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Date:   Wed, 13 Mar 2019 10:18:44 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     Qian Cai <cai@lca.pw>, Jason Gunthorpe <jgg@mellanox.com>,
        "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Mark Rutland <mark.rutland@arm.com>
Subject: Re: [PATCH] mm/debug: add a cast to u64 for atomic64_read()
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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@lca.pw> wrote:
> >
> > On Mon, 2019-03-11 at 12:21 +0000, Jason Gunthorpe wrote:
> > > On Sun, Mar 10, 2019 at 08:58:15PM -0700, Davidlohr Bueso wrote:
> > > > On Sun, 10 Mar 2019, Qian Cai wrote:
> > >
> > > Not saying this patch shouldn't go ahead..
> > >
> > > But is there a special reason the atomic64*'s on ppc don't use the u64
> > > type like other archs? Seems like a better thing to fix than adding
> > > casts all over the place.

s64 if anything, atomic stuff is signed (although since we have -fwrapv
it doesn't matter one whit).

> > A bit of history here,
> >
> > https://patchwork.kernel.org/patch/7344011/#15495901
> 
> Ah, I had already forgotten about that discussion.
> 
> At least the atomic_long part we discussed there has been resolved now
> as part of commit b5d47ef9ea5c ("locking/atomics: Switch to generated
> atomic-long").
> 
> Adding Mark Rutland to Cc, maybe he has some ideas of how to use
> the infrastructure he added to use consistent types for atomic64()
> on the remaining 64-bit architectures.

A quick count shows there's only 5 definitions of atomic64_t in the
tree, it would be trivial to align them on type.

$ git grep "} atomic64_t"
arch/arc/include/asm/atomic.h:} atomic64_t;
arch/arm/include/asm/atomic.h:} atomic64_t;
arch/x86/include/asm/atomic64_32.h:} atomic64_t;
include/asm-generic/atomic64.h:} atomic64_t;
include/linux/types.h:} atomic64_t;

Note that the one used in _most_ cases, is the one from linux/types.h,
and that is using 'long'. The others, all typically on ILP32 platforms,
obviously must use long long.

I have no objection to changing the types.h one to long long or all of
them to s64. It really shouldn't matter at all.