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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Tue, 14 Sep 2021 17:52:59 +0200
Message-ID: <CAMj1kXH_Q4a4Gsi0Xuw=YsV-b7Mu8TQndk3Ei-JFaRV=GSiqUQ@mail.gmail.com>
Subject: Re: [RFC PATCH 5/8] sched: move CPU field back into thread_info if THREAD_INFO_IN_TASK=y
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Keith Packard <keithpac@amazon.com>,
        Russell King <linux@armlinux.org.uk>,
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On Tue, 14 Sept 2021 at 17:49, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Tue, Sep 14, 2021 at 5:11 AM Ard Biesheuvel <ardb@kernel.org> 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.