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From: Octavian Purdila <tavi.purdila@gmail.com>
Date: Wed, 5 Feb 2020 14:24:38 +0200
Message-ID: <CAMoF9u3Jhqyvp3SpA3mUqPhS4zDuXP9GCUu_XsYx2etE0KGkcQ@mail.gmail.com>
Subject: Re: [RFC v3 01/26] asm-generic: atomic64: allow using generic
 atomic64 on 64bit platforms
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To: Peter Zijlstra <peterz@infradead.org>
Cc: linux-arch <linux-arch@vger.kernel.org>, Boqun Feng <boqun.feng@gmail.com>, linux-um <linux-um@lists.infradead.org>, Akira Moroo <retrage01@gmail.com>, linux-kernel-library <linux-kernel-library@freelists.org>, Will Deacon <will@kernel.org>, Hajime Tazaki <thehajime@gmail.com>

On Wed, Feb 5, 2020 at 11:34 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Wed, Feb 05, 2020 at 04:30:10PM +0900, Hajime Tazaki wrote:
> > From: Octavian Purdila <tavi.purdila@gmail.com>
> >
> > With CONFIG_64BIT enabled, atomic64 via CONFIG_GENERIC_ATOMIC64 options
> > are not compiled due to type conflict of atomic64_t defined in
> > linux/type.h.
> >
> > This commit fixes the issue and allow using generic atomic64 ops.
> >
> > Currently, LKL is only the user which defines GENERIC_ATOMIC64
> > (lib/atomic64.c) under CONFIG_64BIT environment.  Thus, there is no
> > issues before this commit.
>
> Uhhhhh, no.
>
> Not allowing GENERIC_ATOMIC64 on 64BIT is a *feature*.
>
> Any 64bit arch that needs GENERIC_ATOMIC64 is an utter piece of crap.
>
> Please explain more.
>

Hi Peter,

I was not aware that not allowing GENERIC_ATOMIC64 was intentional. I
understand your point that a 64 bit architecture that can't handle 64
bit atomic operation is broken.

One way to deal with this in LKL would be to use GCC atomic builtins
or if that doesn't work expose them as host operations. This would
keep LKL as a meta-arch that can run on multiple physical
architectures. I'll give it a try.

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