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

Andrea Parri
2019-05-23T11:20:24Z

> > 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.

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