Subject: Re: [PATCH] powerpc/ppc64: remove __volatile__ in get_current()

From: Benjamin Herrenschmidt <[email-masked]>
Date: Sat, 24 Aug 2013 10:20:10 +1000

On Fri, 2013-08-23 at 18:40 -0500, James Yang wrote:
> Scott's been able to put enough doubt in me to think that this is not 
> entirely safe, even though the testing and code generation show it to 
> work.  Please reject this patch.
> 
> I think there is still value in getting the unnecessary loads to be 
> removed since it would also allow unnecessary conditional branches to 
> be removed.  I'll think about alternate ways to do this.

Hrm, The problem has to do with PACA accesses moving around accross
preempt boundaries, it's a bit tricky, but in the case of "current"
shouldn't be a problem... while the rest of the PACA might change (CPU#
etc...) current remains stable for the point of view of a given thread.

So I think the patch is fine.

Scott ?

Now, we do need some serious rework of PACA accesses. I'm very *VERY*
nervous with what we have now. A bit of grepping shows dozens of cases
where gcc copies r13 into another register or even saves/restores it, it
scares the shit out of me :-)

My thinking is to make r13 a hidden reg like we do (or used to) on ppc32
with r2 and break down paca access into two forms:

 - Direct access of a single field -> asm loads/stores inline

 - Anything else, uses a get_paca/put_paca construct that includes a
preempt_disable/enable (and maybe along with a __get_paca/__put_paca
pair that doesn't). This basically does a mr of r13 into another
register and basically hides the whole lot from gcc.

The former would be used for single fields, the latter, while adding a
potentially unnecessary mr, will be much safer vs. gcc playing games
with r13.

Any volunteer ? Haven't had time to do it myself so far :-)

Cheers,
Ben.