Messages in this thread | | | From | Yafang Shao <> | Date | Thu, 30 Apr 2020 09:31:23 +0800 | Subject | Re: [PATCH 1/2] mm, memcg: Avoid stale protection values when cgroup is above protection |
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On Thu, Apr 30, 2020 at 9:16 AM Chris Down <chris@chrisdown.name> wrote: > > Hi Yafang, > > Yafang Shao writes: > >Would you pls. add some comments above these newly added WRITE_ONCE() ? > >E.g. > >What does them mean to fix ? > >Why do we must add WRITE_ONCE() and READ_ONCE here and there all over > >the memcg protection ? > >Otherwise, it may be harder to understand by the others. > > There is already discussion in the changelogs for previous store tear > improvements. For example, b3a7822e5e75 ("mm, memcg: prevent > mem_cgroup_protected store tearing"). >
I'm sorry that I missed the changelog in the other one. So you'd better add these commit log or comment to this one again.
> WRITE_ONCE and READ_ONCE are standard compiler barriers, in this case, to avoid > store tears from writes in another thread (effective protection caching is > designed by its very nature to permit racing, but tearing is non-ideal). > > You can find out more about them in the "COMPILER BARRIER" section in > Documentation/memory-barriers.txt. I'm not really seeing the value of adding an > extra comment about this specific use of them, unless you have some more > explicit concern.
My concern is why we add these barriers to memcg protection specifically but don't add these barriers to the other memebers like memcg->oom_group which has the same issue ? What is the difference between these members and that members ?
-- Thanks Yafang
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