Re: [RFC PATCH 5/8] sched: move CPU field back into thread_info if THREAD_INFO_IN_TASK=y

[Ard Biesheuvel]

On Tue, 14 Sept 2021 at 17:49, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Tue, Sep 14, 2021 at 5:11 AM Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
> >
> >  static inline unsigned int task_cpu(const struct task_struct *p)
> >  {
> >  #ifdef CONFIG_THREAD_INFO_IN_TASK
> > -       return READ_ONCE(p->cpu);
> > +       return READ_ONCE(p->thread_info.cpu);
> >  #else
> >         return READ_ONCE(task_thread_info(p)->cpu);
> >  #endif
>
> Those two lines look different, but aren't.
>
> Please just remove the CONFIG_THREAD_INFO_IN_TASK conditional, and use
>
>           return READ_ONCE(task_thread_info(p)->cpu);
>
> unconditionally, which now does the right thing regardless.
>

Unfortunately not.

task_cpu() takes a 'const struct task_struct *', whereas
task_thread_info() takes a 'struct task_struct *'.

Since task_thread_info()-><foo> is widely used as an lvalue, I would
need to update task_cpu()'s prototype and fix up all the callers, some
of which take the const flavor themselves. Or introduce
'const_task_thread_info()' which takes the const flavor, and cannot be
used to instantiate lvalues.

Suggestions welcome, but this is the cleanest I could come up with.