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

Mark Rutland
2019-05-23T10:19:38Z

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@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);

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