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

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Date:   Thu, 23 May 2019 13:20:13 +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

> > 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.

Thank you for the clarification, Mark.  I agree with you that it'd be
better to avoid such constructs.  (FWIW, it is not currently possible
to use them in litmus tests for the LKMM...)

Thanks,
  Andrea