linux-kernel - Re: [PATCH] mm/debug: add a cast to u64 for atomic64

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Message-ID: <20190313162903.GB39315@lakrids.cambridge.arm.com>
Date:   Wed, 13 Mar 2019 16:29:04 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Peter Zijlstra <peterz@...radead.org>, Qian Cai <cai@....pw>,
        Jason Gunthorpe <jgg@...lanox.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm/debug: add a cast to u64 for atomic64_read()

On Wed, Mar 13, 2019 at 03:57:42PM +0100, Arnd Bergmann wrote:
> On Wed, Mar 13, 2019 at 3:36 PM Mark Rutland <mark.rutland@....com> wrote:
> > On Wed, Mar 13, 2019 at 10:18:44AM +0100, Peter Zijlstra wrote:
> > > On Mon, Mar 11, 2019 at 03:20:04PM +0100, Arnd Bergmann wrote:
> > > > On Mon, Mar 11, 2019 at 3:00 PM Qian Cai <cai@....pw> wrote:
> >
> > I think that using s64 consistently (with any necessary alignment
> > annotation) makes the most sense. That's unambigious, and what the
> > common headers now use.
> >
> > Now that the scripted atomics are merged, I'd like to move arches over
> > to arch_atomic_*(), so the argument and return types will become s64
> > everywhere.
> 
> Yes, that sounds like the easiest way, especially if we don't touch the
> internal implementation but simply rename all the symbols provided
> by the architectures. Is that what you had in mind, or would you go
> beyond the minimum changes here?

I'd expected to convert arches one-by-one, updating the types during
conversion. I guess it's not strictly necessary to change the internal
types, but it would seem nicer to do that.

I don't think it's possible to do that rename right now, unless we do it
treewide. There are a few core things that need to be fixed up first,
e.g. making <asm-generic/atomic{,64}.h> play nicely with
<asm-generic/atomic-instrumented.h>.

In the end, what I'd like to get to is:

* Arch code provids arch_atomic*_*().

* Common code fleshes out the entire API as raw_atomic*_*(), build atop
  of arch_atomic*_*(). All the ifdeffery lives here.

* Common code builds the instrumented atomic*_*() API atop of the
  raw_atomic*_*() API. No ifdeffery necessary here.

Thanks,
Mark.