[PATCH v3 2/3] hung_task: show the blocker task if the task is hung on semaphore

[Lance Yang <ioworker0@gmail.com>]

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 13 15:18:38 2025] INFO: task cat:1803 blocked for more than 122 seconds.
[Thu Mar 13 15:18:38 2025]       Tainted: G           OE      6.14.0-rc3+ #14
[Thu Mar 13 15:18:38 2025] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[Thu Mar 13 15:18:38 2025] task:cat             state:D stack:0     pid:1803  tgid:1803  ppid:1057   task_flags:0x400000 flags:0x00000004
[Thu Mar 13 15:18:38 2025] Call trace:
[Thu Mar 13 15:18:38 2025]  __switch_to+0x1ec/0x380 (T)
[Thu Mar 13 15:18:38 2025]  __schedule+0xc30/0x44f8
[Thu Mar 13 15:18:38 2025]  schedule+0xb8/0x3b0
[Thu Mar 13 15:18:38 2025]  schedule_timeout+0x1d0/0x208
[Thu Mar 13 15:18:38 2025]  __down_common+0x2d4/0x6f8
[Thu Mar 13 15:18:38 2025]  __down+0x24/0x50
[Thu Mar 13 15:18:38 2025]  down+0xd0/0x140
[Thu Mar 13 15:18:38 2025]  read_dummy+0x3c/0xa0 [hung_task_sem]
[Thu Mar 13 15:18:38 2025]  full_proxy_read+0xfc/0x1d0
[Thu Mar 13 15:18:38 2025]  vfs_read+0x1a0/0x858
[Thu Mar 13 15:18:38 2025]  ksys_read+0x100/0x220
[Thu Mar 13 15:18:38 2025]  __arm64_sys_read+0x78/0xc8
[Thu Mar 13 15:18:38 2025]  invoke_syscall+0xd8/0x278
[Thu Mar 13 15:18:38 2025]  el0_svc_common.constprop.0+0xb8/0x298
[Thu Mar 13 15:18:38 2025]  do_el0_svc+0x4c/0x88
[Thu Mar 13 15:18:38 2025]  el0_svc+0x44/0x108
[Thu Mar 13 15:18:38 2025]  el0t_64_sync_handler+0x134/0x160
[Thu Mar 13 15:18:38 2025]  el0t_64_sync+0x1b8/0x1c0
[Thu Mar 13 15:18:38 2025] INFO: task cat:1803 blocked on a semaphore likely last held by task cat:1802
[Thu Mar 13 15:18:38 2025] task:cat             state:S stack:0     pid:1802  tgid:1802  ppid:1057   task_flags:0x400000 flags:0x00000004
[Thu Mar 13 15:18:38 2025] Call trace:
[Thu Mar 13 15:18:38 2025]  __switch_to+0x1ec/0x380 (T)
[Thu Mar 13 15:18:38 2025]  __schedule+0xc30/0x44f8
[Thu Mar 13 15:18:38 2025]  schedule+0xb8/0x3b0
[Thu Mar 13 15:18:38 2025]  schedule_timeout+0xf4/0x208
[Thu Mar 13 15:18:38 2025]  msleep_interruptible+0x70/0x130
[Thu Mar 13 15:18:38 2025]  read_dummy+0x48/0xa0 [hung_task_sem]
[Thu Mar 13 15:18:38 2025]  full_proxy_read+0xfc/0x1d0
[Thu Mar 13 15:18:38 2025]  vfs_read+0x1a0/0x858
[Thu Mar 13 15:18:38 2025]  ksys_read+0x100/0x220
[Thu Mar 13 15:18:38 2025]  __arm64_sys_read+0x78/0xc8
[Thu Mar 13 15:18:38 2025]  invoke_syscall+0xd8/0x278
[Thu Mar 13 15:18:38 2025]  el0_svc_common.constprop.0+0xb8/0x298
[Thu Mar 13 15:18:38 2025]  do_el0_svc+0x4c/0x88
[Thu Mar 13 15:18:38 2025]  el0_svc+0x44/0x108
[Thu Mar 13 15:18:38 2025]  el0t_64_sync_handler+0x134/0x160
[Thu Mar 13 15:18:38 2025]  el0t_64_sync+0x1b8/0x1c0

[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>
---
 include/linux/semaphore.h  | 15 ++++++++++-
 kernel/hung_task.c         | 45 +++++++++++++++++++++++++--------
 kernel/locking/semaphore.c | 52 +++++++++++++++++++++++++++++++++-----
 3 files changed, 95 insertions(+), 17 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 75dc1048f2c1..1403e71df039 100644
--- a/kernel/hung_task.c
+++ b/kernel/hung_task.c
@@ -99,31 +99,56 @@ 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_lock_type;
 
 	RCU_LOCKDEP_WARN(!rcu_read_lock_held(), "No rcu lock held");
 
 	blocker = READ_ONCE(task->blocker);
-	if (!blocker || !hung_task_blocker_is_type(blocker, BLOCKER_TYPE_MUTEX))
+	if (!blocker)
 		return;
 
-	owner = mutex_get_owner(
-		(struct mutex *)hung_task_blocker_to_lock(blocker));
+	if (hung_task_blocker_is_type(blocker, BLOCKER_TYPE_MUTEX)) {
+		owner = mutex_get_owner(
+			(struct mutex *)hung_task_blocker_to_lock(blocker));
+		blocker_lock_type = BLOCKER_TYPE_MUTEX;
+	} else if (hung_task_blocker_is_type(blocker, BLOCKER_TYPE_SEM)) {
+		owner = sem_last_holder(
+			(struct semaphore *)hung_task_blocker_to_lock(blocker));
+		blocker_lock_type = BLOCKER_TYPE_SEM;
+	} else
+		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_lock_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_lock_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