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

[Waiman Long]

On 2/19/25 9:27 PM, Steven Rostedt wrote:
> 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.

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