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