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

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Message-ID: <20190528104719.GN2623@hirez.programming.kicks-ass.net>
Date:   Tue, 28 May 2019 12:47:19 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andrea Parri <andrea.parri@...rulasolutions.com>
Cc:     Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>,
        linux-kernel@...r.kernel.org, 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,
        gregkh@...uxfoundation.org, jhansen@...are.com, vdasa@...are.com,
        aditr@...are.com, Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 00/18] locking/atomic: atomic64 type cleanup

On Sat, May 25, 2019 at 12:43:40AM +0200, Andrea Parri wrote:
> > ---
> > Subject: Documentation/atomic_t.txt: Clarify pure non-rmw usage
> > 
> > Clarify that pure non-RMW usage of atomic_t is pointless, there is
> > nothing 'magical' about atomic_set() / atomic_read().
> > 
> > This is something that seems to confuse people, because I happen upon it
> > semi-regularly.
> > 
> > Acked-by: Will Deacon <will.deacon@....com>
> > Reviewed-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > ---
> >  Documentation/atomic_t.txt | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> > 
> > diff --git a/Documentation/atomic_t.txt b/Documentation/atomic_t.txt
> > index dca3fb0554db..89eae7f6b360 100644
> > --- a/Documentation/atomic_t.txt
> > +++ b/Documentation/atomic_t.txt
> > @@ -81,9 +81,11 @@ SEMANTICS
> >  
> >  The non-RMW ops are (typically) regular LOADs and STOREs and are canonically
> >  implemented using READ_ONCE(), WRITE_ONCE(), smp_load_acquire() and
> > -smp_store_release() respectively.
> > +smp_store_release() respectively. Therefore, if you find yourself only using
> > +the Non-RMW operations of atomic_t, you do not in fact need atomic_t at all
> > +and are doing it wrong.
> 
> The counterargument (not so theoretic, just look around in the kernel!) is:
> we all 'forget' to use READ_ONCE() and WRITE_ONCE(), it should be difficult
> or more difficult to forget to use atomic_read() and atomic_set()...   IAC,
> I wouldn't call any of them 'wrong'.

I'm thinking you mean that the type system isn't helping us with
READ/WRITE_ONCE() like it does with atomic_t ? And while I agree that
there is room for improvement there, that doesn't mean we should start
using atomic*_t all over the place for that.

Part of the problem with READ/WRITE_ONCE() is that it serves a dual
purpose; we've tried to untangle that at some point, but Linus wasn't
having it.