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 9:27 PM, Steven Rostedt wrote:
> On Wed, 19 Feb 2025 21:15:08 -0500
> Waiman Long <llong@redhat.com> wrote:
>
>> 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
> Do you really need the READ/WRITE_ONCE() with the memory barriers? From
> what I understand, the compiler can't even assume what it read is the same
> after passing a memory barrier like that. So there should be no reason it
> can reread the memory location after a barrier.

You may be right. However, without using a READ_ONCE/WRITE_ONLY, a 
compiler can potentially break up the read/write into multiple smaller 
trunks resulting in partial data. So I will use them to be on the safe 
side. In this particular scenario above, we may not need to use them on 
type as we are going to reread it. I will keep them for lock though.

Cheers,
Longman


> -- Steve
>