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 >
