Re: [PATCH 1/1] hung_task: show the blocker task if the task is hung on semaphore

[Lance Yang]

On Tue, Mar 11, 2025 at 12:20 PM Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
>
> On Thu,  6 Mar 2025 14:05:42 +0800
> Lance Yang <ioworker0@xxxxxxxxx> 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@xxxxxxxxx>
> > + */
> > +#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@xxxxxxxxxx>