) On Mon, Nov 15, 2021 at 10:55 AM Mina Almasry <almasrymina@xxxxxxxxxx> wrote: > > 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? > +Paul & Marco Let's ask the experts. We have a "unsigned long usage" variable that is updated within a lock (hugetlb_lock) but is read without the lock. Q1) I think KCSAN will complain about it and READ_ONCE() in the unlocked read path should be good enough to silent KCSAN. So, the question is should we still use WRITE_ONCE() as well for usage within hugetlb_lock? Q2) Second question is more about 64 bit archs breaking a 64 bit write into two 32 bit writes. Is this a real issue? If yes, then the combination of READ_ONCE()/WRITE_ONCE() are good enough for the given use-case? thanks, Shakeel
