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

Will Deacon
2019-05-24T11:42:34Z

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