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

[Waiman Long <llong@redhat.com>]

On 2/19/25 9:59 PM, Masami Hiramatsu (Google) wrote:
> 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.

That could only happen if the reader can get interrupted/preempted for a 
long time. In that case, we may need to reread the lock again to be sure 
that they are stable.

Cheers,
Longman

>
> Maybe we can run the hung_task in stop_machine()?
> (or introduce common_lock)
>
> Thank you,
>