From 194a9b9e843b4077033be70a07d191107972aa53 Mon Sep 17 00:00:00 2001 From: Lance Yang Date: Mon, 14 Apr 2025 22:59:44 +0800 Subject: [PATCH] hung_task: show the blocker task if the task is hung on semaphore 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: [Tue Apr 8 12:19:07 2025] INFO: task cat:945 blocked for more than 120 seconds. [Tue Apr 8 12:19:07 2025] Tainted: G E 6.14.0-rc6+ #1 [Tue Apr 8 12:19:07 2025] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. [Tue Apr 8 12:19:07 2025] task:cat state:D stack:0 pid:945 tgid:945 ppid:828 task_flags:0x400000 flags:0x00000000 [Tue Apr 8 12:19:07 2025] Call Trace: [Tue Apr 8 12:19:07 2025] [Tue Apr 8 12:19:07 2025] __schedule+0x491/0xbd0 [Tue Apr 8 12:19:07 2025] schedule+0x27/0xf0 [Tue Apr 8 12:19:07 2025] schedule_timeout+0xe3/0xf0 [Tue Apr 8 12:19:07 2025] ? __folio_mod_stat+0x2a/0x80 [Tue Apr 8 12:19:07 2025] ? set_ptes.constprop.0+0x27/0x90 [Tue Apr 8 12:19:07 2025] __down_common+0x155/0x280 [Tue Apr 8 12:19:07 2025] down+0x53/0x70 [Tue Apr 8 12:19:07 2025] read_dummy_semaphore+0x23/0x60 [Tue Apr 8 12:19:07 2025] full_proxy_read+0x5f/0xa0 [Tue Apr 8 12:19:07 2025] vfs_read+0xbc/0x350 [Tue Apr 8 12:19:07 2025] ? __count_memcg_events+0xa5/0x140 [Tue Apr 8 12:19:07 2025] ? count_memcg_events.constprop.0+0x1a/0x30 [Tue Apr 8 12:19:07 2025] ? handle_mm_fault+0x180/0x260 [Tue Apr 8 12:19:07 2025] ksys_read+0x66/0xe0 [Tue Apr 8 12:19:07 2025] do_syscall_64+0x51/0x120 [Tue Apr 8 12:19:07 2025] entry_SYSCALL_64_after_hwframe+0x76/0x7e [Tue Apr 8 12:19:07 2025] RIP: 0033:0x7f419478f46e [Tue Apr 8 12:19:07 2025] RSP: 002b:00007fff1c4d2668 EFLAGS: 00000246 ORIG_RAX: 0000000000000000 [Tue Apr 8 12:19:07 2025] RAX: ffffffffffffffda RBX: 0000000000020000 RCX: 00007f419478f46e [Tue Apr 8 12:19:07 2025] RDX: 0000000000020000 RSI: 00007f4194683000 RDI: 0000000000000003 [Tue Apr 8 12:19:07 2025] RBP: 00007f4194683000 R08: 00007f4194682010 R09: 0000000000000000 [Tue Apr 8 12:19:07 2025] R10: fffffffffffffbc5 R11: 0000000000000246 R12: 0000000000000000 [Tue Apr 8 12:19:07 2025] R13: 0000000000000003 R14: 0000000000020000 R15: 0000000000020000 [Tue Apr 8 12:19:07 2025] [Tue Apr 8 12:19:07 2025] INFO: task cat:945 blocked on a semaphore likely last held by task cat:938 [Tue Apr 8 12:19:07 2025] task:cat state:S stack:0 pid:938 tgid:938 ppid:584 task_flags:0x400000 flags:0x00000000 [Tue Apr 8 12:19:07 2025] Call Trace: [Tue Apr 8 12:19:07 2025] [Tue Apr 8 12:19:07 2025] __schedule+0x491/0xbd0 [Tue Apr 8 12:19:07 2025] ? _raw_spin_unlock_irqrestore+0xe/0x40 [Tue Apr 8 12:19:07 2025] schedule+0x27/0xf0 [Tue Apr 8 12:19:07 2025] schedule_timeout+0x77/0xf0 [Tue Apr 8 12:19:07 2025] ? __pfx_process_timeout+0x10/0x10 [Tue Apr 8 12:19:07 2025] msleep_interruptible+0x49/0x60 [Tue Apr 8 12:19:07 2025] read_dummy_semaphore+0x2d/0x60 [Tue Apr 8 12:19:07 2025] full_proxy_read+0x5f/0xa0 [Tue Apr 8 12:19:07 2025] vfs_read+0xbc/0x350 [Tue Apr 8 12:19:07 2025] ? __count_memcg_events+0xa5/0x140 [Tue Apr 8 12:19:07 2025] ? count_memcg_events.constprop.0+0x1a/0x30 [Tue Apr 8 12:19:07 2025] ? handle_mm_fault+0x180/0x260 [Tue Apr 8 12:19:07 2025] ksys_read+0x66/0xe0 [Tue Apr 8 12:19:07 2025] do_syscall_64+0x51/0x120 [Tue Apr 8 12:19:07 2025] entry_SYSCALL_64_after_hwframe+0x76/0x7e [Tue Apr 8 12:19:07 2025] RIP: 0033:0x7f7c584a646e [Tue Apr 8 12:19:07 2025] RSP: 002b:00007ffdba8ce158 EFLAGS: 00000246 ORIG_RAX: 0000000000000000 [Tue Apr 8 12:19:07 2025] RAX: ffffffffffffffda RBX: 0000000000020000 RCX: 00007f7c584a646e [Tue Apr 8 12:19:07 2025] RDX: 0000000000020000 RSI: 00007f7c5839a000 RDI: 0000000000000003 [Tue Apr 8 12:19:07 2025] RBP: 00007f7c5839a000 R08: 00007f7c58399010 R09: 0000000000000000 [Tue Apr 8 12:19:07 2025] R10: fffffffffffffbc5 R11: 0000000000000246 R12: 0000000000000000 [Tue Apr 8 12:19:07 2025] R13: 0000000000000003 R14: 0000000000020000 R15: 0000000000020000 [Tue Apr 8 12:19:07 2025] [1] https://lore.kernel.org/all/174046694331.2194069.15472952050240807469.stgit@mhiramat.tok.corp.google.com Link: https://lkml.kernel.org/r/20250414145945.84916-3-ioworker0@gmail.com Signed-off-by: Mingzhe Yang Signed-off-by: Lance Yang Suggested-by: Andrew Morton Suggested-by: Masami Hiramatsu (Google) Reviewed-by: Masami Hiramatsu (Google) Cc: Anna Schumaker Cc: Boqun Feng Cc: Ingo Molnar Cc: Joel Granados Cc: John Stultz Cc: Kent Overstreet Cc: Peter Zijlstra Cc: Sergey Senozhatsky Cc: Steven Rostedt Cc: Tomasz Figa Cc: Waiman Long Cc: Will Deacon Cc: Yongliang Gao Cc: Zi Li Signed-off-by: Andrew Morton --- include/linux/semaphore.h | 15 +++++++++- kernel/hung_task.c | 52 ++++++++++++++++++++++++++-------- kernel/locking/semaphore.c | 57 ++++++++++++++++++++++++++++++++++---- 3 files changed, 106 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 de9117c0e671..3ef032e22f7e 100644 --- a/kernel/locking/semaphore.c +++ b/kernel/locking/semaphore.c @@ -34,6 +34,7 @@ #include #include #include +#include static noinline void __down(struct semaphore *sem); static noinline int __down_interruptible(struct semaphore *sem); @@ -41,6 +42,41 @@ 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, struct wake_q_head *wake_q); +#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 * @sem: the semaphore to be acquired @@ -59,7 +95,7 @@ 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); @@ -83,7 +119,7 @@ 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); @@ -110,7 +146,7 @@ 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); @@ -140,7 +176,7 @@ 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); @@ -165,7 +201,7 @@ 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); @@ -187,6 +223,9 @@ void __sched up(struct semaphore *sem) DEFINE_WAKE_Q(wake_q); raw_spin_lock_irqsave(&sem->lock, flags); + + hung_task_sem_clear_if_holder(sem); + if (likely(list_empty(&sem->wait_list))) sem->count++; else @@ -228,8 +267,10 @@ 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) { + hung_task_sem_set_holder(sem); return 0; + } } timed_out: @@ -246,10 +287,14 @@ static inline int __sched __down_common(struct semaphore *sem, long state, { int ret; + hung_task_set_blocker(sem, BLOCKER_TYPE_SEM); + trace_contention_begin(sem, 0); ret = ___down_common(sem, state, timeout); trace_contention_end(sem, ret); + hung_task_clear_blocker(); + return ret; } -- 2.25.1