+Paul & Marco On Tue, Feb 21, 2023 at 5:51 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > 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. > Thanks Willy for pointing this out. If the compiler can really do this then [READ|WRITE]_ONCE are required here. I always have big bad compiler lwn article open in a tab. I couldn't map this transformation to ones mentioned in that article. Do we have name of this one? thanks, Shakeel
