Re: [PATCH v2] powerpc: slightly improve cache helpers

On Fri, Jul 19, 2019 at 09:04:55AM -0700, Nathan Chancellor wrote:

On Fri, Jul 19, 2019 at 10:23:03AM -0500, Segher Boessenkool wrote:

quoted

On Thu, Jul 18, 2019 at 08:24:56PM -0700, Nathan Chancellor wrote:

quoted

On Mon, Jul 08, 2019 at 11:49:52PM -0700, Nathan Chancellor wrote:

quoted

On Tue, Jul 09, 2019 at 07:04:43AM +0200, Christophe Leroy wrote:

quoted

Is that a Clang bug ?

No idea, it happens with clang-8 and clang-9 though (pretty sure there
were fixes for PowerPC in clang-8 so something before it probably won't
work but I haven't tried).

quoted

Do you have a disassembly of the code both with and without this patch in
order to compare ?

I can give you whatever disassembly you want (or I can upload the raw
files if that is easier).

What disassembly/files did you need to start taking a look at this? I
can upload/send whatever you need.

A before and after of *only this patch*.  And then look at what changed;
it maybe be obvious what is the problem to you, as well, so look at it
yourself first?


Segher

Hi Segher,

Looks like the problematic function is dcbz, as there is no segfault
when only that function is reverted to a
pre-6c5875843b87c3adea2beade9d1b8b3d4523900a state.

I was able to expose a singular problematic callsite using the attached
patch (let me know if that is insufficient).

I have attached the disassembly of arch/powerpc/kernel/mem.o with
clear_page (working) and broken_clear_page (broken), along with the side
by side diff. My assembly knowledge is fairly limited as it stands and
it is certainly not up to snuff on PowerPC so I have no idea what I am
looking for. Please let me know if anything immediately looks off or if
there is anything else I can do to help out.

Cheers,
Nathan