Re: [PATCH 1/2] hung_task: Show the blocker task if the task is hung on mutex

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Wed, 19 Feb 2025 21:15:08 -0500
Waiman Long <llong@redhat.com> wrote:

> 
> On 2/19/25 8:41 PM, Steven Rostedt wrote:
> > On Wed, 19 Feb 2025 20:36:13 -0500
> > Waiman Long <llong@redhat.com> wrote:
> >
> >   
> >>>>>> this field, we don't need to take lock, though taking the wait_lock may
> >>>>>> still be needed to examine other information inside the mutex.
> >>> Do we need to take it just for accessing owner, which is in an atomic?
> >> Right. I forgot it is an atomic_long_t. In that case, no lock should be
> >> needed.
> > Now if we have a two fields to read:
> >
> > 	block_flags (for the type of lock) and blocked_on (for the lock)
> >
> > We need a way to synchronize the two. What happens if we read the type, and
> > the task wakes up and and then blocks on a different type of lock?
> >
> > Then the lock read from blocked_on could be a different type of lock than
> > what is expected.
> 
> That is different from reading the owner. In this case, we need to use 
> smp_rmb()/wmb() to sequence the read and write operations unless it is 
> guaranteed that they are in the same cacheline. One possible way is as 
> follows:
> 
> Writer - setting them:
> 
>      WRITE_ONCE(lock)
>      smp_wmb()
>      WRITE_ONCE(type)
> 
> Clearing them:
> 
>      WRITE_ONCE(type, 0)
>      smp_wmb()
>      WRITE_ONCE(lock, NULL)
> 
> Reader:
> 
>      READ_ONCE(type)
> again:
>      smp_rmb()
>      READ_ONCE(lock)
>      smp_rmb()
>      if (READ_ONCE(type) != type)
>          goto again

What about mutex-rwsem-mutex case?

mutex_lock(&lock1);
down_read(&lock2);
mutex_lock(&lock3);

The worst scenario is;

WRITE_ONCE(lock, &lock1)
smp_wmb()
WRITE_ONCE(type, MUTEX)     READ_ONCE(type) -> MUTEX
WRITE_ONCE(type, 0)
smp_wmb()
WRITE_ONCE(lock, NULL)
WRITE_ONCE(lock, &lock2)    READ_ONCE(lock) -> &lock2
smp_wmb()
WRITE_ONCE(type, RWSEM)
WRITE_ONCE(type, 0)
smp_wmb()
WRITE_ONCE(lock, NULL)
WRITE_ONCE(lock, &lock3)
smp_wmb()
WRITE_ONCE(type, MUTEX)     READ_ONCE(type) -> MUTEX == MUTEX
WRITE_ONCE(type, 0)
smp_wmb()
WRITE_ONCE(lock, NULL)

                            "OK, lock2 is a MUTEX!"

So unless stopping the blocker task, we can not ensure this works.
But unless decode the lock, we don't know the blocker task.

Maybe we can run the hung_task in stop_machine()?
(or introduce common_lock)

Thank you,

-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>