linux-kernel - Re: [PATCH 00/18] locking/atomic: atomic64 type cleanup

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Message-ID: <20190523083013.GA4616@andrea>
Date:   Thu, 23 May 2019 10:30:13 +0200
From:   Andrea Parri <andrea.parri@...rulasolutions.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        will.deacon@....com, aou@...s.berkeley.edu, arnd@...db.de,
        bp@...en8.de, catalin.marinas@....com, davem@...emloft.net,
        fenghua.yu@...el.com, heiko.carstens@...ibm.com,
        herbert@...dor.apana.org.au, ink@...assic.park.msu.ru,
        jhogan@...nel.org, linux@...linux.org.uk, mattst88@...il.com,
        mingo@...nel.org, mpe@...erman.id.au, palmer@...ive.com,
        paul.burton@...s.com, paulus@...ba.org, ralf@...ux-mips.org,
        rth@...ddle.net, stable@...r.kernel.org, tglx@...utronix.de,
        tony.luck@...el.com, vgupta@...opsys.com
Subject: Re: [PATCH 00/18] locking/atomic: atomic64 type cleanup

Hi Mark,

On Wed, May 22, 2019 at 02:22:32PM +0100, Mark Rutland wrote:
> Currently architectures return inconsistent types for atomic64 ops. Some return
> long (e..g. powerpc), some return long long (e.g. arc), and some return s64
> (e.g. x86).

(only partially related, but probably worth asking:)

While reading the series, I realized that the following expression:

	atomic64_t v;
        ...
	typeof(v.counter) my_val = atomic64_set(&v, VAL);

is a valid expression on some architectures (in part., on architectures
which #define atomic64_set() to WRITE_ONCE()) but is invalid on others.
(This is due to the fact that WRITE_ONCE() can be used as an rvalue in
the above assignment; TBH, I ignore the reasons for having such rvalue?)

IIUC, similar considerations hold for atomic_set().

The question is whether this is a known/"expected" inconsistency in the
implementation of atomic64_set() or if this would also need to be fixed
/addressed (say in a different patchset)?

Thanks,
  Andrea