[RFC PATCH 2/8] thread_info: allow custom in-task thread_info

[Andy Lutomirski]

On Sep 21, 2016 12:28 AM, "Mark Rutland" <mark.rutland at arm.com> wrote:
>
> Hi Andy,
>
> On Fri, Sep 16, 2016 at 08:11:14AM -0700, Andy Lutomirski wrote:
> > > On Thu, Sep 15, 2016 at 11:37:47AM -0700, Andy Lutomirski wrote:
> > > Just to check, what do you mean to happen with the flags field? Should
> > > that always be in the generic thread_info? e.g.
> > >
> > > struct thread_info {
> > >         u32 flags;
> > > #ifdef arch_thread_info
> > >         struct arch_thread_info arch_ti;
> > > #endif
> > > };
> >
> > Exactly.  Possibly with a comment that using thread_struct should be
> > preferred and that arch_thread_info should be used only if some header
> > file requires access via current_thread_info() or task_thread_info().
>
> While fixing up these patches, I realised that I'm somewhat concerned by
> flags becoming a u32 (where it was previously an unsigned long for
> arm64).
>
> The generic {test,set,*}_ti_thread_flag() helpers use the usual bitops,
> which perform accesses of sizeof(unsigned long) at a time, and for arm64
> these need to be naturally-aligned.
>
> We happen to get that alignment from subsequent fields in task_struct
> and/or thread_info, and for arm64 we don't seem to have a problem with
> tearing, but it feels somewhat fragile, and leaves me uneasy.
>
> Looking at the git log, it seems that x86 also use unsigned long until
> commit affa219b60a11b32 ("x86: change thread_info's flag field back to
> 32 bits"), where if I'm reading correctly, this was done to get rid of
> unnecessary padding. With THREAD_INFO_IN_STACK, thread_info::flags is
> immediately followed by a long on x86, so we save no padding.
>
> Given all that, can we make the generic thread_info::flags an unsigned
> long, matching what the thread flag helpers implicitly assume?
>

Yes.  Want to send the patch or should I?

--Andy