ONCE(current->blocker));
+
+ WRITE_ONCE(current->blocker, 0UL);
+}
+
+/*
+ * hung_task_get_blocker_type - Extracts blocker type from encoded blocker
+ * address.
+ *
+ * @blocker: Blocker pointer with encoded type (via LSB bits)
+ *
+ * Returns: BLOCKER_TYPE_MUTEX, BLOCKER_TYPE_SEM, etc.
+ */
+static inline unsigned long hung_task_get_blocker_type(unsigned long blocker)
+{
+ WARN_ON_ONCE(!blocker);
+
+ return blocker & BLOCKER_TYPE_MASK;
+}
+
+static inline void *hung_task_blocker_to_lock(unsigned long blocker)
+{
+ WARN_ON_ONCE(!blocker);
+
+ return (void *)(blocker & ~BLOCKER_TYPE_MASK);
+}
+#else
+static inline void hung_task_set_blocker(void *lock, unsigned long type)
+{
+}
+static inline void hung_task_clear_blocker(void)
+{
+}
+static inline unsigned long hung_task_get_blocker_type(unsigned long blocker)
+{
+ return 0UL;
+}
+static inline void *hung_task_blocker_to_lock(unsigned long blocker)
+{
+ return NULL;
+}
+#endif
+
+#endif /* __LINUX_HUNG_TASK_H */
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 1419d94c8e87..f27060dac499 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1218,7 +1218,7 @@ struct task_struct {
#endif
#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
- struct mutex *blocker_mutex;
+ unsigned long blocker;
#endif
#ifdef CONFIG_DEBUG_ATOMIC_SLEEP
diff --git a/kernel/hung_task.c b/kernel/hung_task.c
index dc898ec93463..79558d76ef06 100644
--- a/kernel/hung_task.c
+++ b/kernel/hung_task.c
@@ -22,6 +22,7 @@
#include <linux/sched/signal.h>
#include <linux/sched/debug.h>
#include <linux/sched/sysctl.h>
+#include <linux/hung_task.h>
#include <trace/events/sched.h>
@@ -98,16 +99,18 @@ static struct notifier_block panic_block = {
static void debug_show_blocker(struct task_struct *task)
{
struct task_struct *g, *t;
- unsigned long owner;
- struct mutex *lock;
+ unsigned long owner, blocker;
RCU_LOCKDEP_WARN(!rcu_read_lock_held(), "No rcu lock held");
- lock = READ_ONCE(task->blocker_mutex);
- if (!lock)
+ blocker = READ_ONCE(task->blocker);
+ if (!blocker ||
+ hung_task_get_blocker_type(blocker) != BLOCKER_TYPE_MUTEX)
return;
- owner = mutex_get_owner(lock);
+ owner = mutex_get_owner(
+ (struct mutex *)hung_task_blocker_to_lock(blocker));
+
if (unlikely(!owner)) {
pr_err("INFO: task %s:%d is blocked on a mutex, but the owner is not found.\n",
task->comm, task->pid);
diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index 6a543c204a14..e9ef70a6cb5f 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -29,6 +29,7 @@
#include <linux/interrupt.h>
#include <linux/debug_locks.h>
#include <linux/osq_lock.h>
+#include <linux/hung_task.h>
#define CREATE_TRACE_POINTS
#include <trace/events/lock.h>
@@ -189,7 +190,7 @@ __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);
+ hung_task_set_blocker(lock, BLOCKER_TYPE_MUTEX);
#endif
debug_mutex_add_waiter(lock, waiter, current);
@@ -207,7 +208,7 @@ __mutex_remove_waiter(struct mutex *lock, struct mutex_waiter *waiter)
debug_mutex_remove_waiter(lock, waiter, current);
#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
- WRITE_ONCE(current->blocker_mutex, NULL);
+ hung_task_clear_blocker();
#endif
}
--
2.45.2
^ permalink raw reply [flat|nested] 10+ messages in thread* Re: [PATCH v4 1/3] hung_task: replace blocker_mutex with encoded blocker
2025-03-20 6:49 ` [PATCH v4 1/3] hung_task: replace blocker_mutex with encoded blocker Lance Yang
@ 2025-04-07 20:08 ` Andrew Morton
2025-04-08 12:29 ` Lance Yang
0 siblings, 1 reply; 10+ messages in thread
From: Andrew Morton @ 2025-04-07 20:08 UTC (permalink / raw)
To: Lance Yang
Cc: will, peterz, mingo, longman, mhiramat, anna.schumaker,
boqun.feng, joel.granados, kent.overstreet, leonylgao,
linux-kernel, rostedt, senozhatsky, tfiga, amaindex, jstultz,
Mingzhe Yang
On Thu, 20 Mar 2025 14:49:21 +0800 Lance Yang <ioworker0@gmail.com> wrote:
> This patch replaces 'struct mutex *blocker_mutex' with 'unsigned long
> blocker', as only one blocker is active at a time.
>
> The blocker filed can store both the lock addrees and the lock type, with
> LSB used to encode the type as Masami suggested, making it easier to extend
> the feature to cover other types of locks.
>
> Also, once the lock type is determined, we can directly extract the address
> and cast it to a lock pointer ;)
>
> ...
>
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1218,7 +1218,7 @@ struct task_struct {
> #endif
>
> #ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
> - struct mutex *blocker_mutex;
> + unsigned long blocker;
-ENOCOMMENT
^ permalink raw reply [flat|nested] 10+ messages in thread* Re: [PATCH v4 1/3] hung_task: replace blocker_mutex with encoded blocker
2025-04-07 20:08 ` Andrew Morton
@ 2025-04-08 12:29 ` Lance Yang
0 siblings, 0 replies; 10+ messages in thread
From: Lance Yang @ 2025-04-08 12:29 UTC (permalink / raw)
To: akpm
Cc: amaindex, anna.schumaker, boqun.feng, ioworker0, joel.granados,
jstultz, kent.overstreet, leonylgao, linux-kernel, longman,
mhiramat, mingo, mingzhe.yang, peterz, rostedt, senozhatsky,
tfiga, will
On Tue, Apr 8, 2025 at 4:08 AM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Thu, 20 Mar 2025 14:49:21 +0800 Lance Yang <ioworker0@gmail.com> wrote:
>
> > This patch replaces 'struct mutex *blocker_mutex' with 'unsigned long
> > blocker', as only one blocker is active at a time.
> >
> > The blocker filed can store both the lock addrees and the lock type, with
> > LSB used to encode the type as Masami suggested, making it easier to extend
> > the feature to cover other types of locks.
> >
> > Also, once the lock type is determined, we can directly extract the address
> > and cast it to a lock pointer ;)
> >
> > ...
> >
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -1218,7 +1218,7 @@ struct task_struct {
> > #endif
> >
> > #ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
> > - struct mutex *blocker_mutex;
> > + unsigned long blocker;
>
> -ENOCOMMENT
>
Got it. Does the following change make sense and is it clear?
diff --git a/include/linux/sched.h b/include/linux/sched.h
index f27060dac499..27dad9aa99a0 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1218,6 +1218,10 @@ struct task_struct {
#endif
#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
+ /*
+ * Encoded lock address causing task block (lower 2 bits = type from
+ * <linux/hung_task.h>). Accessed via hung_task_*() helpers.
+ */
unsigned long blocker;
#endif
---
Thanks,
Lance
^ permalink raw reply [flat|nested] 10+ messages in thread
* [PATCH v4 2/3] hung_task: show the blocker task if the task is hung on semaphore
2025-03-20 6:49 [PATCH v4 0/3] hung_task: extend blocking task stacktrace dump to semaphore Lance Yang
2025-03-20 6:49 ` [PATCH v4 1/3] hung_task: replace blocker_mutex with encoded blocker Lance Yang
@ 2025-03-20 6:49 ` Lance Yang
2025-04-07 20:08 ` Andrew Morton
2025-04-07 20:08 ` Andrew Morton
2025-03-20 6:49 ` [PATCH v4 3/3] samples: extend hung_task detector test with semaphore support Lance Yang
2025-04-07 20:08 ` [PATCH v4 0/3] hung_task: extend blocking task stacktrace dump to semaphore Andrew Morton
3 siblings, 2 replies; 10+ messages in thread
From: Lance Yang @ 2025-03-20 6:49 UTC (permalink / raw)
To: akpm
Cc: will, peterz, mingo, longman, mhiramat, anna.schumaker,
boqun.feng, joel.granados, kent.overstreet, leonylgao,
linux-kernel, rostedt, senozhatsky, tfiga, amaindex, jstultz,
Lance Yang, Mingzhe Yang
Inspired by mutex blocker tracking[1], this patch makes a trade-off to
balance the overhead and utility of the hung task detector.
Unlike mutexes, semaphores lack explicit ownership tracking, making it
challenging to identify the root cause of hangs. To address this, we
introduce a last_holder field to the semaphore structure, which is
updated when a task successfully calls down() and cleared during up().
The assumption is that if a task is blocked on a semaphore, the holders
must not have released it. While this does not guarantee that the last
holder is one of the current blockers, it likely provides a practical hint
for diagnosing semaphore-related stalls.
With this change, the hung task detector can now show blocker task's info
like below:
[Thu Mar 20 04:52:21 2025] INFO: task cat:955 blocked for more than 120 seconds.
[Thu Mar 20 04:52:21 2025] Tainted: G E 6.14.0-rc6+ #1
[Thu Mar 20 04:52:21 2025] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[Thu Mar 20 04:52:21 2025] task:cat state:D stack:0 pid:955 tgid:955 ppid:917 task_flags:0x400000 flags:0x00000000
[Thu Mar 20 04:52:21 2025] Call Trace:
[Thu Mar 20 04:52:21 2025] <TASK>
[Thu Mar 20 04:52:21 2025] __schedule+0x491/0xbd0
[Thu Mar 20 04:52:21 2025] schedule+0x27/0xf0
[Thu Mar 20 04:52:21 2025] schedule_timeout+0xe3/0xf0
[Thu Mar 20 04:52:21 2025] ? __folio_mod_stat+0x2a/0x80
[Thu Mar 20 04:52:21 2025] ? set_ptes.constprop.0+0x27/0x90
[Thu Mar 20 04:52:21 2025] __down_common+0x155/0x280
[Thu Mar 20 04:52:21 2025] down+0x53/0x70
[Thu Mar 20 04:52:21 2025] read_dummy_semaphore+0x23/0x60
[Thu Mar 20 04:52:21 2025] full_proxy_read+0x5f/0xa0
[Thu Mar 20 04:52:21 2025] vfs_read+0xbc/0x350
[Thu Mar 20 04:52:21 2025] ? __count_memcg_events+0xa5/0x140
[Thu Mar 20 04:52:21 2025] ? count_memcg_events.constprop.0+0x1a/0x30
[Thu Mar 20 04:52:21 2025] ? handle_mm_fault+0x180/0x260
[Thu Mar 20 04:52:21 2025] ksys_read+0x66/0xe0
[Thu Mar 20 04:52:21 2025] do_syscall_64+0x51/0x120
[Thu Mar 20 04:52:21 2025] entry_SYSCALL_64_after_hwframe+0x76/0x7e
[Thu Mar 20 04:52:21 2025] RIP: 0033:0x7ff96d4ab46e
[Thu Mar 20 04:52:21 2025] RSP: 002b:00007ffe2f47f3a8 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
[Thu Mar 20 04:52:21 2025] RAX: ffffffffffffffda RBX: 0000000000020000 RCX: 00007ff96d4ab46e
[Thu Mar 20 04:52:21 2025] RDX: 0000000000020000 RSI: 00007ff96d39f000 RDI: 0000000000000003
[Thu Mar 20 04:52:21 2025] RBP: 00007ff96d39f000 R08: 00007ff96d39e010 R09: 0000000000000000
[Thu Mar 20 04:52:21 2025] R10: fffffffffffffbc5 R11: 0000000000000246 R12: 0000000000000000
[Thu Mar 20 04:52:21 2025] R13: 0000000000000003 R14: 0000000000020000 R15: 0000000000020000
[Thu Mar 20 04:52:21 2025] </TASK>
[Thu Mar 20 04:52:21 2025] INFO: task cat:955 blocked on a semaphore likely last held by task cat:909
[Thu Mar 20 04:52:21 2025] task:cat state:S stack:0 pid:909 tgid:909 ppid:771 task_flags:0x400000 flags:0x00000000
[Thu Mar 20 04:52:21 2025] Call Trace:
[Thu Mar 20 04:52:21 2025] <TASK>
[Thu Mar 20 04:52:21 2025] __schedule+0x491/0xbd0
[Thu Mar 20 04:52:21 2025] ? _raw_spin_unlock_irqrestore+0xe/0x40
[Thu Mar 20 04:52:21 2025] schedule+0x27/0xf0
[Thu Mar 20 04:52:21 2025] schedule_timeout+0x77/0xf0
[Thu Mar 20 04:52:21 2025] ? __pfx_process_timeout+0x10/0x10
[Thu Mar 20 04:52:21 2025] msleep_interruptible+0x49/0x60
[Thu Mar 20 04:52:21 2025] read_dummy_semaphore+0x2d/0x60
[Thu Mar 20 04:52:21 2025] full_proxy_read+0x5f/0xa0
[Thu Mar 20 04:52:21 2025] vfs_read+0xbc/0x350
[Thu Mar 20 04:52:21 2025] ? __count_memcg_events+0xa5/0x140
[Thu Mar 20 04:52:21 2025] ? count_memcg_events.constprop.0+0x1a/0x30
[Thu Mar 20 04:52:21 2025] ? handle_mm_fault+0x180/0x260
[Thu Mar 20 04:52:21 2025] ksys_read+0x66/0xe0
[Thu Mar 20 04:52:21 2025] do_syscall_64+0x51/0x120
[Thu Mar 20 04:52:21 2025] entry_SYSCALL_64_after_hwframe+0x76/0x7e
[Thu Mar 20 04:52:21 2025] RIP: 0033:0x7fe6bf7a046e
[Thu Mar 20 04:52:21 2025] RSP: 002b:00007ffd6e1a4028 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
[Thu Mar 20 04:52:21 2025] RAX: ffffffffffffffda RBX: 0000000000020000 RCX: 00007fe6bf7a046e
[Thu Mar 20 04:52:21 2025] RDX: 0000000000020000 RSI: 00007fe6bf694000 RDI: 0000000000000003
[Thu Mar 20 04:52:21 2025] RBP: 00007fe6bf694000 R08: 00007fe6bf693010 R09: 0000000000000000
[Thu Mar 20 04:52:21 2025] R10: fffffffffffffbc5 R11: 0000000000000246 R12: 0000000000000000
[Thu Mar 20 04:52:21 2025] R13: 0000000000000003 R14: 0000000000020000 R15: 0000000000020000
[1] https://lore.kernel.org/all/174046694331.2194069.15472952050240807469.stgit@mhiramat.tok.corp.google.com
Suggested-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Mingzhe Yang <mingzhe.yang@ly.com>
Signed-off-by: Lance Yang <ioworker0@gmail.com>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
---
include/linux/semaphore.h | 15 ++++++++++-
kernel/hung_task.c | 52 ++++++++++++++++++++++++++++++--------
kernel/locking/semaphore.c | 52 +++++++++++++++++++++++++++++++++-----
3 files changed, 101 insertions(+), 18 deletions(-)
diff --git a/include/linux/semaphore.h b/include/linux/semaphore.h
index 04655faadc2d..89706157e622 100644
--- a/include/linux/semaphore.h
+++ b/include/linux/semaphore.h
@@ -16,13 +16,25 @@ struct semaphore {
raw_spinlock_t lock;
unsigned int count;
struct list_head wait_list;
+
+#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
+ unsigned long last_holder;
+#endif
};
+#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
+#define __LAST_HOLDER_SEMAPHORE_INITIALIZER \
+ , .last_holder = 0UL
+#else
+#define __LAST_HOLDER_SEMAPHORE_INITIALIZER
+#endif
+
#define __SEMAPHORE_INITIALIZER(name, n) \
{ \
.lock = __RAW_SPIN_LOCK_UNLOCKED((name).lock), \
.count = n, \
- .wait_list = LIST_HEAD_INIT((name).wait_list), \
+ .wait_list = LIST_HEAD_INIT((name).wait_list) \
+ __LAST_HOLDER_SEMAPHORE_INITIALIZER \
}
/*
@@ -47,5 +59,6 @@ extern int __must_check down_killable(struct semaphore *sem);
extern int __must_check down_trylock(struct semaphore *sem);
extern int __must_check down_timeout(struct semaphore *sem, long jiffies);
extern void up(struct semaphore *sem);
+extern unsigned long sem_last_holder(struct semaphore *sem);
#endif /* __LINUX_SEMAPHORE_H */
diff --git a/kernel/hung_task.c b/kernel/hung_task.c
index 79558d76ef06..d2432df2b905 100644
--- a/kernel/hung_task.c
+++ b/kernel/hung_task.c
@@ -99,32 +99,62 @@ static struct notifier_block panic_block = {
static void debug_show_blocker(struct task_struct *task)
{
struct task_struct *g, *t;
- unsigned long owner, blocker;
+ unsigned long owner, blocker, blocker_type;
RCU_LOCKDEP_WARN(!rcu_read_lock_held(), "No rcu lock held");
blocker = READ_ONCE(task->blocker);
- if (!blocker ||
- hung_task_get_blocker_type(blocker) != BLOCKER_TYPE_MUTEX)
+ if (!blocker)
return;
- owner = mutex_get_owner(
- (struct mutex *)hung_task_blocker_to_lock(blocker));
+ blocker_type = hung_task_get_blocker_type(blocker);
+
+ switch (blocker_type) {
+ case BLOCKER_TYPE_MUTEX:
+ owner = mutex_get_owner(
+ (struct mutex *)hung_task_blocker_to_lock(blocker));
+ break;
+ case BLOCKER_TYPE_SEM:
+ owner = sem_last_holder(
+ (struct semaphore *)hung_task_blocker_to_lock(blocker));
+ break;
+ default:
+ WARN_ON_ONCE(1);
+ return;
+ }
+
if (unlikely(!owner)) {
- pr_err("INFO: task %s:%d is blocked on a mutex, but the owner is not found.\n",
- task->comm, task->pid);
+ switch (blocker_type) {
+ case BLOCKER_TYPE_MUTEX:
+ pr_err("INFO: task %s:%d is blocked on a mutex, but the owner is not found.\n",
+ task->comm, task->pid);
+ break;
+ case BLOCKER_TYPE_SEM:
+ pr_err("INFO: task %s:%d is blocked on a semaphore, but the last holder is not found.\n",
+ task->comm, task->pid);
+ break;
+ }
return;
}
/* Ensure the owner information is correct. */
for_each_process_thread(g, t) {
- if ((unsigned long)t == owner) {
+ if ((unsigned long)t != owner)
+ continue;
+
+ switch (blocker_type) {
+ case BLOCKER_TYPE_MUTEX:
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;
+ task->comm, task->pid, t->comm, t->pid);
+ break;
+ case BLOCKER_TYPE_SEM:
+ pr_err("INFO: task %s:%d blocked on a semaphore likely last held by task %s:%d\n",
+ task->comm, task->pid, t->comm, t->pid);
+ break;
}
+ sched_show_task(t);
+ return;
}
}
#else
diff --git a/kernel/locking/semaphore.c b/kernel/locking/semaphore.c
index 34bfae72f295..3d06d4adc05b 100644
--- a/kernel/locking/semaphore.c
+++ b/kernel/locking/semaphore.c
@@ -33,12 +33,14 @@
#include <linux/spinlock.h>
#include <linux/ftrace.h>
#include <trace/events/lock.h>
+#include <linux/hung_task.h>
static noinline void __down(struct semaphore *sem);
static noinline int __down_interruptible(struct semaphore *sem);
static noinline int __down_killable(struct semaphore *sem);
static noinline int __down_timeout(struct semaphore *sem, long timeout);
static noinline void __up(struct semaphore *sem);
+static inline void __sem_acquire(struct semaphore *sem);
/**
* down - acquire the semaphore
@@ -58,7 +60,7 @@ void __sched down(struct semaphore *sem)
might_sleep();
raw_spin_lock_irqsave(&sem->lock, flags);
if (likely(sem->count > 0))
- sem->count--;
+ __sem_acquire(sem);
else
__down(sem);
raw_spin_unlock_irqrestore(&sem->lock, flags);
@@ -82,7 +84,7 @@ int __sched down_interruptible(struct semaphore *sem)
might_sleep();
raw_spin_lock_irqsave(&sem->lock, flags);
if (likely(sem->count > 0))
- sem->count--;
+ __sem_acquire(sem);
else
result = __down_interruptible(sem);
raw_spin_unlock_irqrestore(&sem->lock, flags);
@@ -109,7 +111,7 @@ int __sched down_killable(struct semaphore *sem)
might_sleep();
raw_spin_lock_irqsave(&sem->lock, flags);
if (likely(sem->count > 0))
- sem->count--;
+ __sem_acquire(sem);
else
result = __down_killable(sem);
raw_spin_unlock_irqrestore(&sem->lock, flags);
@@ -139,7 +141,7 @@ int __sched down_trylock(struct semaphore *sem)
raw_spin_lock_irqsave(&sem->lock, flags);
count = sem->count - 1;
if (likely(count >= 0))
- sem->count = count;
+ __sem_acquire(sem);
raw_spin_unlock_irqrestore(&sem->lock, flags);
return (count < 0);
@@ -164,7 +166,7 @@ int __sched down_timeout(struct semaphore *sem, long timeout)
might_sleep();
raw_spin_lock_irqsave(&sem->lock, flags);
if (likely(sem->count > 0))
- sem->count--;
+ __sem_acquire(sem);
else
result = __down_timeout(sem, timeout);
raw_spin_unlock_irqrestore(&sem->lock, flags);
@@ -185,6 +187,12 @@ void __sched up(struct semaphore *sem)
unsigned long flags;
raw_spin_lock_irqsave(&sem->lock, flags);
+
+#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
+ if (READ_ONCE(sem->last_holder) == (unsigned long)current)
+ WRITE_ONCE(sem->last_holder, 0UL);
+#endif
+
if (likely(list_empty(&sem->wait_list)))
sem->count++;
else
@@ -224,8 +232,12 @@ static inline int __sched ___down_common(struct semaphore *sem, long state,
raw_spin_unlock_irq(&sem->lock);
timeout = schedule_timeout(timeout);
raw_spin_lock_irq(&sem->lock);
- if (waiter.up)
+ if (waiter.up) {
+#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
+ WRITE_ONCE(sem->last_holder, (unsigned long)current);
+#endif
return 0;
+ }
}
timed_out:
@@ -242,10 +254,18 @@ static inline int __sched __down_common(struct semaphore *sem, long state,
{
int ret;
+#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
+ hung_task_set_blocker(sem, BLOCKER_TYPE_SEM);
+#endif
+
trace_contention_begin(sem, 0);
ret = ___down_common(sem, state, timeout);
trace_contention_end(sem, ret);
+#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
+ hung_task_clear_blocker();
+#endif
+
return ret;
}
@@ -277,3 +297,23 @@ static noinline void __sched __up(struct semaphore *sem)
waiter->up = true;
wake_up_process(waiter->task);
}
+
+#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
+unsigned long sem_last_holder(struct semaphore *sem)
+{
+ return READ_ONCE(sem->last_holder);
+}
+#else
+unsigned long sem_last_holder(struct semaphore *sem)
+{
+ return 0UL;
+}
+#endif
+
+static inline void __sem_acquire(struct semaphore *sem)
+{
+ sem->count--;
+#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
+ WRITE_ONCE(sem->last_holder, (unsigned long)current);
+#endif
+}
--
2.45.2
^ permalink raw reply [flat|nested] 10+ messages in thread* Re: [PATCH v4 2/3] hung_task: show the blocker task if the task is hung on semaphore
2025-03-20 6:49 ` [PATCH v4 2/3] hung_task: show the blocker task if the task is hung on semaphore Lance Yang
@ 2025-04-07 20:08 ` Andrew Morton
2025-04-07 20:08 ` Andrew Morton
1 sibling, 0 replies; 10+ messages in thread
From: Andrew Morton @ 2025-04-07 20:08 UTC (permalink / raw)
To: Lance Yang
Cc: will, peterz, mingo, longman, mhiramat, anna.schumaker,
boqun.feng, joel.granados, kent.overstreet, leonylgao,
linux-kernel, rostedt, senozhatsky, tfiga, amaindex, jstultz,
Mingzhe Yang
On Thu, 20 Mar 2025 14:49:22 +0800 Lance Yang <ioworker0@gmail.com> wrote:
> Inspired by mutex blocker tracking[1], this patch makes a trade-off to
> balance the overhead and utility of the hung task detector.
>
> Unlike mutexes, semaphores lack explicit ownership tracking, making it
> challenging to identify the root cause of hangs. To address this, we
> introduce a last_holder field to the semaphore structure, which is
> updated when a task successfully calls down() and cleared during up().
>
> The assumption is that if a task is blocked on a semaphore, the holders
> must not have released it. While this does not guarantee that the last
> holder is one of the current blockers, it likely provides a practical hint
> for diagnosing semaphore-related stalls.
>
> With this change, the hung task detector can now show blocker task's info
> like below:
+#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
+#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
+#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
+#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
+#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
+#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
+#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
+#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
It looks pretty simple to reduce the amount of ifdeffery which this
patch adds.
^ permalink raw reply [flat|nested] 10+ messages in thread
* Re: [PATCH v4 2/3] hung_task: show the blocker task if the task is hung on semaphore
2025-03-20 6:49 ` [PATCH v4 2/3] hung_task: show the blocker task if the task is hung on semaphore Lance Yang
2025-04-07 20:08 ` Andrew Morton
@ 2025-04-07 20:08 ` Andrew Morton
2025-04-08 12:27 ` Lance Yang
1 sibling, 1 reply; 10+ messages in thread
From: Andrew Morton @ 2025-04-07 20:08 UTC (permalink / raw)
To: Lance Yang
Cc: will, peterz, mingo, longman, mhiramat, anna.schumaker,
boqun.feng, joel.granados, kent.overstreet, leonylgao,
linux-kernel, rostedt, senozhatsky, tfiga, amaindex, jstultz,
Mingzhe Yang
On Thu, 20 Mar 2025 14:49:22 +0800 Lance Yang <ioworker0@gmail.com> wrote:
> --- a/kernel/locking/semaphore.c
> +++ b/kernel/locking/semaphore.c
> @@ -33,12 +33,14 @@
> #include <linux/spinlock.h>
> #include <linux/ftrace.h>
> #include <trace/events/lock.h>
> +#include <linux/hung_task.h>
>
> static noinline void __down(struct semaphore *sem);
> static noinline int __down_interruptible(struct semaphore *sem);
> static noinline int __down_killable(struct semaphore *sem);
> static noinline int __down_timeout(struct semaphore *sem, long timeout);
> static noinline void __up(struct semaphore *sem);
> +static inline void __sem_acquire(struct semaphore *sem);
It feels Just Weird to forward declare a static inline. Is there a
special reason for doing this?
^ permalink raw reply [flat|nested] 10+ messages in thread
* Re: [PATCH v4 2/3] hung_task: show the blocker task if the task is hung on semaphore
2025-04-07 20:08 ` Andrew Morton
@ 2025-04-08 12:27 ` Lance Yang
0 siblings, 0 replies; 10+ messages in thread
From: Lance Yang @ 2025-04-08 12:27 UTC (permalink / raw)
To: akpm
Cc: amaindex, anna.schumaker, boqun.feng, ioworker0, joel.granados,
jstultz, kent.overstreet, leonylgao, linux-kernel, longman,
mhiramat, mingo, mingzhe.yang, peterz, rostedt, senozhatsky,
tfiga, will
Hi Andrew,
Thanks a lot for taking time to review!
On Tue, Apr 8, 2025 at 4:08 AM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Thu, 20 Mar 2025 14:49:22 +0800 Lance Yang <ioworker0@gmail.com> wrote:
>
> > Inspired by mutex blocker tracking[1], this patch makes a trade-off to
> > balance the overhead and utility of the hung task detector.
> >
> > Unlike mutexes, semaphores lack explicit ownership tracking, making it
> > challenging to identify the root cause of hangs. To address this, we
> > introduce a last_holder field to the semaphore structure, which is
> > updated when a task successfully calls down() and cleared during up().
> >
> > The assumption is that if a task is blocked on a semaphore, the holders
> > must not have released it. While this does not guarantee that the last
> > holder is one of the current blockers, it likely provides a practical hint
> > for diagnosing semaphore-related stalls.
> >
> > With this change, the hung task detector can now show blocker task's info
> > like below:
>
> +#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
> +#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
> +#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
> +#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
> +#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
> +#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
> +#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
> +#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
>
> It looks pretty simple to reduce the amount of ifdeffery which this
> patch adds.
Good catch! We can reduce five ifdeffery with the following change ;)
diff --git a/kernel/locking/semaphore.c b/kernel/locking/semaphore.c
index 3d06d4adc05b..db8a8f696f50 100644
--- a/kernel/locking/semaphore.c
+++ b/kernel/locking/semaphore.c
@@ -40,7 +40,41 @@ static noinline int __down_interruptible(struct semaphore *sem);
static noinline int __down_killable(struct semaphore *sem);
static noinline int __down_timeout(struct semaphore *sem, long timeout);
static noinline void __up(struct semaphore *sem);
-static inline void __sem_acquire(struct semaphore *sem);
+
+#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
+static inline void hung_task_sem_set_holder(struct semaphore *sem)
+{
+ WRITE_ONCE((sem)->last_holder, (unsigned long)current);
+}
+
+static inline void hung_task_sem_clear_if_holder(struct semaphore *sem)
+{
+ if (READ_ONCE((sem)->last_holder) == (unsigned long)current)
+ WRITE_ONCE((sem)->last_holder, 0UL);
+}
+
+unsigned long sem_last_holder(struct semaphore *sem)
+{
+ return READ_ONCE(sem->last_holder);
+}
+#else
+static inline void hung_task_sem_set_holder(struct semaphore *sem)
+{
+}
+static inline void hung_task_sem_clear_if_holder(struct semaphore *sem)
+{
+}
+unsigned long sem_last_holder(struct semaphore *sem)
+{
+ return 0UL;
+}
+#endif
+
+static inline void __sem_acquire(struct semaphore *sem)
+{
+ sem->count--;
+ hung_task_sem_set_holder(sem);
+}
/**
* down - acquire the semaphore
@@ -188,10 +222,7 @@ void __sched up(struct semaphore *sem)
raw_spin_lock_irqsave(&sem->lock, flags);
-#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
- if (READ_ONCE(sem->last_holder) == (unsigned long)current)
- WRITE_ONCE(sem->last_holder, 0UL);
-#endif
+ hung_task_sem_clear_if_holder(sem);
if (likely(list_empty(&sem->wait_list)))
sem->count++;
@@ -233,9 +264,7 @@ static inline int __sched ___down_common(struct semaphore *sem, long state,
timeout = schedule_timeout(timeout);
raw_spin_lock_irq(&sem->lock);
if (waiter.up) {
-#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
- WRITE_ONCE(sem->last_holder, (unsigned long)current);
-#endif
+ hung_task_sem_set_holder(sem);
return 0;
}
}
@@ -254,17 +283,13 @@ static inline int __sched __down_common(struct semaphore *sem, long state,
{
int ret;
-#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
hung_task_set_blocker(sem, BLOCKER_TYPE_SEM);
-#endif
trace_contention_begin(sem, 0);
ret = ___down_common(sem, state, timeout);
trace_contention_end(sem, ret);
-#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
hung_task_clear_blocker();
-#endif
return ret;
}
@@ -297,23 +322,3 @@ static noinline void __sched __up(struct semaphore *sem)
waiter->up = true;
wake_up_process(waiter->task);
}
-
-#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
-unsigned long sem_last_holder(struct semaphore *sem)
-{
- return READ_ONCE(sem->last_holder);
-}
-#else
-unsigned long sem_last_holder(struct semaphore *sem)
-{
- return 0UL;
-}
-#endif
-
-static inline void __sem_acquire(struct semaphore *sem)
-{
- sem->count--;
-#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
- WRITE_ONCE(sem->last_holder, (unsigned long)current);
-#endif
-}
---
[...]
> > static noinline void __down(struct semaphore *sem);
> > static noinline int __down_interruptible(struct semaphore *sem);
> > static noinline int __down_killable(struct semaphore *sem);
> > static noinline int __down_timeout(struct semaphore *sem, long timeout);
> > static noinline void __up(struct semaphore *sem);
> > +static inline void __sem_acquire(struct semaphore *sem);
>
> It feels Just Weird to forward declare a static inline. Is there a
> special reason for doing this?
Thanks for pointing this out.
Indeed, the forward declaratio was weird :(
Fixed by removing it as shown in the diff above.
Thanks,
Lance
^ permalink raw reply [flat|nested] 10+ messages in thread
* [PATCH v4 3/3] samples: extend hung_task detector test with semaphore support
2025-03-20 6:49 [PATCH v4 0/3] hung_task: extend blocking task stacktrace dump to semaphore Lance Yang
2025-03-20 6:49 ` [PATCH v4 1/3] hung_task: replace blocker_mutex with encoded blocker Lance Yang
2025-03-20 6:49 ` [PATCH v4 2/3] hung_task: show the blocker task if the task is hung on semaphore Lance Yang
@ 2025-03-20 6:49 ` Lance Yang
2025-04-07 20:08 ` [PATCH v4 0/3] hung_task: extend blocking task stacktrace dump to semaphore Andrew Morton
3 siblings, 0 replies; 10+ messages in thread
From: Lance Yang @ 2025-03-20 6:49 UTC (permalink / raw)
To: akpm
Cc: will, peterz, mingo, longman, mhiramat, anna.schumaker,
boqun.feng, joel.granados, kent.overstreet, leonylgao,
linux-kernel, rostedt, senozhatsky, tfiga, amaindex, jstultz,
Lance Yang
From: Zi Li <amaindex@outlook.com>
Extend the existing hung_task detector test module to support multiple lock
types, including mutex and semaphore, with room for future additions (e.g.,
spinlock, etc.). This module creates dummy files under <debugfs>/hung_task,
such as 'mutex' and 'semaphore'. The read process on any of these files
will sleep for enough long time (256 seconds) while holding the respective
lock. As a result, the second process will wait on the lock for a prolonged
duration and be detected by the hung_task detector.
This change unifies the previous mutex-only sample into a single,
extensible hung_task_tests module, reducing code duplication and improving
maintainability.
Usage is:
> cd /sys/kernel/debug/hung_task
> cat mutex & cat mutex # Test mutex blocking
> cat semaphore & cat semaphore # Test semaphore blocking
Update the Kconfig description to reflect multiple debugfs files support.
Suggested-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Lance Yang <ioworker0@gmail.com>
Signed-off-by: Zi Li <amaindex@outlook.com>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
---
samples/Kconfig | 9 +--
samples/hung_task/Makefile | 2 +-
samples/hung_task/hung_task_mutex.c | 66 --------------------
samples/hung_task/hung_task_tests.c | 97 +++++++++++++++++++++++++++++
4 files changed, 103 insertions(+), 71 deletions(-)
delete mode 100644 samples/hung_task/hung_task_mutex.c
create mode 100644 samples/hung_task/hung_task_tests.c
diff --git a/samples/Kconfig b/samples/Kconfig
index 09011be2391a..753ed1f170b5 100644
--- a/samples/Kconfig
+++ b/samples/Kconfig
@@ -304,10 +304,11 @@ 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 a module that provides debugfs files (e.g., mutex, semaphore,
+ etc.) under <debugfs>/hung_task. If user reads one of these files,
+ it will sleep long time (256 seconds) with holding a lock. Thus,
+ if 2 or more processes read the same file concurrently, it will
+ be detected by the hung_task watchdog.
source "samples/rust/Kconfig"
diff --git a/samples/hung_task/Makefile b/samples/hung_task/Makefile
index f4d6ab563488..86036f1a204d 100644
--- a/samples/hung_task/Makefile
+++ b/samples/hung_task/Makefile
@@ -1,2 +1,2 @@
# SPDX-License-Identifier: GPL-2.0-only
-obj-$(CONFIG_SAMPLE_HUNG_TASK) += hung_task_mutex.o
+obj-$(CONFIG_SAMPLE_HUNG_TASK) += hung_task_tests.o
diff --git a/samples/hung_task/hung_task_mutex.c b/samples/hung_task/hung_task_mutex.c
deleted file mode 100644
index 47ed38239ea3..000000000000
--- a/samples/hung_task/hung_task_mutex.c
+++ /dev/null
@@ -1,66 +0,0 @@
-// 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);
-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 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");
diff --git a/samples/hung_task/hung_task_tests.c b/samples/hung_task/hung_task_tests.c
new file mode 100644
index 000000000000..a5c09bd3a47d
--- /dev/null
+++ b/samples/hung_task/hung_task_tests.c
@@ -0,0 +1,97 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * hung_task_tests.c - Sample code for testing hung tasks with mutex,
+ * semaphore, etc.
+ *
+ * Usage: Load this module and read `<debugfs>/hung_task/mutex`,
+ * `<debugfs>/hung_task/semaphore`, etc., with 2 or more processes.
+ *
+ * This is for testing kernel hung_task error messages with various locking
+ * mechanisms (e.g., mutex, semaphore, etc.). Note that this may freeze
+ * your system or cause a panic. Use only for testing purposes.
+ */
+
+#include <linux/debugfs.h>
+#include <linux/delay.h>
+#include <linux/fs.h>
+#include <linux/module.h>
+#include <linux/mutex.h>
+#include <linux/semaphore.h>
+
+#define HUNG_TASK_DIR "hung_task"
+#define HUNG_TASK_MUTEX_FILE "mutex"
+#define HUNG_TASK_SEM_FILE "semaphore"
+#define SLEEP_SECOND 256
+
+static const char dummy_string[] = "This is a dummy string.";
+static DEFINE_MUTEX(dummy_mutex);
+static DEFINE_SEMAPHORE(dummy_sem, 1);
+static struct dentry *hung_task_dir;
+
+/* Mutex-based read function */
+static ssize_t read_dummy_mutex(struct file *file, char __user *user_buf,
+ size_t count, loff_t *ppos)
+{
+ /* Second task waits on mutex, entering uninterruptible sleep */
+ guard(mutex)(&dummy_mutex);
+
+ /* First task sleeps here, interruptible */
+ msleep_interruptible(SLEEP_SECOND * 1000);
+
+ return simple_read_from_buffer(user_buf, count, ppos, dummy_string,
+ sizeof(dummy_string));
+}
+
+/* Semaphore-based read function */
+static ssize_t read_dummy_semaphore(struct file *file, char __user *user_buf,
+ size_t count, loff_t *ppos)
+{
+ /* Second task waits on semaphore, entering uninterruptible sleep */
+ down(&dummy_sem);
+
+ /* First task sleeps here, interruptible */
+ msleep_interruptible(SLEEP_SECOND * 1000);
+
+ up(&dummy_sem);
+
+ return simple_read_from_buffer(user_buf, count, ppos, dummy_string,
+ sizeof(dummy_string));
+}
+
+/* File operations for mutex */
+static const struct file_operations hung_task_mutex_fops = {
+ .read = read_dummy_mutex,
+};
+
+/* File operations for semaphore */
+static const struct file_operations hung_task_sem_fops = {
+ .read = read_dummy_semaphore,
+};
+
+static int __init hung_task_tests_init(void)
+{
+ hung_task_dir = debugfs_create_dir(HUNG_TASK_DIR, NULL);
+ if (IS_ERR(hung_task_dir))
+ return PTR_ERR(hung_task_dir);
+
+ /* Create debugfs files for mutex and semaphore tests */
+ debugfs_create_file(HUNG_TASK_MUTEX_FILE, 0400, hung_task_dir, NULL,
+ &hung_task_mutex_fops);
+ debugfs_create_file(HUNG_TASK_SEM_FILE, 0400, hung_task_dir, NULL,
+ &hung_task_sem_fops);
+
+ return 0;
+}
+
+static void __exit hung_task_tests_exit(void)
+{
+ debugfs_remove_recursive(hung_task_dir);
+}
+
+module_init(hung_task_tests_init);
+module_exit(hung_task_tests_exit);
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Masami Hiramatsu <mhiramat@kernel.org>");
+MODULE_AUTHOR("Zi Li <amaindex@outlook.com>");
+MODULE_DESCRIPTION("Simple sleep under lock files for testing hung task");
--
2.45.2
^ permalink raw reply [flat|nested] 10+ messages in thread* Re: [PATCH v4 0/3] hung_task: extend blocking task stacktrace dump to semaphore
2025-03-20 6:49 [PATCH v4 0/3] hung_task: extend blocking task stacktrace dump to semaphore Lance Yang
` (2 preceding siblings ...)
2025-03-20 6:49 ` [PATCH v4 3/3] samples: extend hung_task detector test with semaphore support Lance Yang
@ 2025-04-07 20:08 ` Andrew Morton
3 siblings, 0 replies; 10+ messages in thread
From: Andrew Morton @ 2025-04-07 20:08 UTC (permalink / raw)
To: Lance Yang
Cc: will, peterz, mingo, longman, mhiramat, anna.schumaker,
boqun.feng, joel.granados, kent.overstreet, leonylgao,
linux-kernel, rostedt, senozhatsky, tfiga, amaindex, jstultz
On Thu, 20 Mar 2025 14:49:20 +0800 Lance Yang <ioworker0@gmail.com> wrote:
> Inspired by mutex blocker tracking[1], this patch series extend the
> feature to not only dump the blocker task holding a mutex but also to
> support semaphores. Unlike mutexes, semaphores lack explicit ownership
> tracking, making it challenging to identify the root cause of hangs. To
> address this, we introduce a last_holder field to the semaphore structure,
> which is updated when a task successfully calls down() and cleared during
> up().
>
> The assumption is that if a task is blocked on a semaphore, the holders
> must not have released it. While this does not guarantee that the last
> holder is one of the current blockers, it likely provides a practical hint
> for diagnosing semaphore-related stalls.
>
> With this change, the hung task detector can now show blocker task's info
> like below:
Seems useful, but the semaphore code isn't really an akpm thing.
Peter, could you please comment?
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