Re: [PATCH v6] hugetlb: Add hugetlb.*.numa_stat file

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On Mon, Nov 15, 2021 at 10:22 AM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
>
> Subject:   Re: [PATCH v6] hugetlb: Add hugetlb.*.numa_stat file
>
> To:        Muchun Song <songmuchun@xxxxxxxxxxxxx>, Shakeel Butt <shakeelb@xxxxxxxxxx>, Mina Almasry <almasrymina@xxxxxxxxxx>
>
> Cc:        Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>, Shuah Khan <shuah@xxxxxxxxxx>, Miaohe Lin <linmiaohe@xxxxxxxxxx>, Oscar Salvador <osalvador@xxxxxxx>, Michal Hocko <mhocko@xxxxxxxx>, David Rientjes <rientjes@xxxxxxxxxx>, Jue Wang <juew@xxxxxxxxxx>, Yang Yao <ygyao@xxxxxxxxxx>, Joanna Li <joannali@xxxxxxxxxx>, Cannon Matthews <cannonmatthews@xxxxxxxxxx>, Linux Memory Management List <linux-mm@xxxxxxxxx>, LKML <linux-kernel@xxxxxxxxxxxxxxx>
>
> Bcc:
>
> -=-=-=-=-=-=-=-=-=# Don't remove this line #=-=-=-=-=-=-=-=-=-
>
> On 11/14/21 5:43 AM, Muchun Song wrote:
>
> > On Sun, Nov 14, 2021 at 3:15 AM Shakeel Butt <shakeelb@xxxxxxxxxx> wrote:
>
> >> On Sat, Nov 13, 2021 at 6:48 AM Mina Almasry <almasrymina@xxxxxxxxxx> wrote:
>
> >>> On Fri, Nov 12, 2021 at 6:45 PM Muchun Song <songmuchun@xxxxxxxxxxxxx> wrote:
>
> >>>> On Sat, Nov 13, 2021 at 7:36 AM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
>
> >> We have following options:
>
> >>
>
> >> 1) Use atomic type for usage.
>
> >> 2) Use "unsigned long" for usage along with WRITE_ONCE/READ_ONCE.
>
> >> 3) Use hugetlb_lock for hugetlb_cgroup_read_numa_stat as well.
>
> >>
>
> >> All options are valid but we would like to avoid (3).
>
> >>
>
> >> What if we use "unsigned long" type but without READ_ONCE/WRITE_ONCE.
>
> >> The potential issues with that are KCSAN will report this as race and
>
> >> possible garbage value on archs which do not support atomic writes to
>
> >> unsigned long.
>
> >
>
> > At least I totally agree with you. Thanks for your detailed explanation.
>
> >
>
>
>
> Thanks everyone.  This makes sense.
>
>
>
> However, I should note that this same situation (updates to unsigned
>
> long variables under lock and reads of the the same variable without
>
> lock or READ/WRITE_ONCE) exists in hugetlb sysfs files today.  Not
>
> suggesting that this makes it OK to ignore the potential issue.  Just
>
> wanted to point this out.
>

Sorry I'm still a bit confused. READ_ONCE/WRITE_ONCE isn't documented
to provide atomicity to the write or read, just prevents the compiler
from re-ordering them. Is there something I'm missing, or is the
suggestion to add READ_ONCE/WRITE_ONCE simply to supress the KCSAN
warnings?

> --
>
> Mike Kravetz
>





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