On Wed, Feb 22, 2023 at 01:00:00AM +0800, Martin Zhao wrote: > On Tue, Feb 21, 2023 at 1:17 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? > > > > 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. > > Thanks for your watching! > This topic is found in code review by coincidence, so no real issues > recorded for now. I checked other read/write callbacks about other knobs, > most of them use READ_ONCE/WRITE_ONCE on the user setting variable. Sorry, which knobs are you talking about? I actually don't see any user knobs in mm/memcontrol.c which are using WRITE_ONCE(). I see some of them using xchg(), but it's a different thing. > Actually I am curious as well why this interface doesn't use > READ_ONCE/WRITE_ONCE, is there any other synchronization mechanism I > didn't notice yet? Because nobody saw any issues with the current code? And again if it's something that makes any automated verifiers/tooling unhappy, I'm totally fine for fixing it, just let make it clear (and also fix the commit title, which is not true). Thanks!
