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Message-ID: <CAK1f24kbb5w76CeFVDJGN79OYuJry-irxp04Jxf=opKQZA3shQ@mail.gmail.com>
Date: Tue, 11 Mar 2025 12:48:33 +0800
From: Lance Yang <ioworker0@...il.com>
To: Masami Hiramatsu <mhiramat@...nel.org>
Cc: akpm@...ux-foundation.org, anna.schumaker@...cle.com, boqun.feng@...il.com,
joel.granados@...nel.org, kent.overstreet@...ux.dev, leonylgao@...cent.com,
linux-kernel@...r.kernel.org, longman@...hat.com, mingo@...hat.com,
mingzhe.yang@...com, peterz@...radead.org, rostedt@...dmis.org,
senozhatsky@...omium.org, tfiga@...omium.org, will@...nel.org
Subject: Re: [PATCH 1/1] hung_task: show the blocker task if the task is hung
on semaphore
On Tue, Mar 11, 2025 at 12:20 PM Masami Hiramatsu <mhiramat@...nel.org> wrote:
>
> On Thu, 6 Mar 2025 14:05:42 +0800
> Lance Yang <ioworker0@...il.com> wrote:
>
> > Hi Masami,
> >
> > Could you give me a quick feedback before sending out the v2?
> >
> > It seems unnecessary to make 'blocker' a union. I replaced 'struct mutex
> > *blocker_mutex' with 'unsigned long blocker', as only one blocker is active
> > at a time, IIUC ;)
> >
> > The blocker filed can store both the lock addrees and the lock type, with
> > LSB used to encode the type as you 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 ;)
>
> Hi Lance,
>
> Sorry I missed to reply.
No worries at all ;p
>
> >
> > ---
> > include/linux/hung_task.h | 82 +++++++++++++++++++++++++++++++++++++++
> > include/linux/sched.h | 2 +-
> > kernel/hung_task.c | 15 ++++---
> > kernel/locking/mutex.c | 8 +++-
> > 4 files changed, 99 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..569d2e579f36
> > --- /dev/null
> > +++ b/include/linux/hung_task.h
> > @@ -0,0 +1,82 @@
> > +/* SPDX-License-Identifier: GPL-2.0-only */
> > +/*
> > + * Detect Hung Task: detecting tasks stuck in D state
> > + *
> > + * Copyright 2025 Lance Yang <ioworker0@...il.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 rw-mutex (BLOCKER_TYPE_RWMUTEX)
>
> ^ RT-Mutex?
Good catch! It's a typo, sorry ;(
>
> > + * 11 - Blocked on rw-semaphore (BLOCKER_TYPE_RWSEM)
> > + */
> > +#define BLOCKER_TYPE_MUTEX 0x00UL
> > +#define BLOCKER_TYPE_SEM 0x01UL
> > +#define BLOCKER_TYPE_RWMUTEX 0x02UL
>
> Ditto.
Yes. It should have been called BLOCKER_TYPE_RTMUTEX ;)
>
> > +#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(lock_ptr & BLOCKER_TYPE_MASK);
>
> If it founds the lock_ptr & BLOCKER_TYPE_MASK is set, it should return
> without writing anything. (WARN_ON_ONCE() is OK)
Yeah, it makes sense to me. Will adjust as you suggested.
>
> > + WARN_ON_ONCE(READ_ONCE(current->blocker));
> > +
> > + 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);
> > +}
> > +
> > +static inline bool hung_task_blocker_is_type(unsigned long blocker,
> > + unsigned long type)
> > +{
> > + WARN_ON_ONCE(!blocker);
> > +
> > + return (blocker & BLOCKER_TYPE_MASK) == type;
> > +}
> > +
> > +static inline void *hung_task_blocker_to_lock(unsigned long blocker)
> > +{
> > + WARN_ON_ONCE(!blocker);
> > +
> > + return (void *)(blocker & ~BLOCKER_TYPE_MASK);
> > +}
> > +#else
> > +#define hung_task_set_blocker(lock, type) do {} while (0)
> > +#define hung_task_clear_blocker() do {} while (0)
> > +static inline bool hung_task_blocker_is_type(unsigned long blocker,
> > + unsigned long type)
> > +{
> > + return false;
> > +}
> > +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 ccd7217fcec1..f7fa832261c8 100644
> > --- a/kernel/hung_task.c
> > +++ b/kernel/hung_task.c
> > @@ -25,6 +25,10 @@
> >
> > #include <trace/events/sched.h>
> >
> > +#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
> > +#include <linux/hung_task.h>
> > +#endif
> > +
> > /*
> > * The number of tasks checked:
> > */
> > @@ -98,16 +102,17 @@ 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_blocker_is_type(blocker, BLOCKER_TYPE_MUTEX))
> > return;
> >
> > - owner = mutex_get_owner(lock);
> > + owner = mutex_get_owner(
> > + (struct mutex *)hung_task_blocker_to_lock(blocker));
>
> Yeah, others look good to me.
Thanks a lot for taking time to review!
Lance
>
> Thank you!
>
>
> > +
> > 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..642d6398e0dd 100644
> > --- a/kernel/locking/mutex.c
> > +++ b/kernel/locking/mutex.c
> > @@ -42,6 +42,10 @@
> > # define MUTEX_WARN_ON(cond)
> > #endif
> >
> > +#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
> > +#include <linux/hung_task.h>
> > +#endif
> > +
> > void
> > __mutex_init(struct mutex *lock, const char *name, struct lock_class_key *key)
> > {
> > @@ -189,7 +193,7 @@ __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 +211,7 @@ __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
> > }
> >
> > --
> >
> > Thanks,
> > Lance
>
>
> --
> Masami Hiramatsu (Google) <mhiramat@...nel.org>
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