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

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Message-ID: <20190523101926.GA3370@lakrids.cambridge.arm.com>
Date:   Thu, 23 May 2019 11:19:26 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Andrea Parri <andrea.parri@...rulasolutions.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

On Thu, May 23, 2019 at 10:30:13AM +0200, Andrea Parri wrote:
> Hi Mark,

Hi Andrea,

> 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)?

In either case, I don't think the intent is that they should be used that way,
and from a quick scan, I can only fine a single relevant instance today:

[mark@...rids:~/src/linux]% git grep '\(return\|=\)\s\+atomic\(64\)\?_set'
include/linux/vmw_vmci_defs.h:  return atomic_set((atomic_t *)var, (u32)new_val);
include/linux/vmw_vmci_defs.h:  return atomic64_set(var, new_val);


[mark@...rids:~/src/linux]% git grep '=\s+atomic_set' | wc -l
0
[mark@...rids:~/src/linux]% git grep '=\s+atomic64_set' | wc -l
0

Any architectures implementing arch_atomic_* will have both of these functions
returning void. Currently that's x86 and arm64, but (time permitting) I intend
to migrate other architectures, so I guess we'll have to fix the above up as
required.

I think it's best to avoid the construct above.

Thanks,
Mark.