On Tue, Feb 21, 2023 at 03:22:32PM +0800, Muchun Song wrote: > > > > On Feb 21, 2023, at 13:17, 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? > > > > 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. > > I think there is no difference in the assembly code between them in most > cases. The only intention that I can think of is to avoid the potential > complaint (data race) emitted by KCSAN. +1 And it might be a totally good reason for this change, let's just make it clear, instead of pretending to fix non-existing concurrency problems. Thanks!
