On 2019-10-14, Tejun Heo wrote: > Hello, Aleksa. > > On Tue, Oct 15, 2019 at 02:59:31AM +1100, Aleksa Sarai wrote: > > On 2019-10-14, Tejun Heo wrote: > > > On Sat, Oct 12, 2019 at 12:05:39PM +1100, Aleksa Sarai wrote: > > > > Because pids->limit can be changed concurrently (but we don't want to > > > > take a lock because it would be needlessly expensive), use the > > > > appropriate memory barriers. > > > > > > I can't quite tell what problem it's fixing. Can you elaborate a > > > scenario where the current code would break that your patch fixes? > > > > As far as I can tell, not using *_ONCE() here means that if you had a > > process changing pids->limit from A to B, a process might be able to > > temporarily exceed pids->limit -- because pids->limit accesses are not > > protected by mutexes and the C compiler can produce confusing > > intermediate values for pids->limit[1]. > > > > But this is more of a correctness fix than one fixing an actually > > exploitable bug -- given the kernel memory model work, it seems like a > > good idea to just use READ_ONCE() and WRITE_ONCE() for shared memory > > access. > > READ/WRITE_ONCE provides protection against compiler generating > multiple accesses for a single operation. It won't prevent split > writes / reads of 64bit variables on 32bit machines. For that, you'd > have to switch them to atomic64_t's. 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. > 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. [1]: https://lwn.net/Articles/793253/ -- Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH