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

[Steven Rostedt]

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

-- Steve