On Mon, Feb 20, 2023 at 10:52:10PM -0800, Shakeel Butt wrote: > 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. Wouldn't a compiler be within its rights to implement a one byte store as: load-word modify-byte-in-word store-word and if this is a lockless store to a word which has an adjacent byte also being modified by another CPU, one of those CPUs can lose its store? And WRITE_ONCE would prevent the compiler from implementing the store in that way.
