Hello, Aleksa.
On Wed, Oct 16, 2019 at 07:32:19PM +1100, Aleksa Sarai wrote:
Maybe I'm misunderstanding Documentation/atomic_t.txt, but it looks to me like it's explicitly saying that I shouldn't use atomic64_t if I'm just using it for fetching and assignment.
Hah, where is it saying that? The alternative would be seqlock or u64_stats or straight-up locking but idk for this atomic64_t should be fine.
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. 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.
As for 64-bit on 32-bit machines -- that is a separate issue, but from [1] it seems to me like there are more problems that *_ONCE() fixes than just split reads and writes.
Your explanations are too wishy washy. If you wanna fix it, please do it correctly. R/W ONCE isn't the right solution here.
Thanks.