On Mon, Feb 20, 2023 at 9:17 PM Roman Gushchin <roman.gushchin@xxxxxxxxx> wrote: > > > On Feb 20, 2023, at 3:06 PM, Shakeel Butt <shakeelb@xxxxxxxxxx> wrote: > > > > On Mon, Feb 20, 2023 at 01:09:44PM -0800, Roman Gushchin wrote: > >>> On Mon, Feb 20, 2023 at 11:16:38PM +0800, Yue Zhao wrote: > >>> The knob for cgroup v2 memory controller: memory.oom.group > >>> will be read and written simultaneously by user space > >>> programs, thus we'd better change memcg->oom_group access > >>> with atomic operations to avoid concurrency problems. > >>> > >>> Signed-off-by: Yue Zhao <findns94@xxxxxxxxx> > >> > >> Hi Yue! > >> > >> I'm curious, have any seen any real issues which your patch is solving? > >> Can you, please, provide a bit more details. > >> > > > > IMHO such details are not needed. oom_group is being accessed > > concurrently and one of them can be a write access. At least > > READ_ONCE/WRITE_ONCE is needed here. > > Needed for what? For this particular case, documenting such an access. Though I don't think there are any architectures which may tear a one byte read/write and merging/refetching is not an issue for this. > > I mean it’s obviously not a big deal to put READ_ONCE()/WRITE_ONCE() here, but I struggle to imagine a scenario when it will make any difference. IMHO it’s easier to justify a proper atomic operation here, even if it’s most likely an overkill. > > My question is very simple: the commit log mentions “… to avoid concurrency problems”, so I wonder what problems are these. > > Also there are other similar cgroup interfaces without READ_ONCE()/WRITE_ONCE() Yeah and those are v1 interfaces e.g. oom_kill_disable, swappiness, soft_limit. These definitely need [READ|WRITE]_ONCE primitive. Yue, can you update your patch and convert all accesses to these fields through [READ|WRITE]_ONCE ?
