On Thu, Jun 23, 2016 at 12:17 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> With the goal being that I'm hoping that we can then actually get rid
> of this (at least on x86-64, even if we leave it in some other
> architectures) in 4.8.

The context here was that we could almost get rid of thread-info
entirely, at least for x86-64, by moving it into struct task_struct.

It turns out that we're not *that* far off after the obvious cleanups
I already committed, but I couldn't get things quite to work.

I'm attaching a patch that I wrote today that doesn't boot, but "looks
right". The reason I'm attaching it is because I'm hoping somebody
wants to take a look and maybe see what else I missed, but mostly
because I think the patch is interesting in a couple of cases where we
just do incredibly ugly things.

First off, some code that Andy wrote when he re-organized the entry path.

Oh Gods, Andy. That pt_regs_to_thread_info() thing made me want to do
unspeakable acts on a poor innocent wax figure that looked _exactly_
like you.

I just got rid of pt_regs_to_thread_info() entirely, and just replaced
it with current_thread_info().  I'm not at all convinced that trying
to be that clever was really a good idea.

Secondly, the x86-64 ret_from_fork calling convention was documented
wrongly. It says %rdi contains the previous task pointer. Yes it does,
but it doesn't mention that %r8 is supposed to contain the new
thread_info. That was fun to find.

And thirdly, the stack size games that asm/kprobes.h plays are just
disgusting. I stared at that code for much too long. I may in fact be
going blind as a result.

The rest was fairly straightforward, although since the end result
doesn't actually work, that "straightforward" may be broken too. But
the basic approach _looks_ sane.

Comments? Anybody want to play with this and see where I went wrong?

(Note - this patch was written on top of the two thread-info removal
patches I committed in

   da01e18a37a5 x86: avoid avoid passing around 'thread_info' in stack
dumping code
   6720a305df74 locking: avoid passing around 'thread_info' in mutex
debugging code

and depends on them, since "ti->task" no longer exists with
CONFIG_THREAD_INFO_IN_TASK. "ti" and "task" will have the same value).

                 Linus