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Date:   Fri, 24 May 2019 12:42:20 +0100
From:   Will Deacon <will.deacon@arm.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Mark Rutland <mark.rutland@arm.com>,
        Andrea Parri <andrea.parri@amarulasolutions.com>,
        linux-kernel@vger.kernel.org, aou@eecs.berkeley.edu, arnd@arndb.de,
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        aditr@vmware.com, Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH 00/18] locking/atomic: atomic64 type cleanup
Message-ID: <20190524114220.GA4260@fuggles.cambridge.arm.com>
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On Fri, May 24, 2019 at 01:18:07PM +0200, Peter Zijlstra wrote:
> On Fri, May 24, 2019 at 12:37:31PM +0200, Peter Zijlstra wrote:
> > On Thu, May 23, 2019 at 11:19:26AM +0100, Mark Rutland wrote:
> > 
> > > [mark@lakrids:~/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);
> > > 
> > 
> > Oh boy, what a load of crap you just did find.
> > 
> > How about something like the below? I've not read how that buffer is
> > used, but the below preserves all broken without using atomic*_t.
> 
> Clarified by something along these lines?
> 
> ---
>  Documentation/atomic_t.txt | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/Documentation/atomic_t.txt b/Documentation/atomic_t.txt
> index dca3fb0554db..125c95ddbbc0 100644
> --- a/Documentation/atomic_t.txt
> +++ b/Documentation/atomic_t.txt
> @@ -83,6 +83,9 @@ 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.
>  

Not sure you need a new paragraph here.

> +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.
> +

That makes sense to me, although I now find that the sentence below is a bit
confusing because it sounds like it's a caveat relating to only using
Non-RMW ops.

>  The one detail to this is that atomic_set{}() should be observable to the RMW
>  ops. That is:

How about changing this to be:

  "A subtle detail of atomic_set{}() is that it should be observable..."

With that:

Acked-by: Will Deacon <will.deacon@arm.com>

Will