Re: [RFC PATCH] mm: thp: grab the lock before manipulation defer list

From: Michal Hocko
Date: Thu Jan 09 2020 - 03:36:48 EST


On Thu 09-01-20 11:18:21, Wei Yang wrote:
> On Wed, Jan 08, 2020 at 10:40:41AM +0100, Michal Hocko wrote:
> >On Wed 08-01-20 08:35:43, Wei Yang wrote:
> >> On Tue, Jan 07, 2020 at 09:38:08AM +0100, Michal Hocko wrote:
> >> >On Tue 07-01-20 09:22:41, Wei Yang wrote:
> >> >> On Mon, Jan 06, 2020 at 11:23:45AM +0100, Michal Hocko wrote:
> >> >> >On Fri 03-01-20 22:34:07, Wei Yang wrote:
> >> >> >> As all the other places, we grab the lock before manipulate the defer list.
> >> >> >> Current implementation may face a race condition.
> >> >> >
> >> >> >Please always make sure to describe the effect of the change. Why a racy
> >> >> >list_empty check matters?
> >> >> >
> >> >>
> >> >> Hmm... access the list without proper lock leads to many bad behaviors.
> >> >
> >> >My point is that the changelog should describe that bad behavior.
> >> >
> >> >> For example, if we grab the lock after checking list_empty, the page may
> >> >> already be removed from list in split_huge_page_list. And then list_del_init
> >> >> would trigger bug.
> >> >
> >> >And how does list_empty check under the lock guarantee that the page is
> >> >on the deferred list?
> >>
> >> Just one confusion, is this kind of description basic concept of concurrent
> >> programming? How detail level we need to describe the effect?
> >
> >When I write changelogs for patches like this I usually describe, what
> >is the potential race - e.g.
> > CPU1 CPU2
> > path1 path2
> > check lock
> > operation2
> > unlock
> > lock
> > # check might not hold anymore
> > operation1
> > unlock
> >
> >and what is the effect of the race - e.g. a crash, data corruption,
> >pointless attempt for operation1 which fails with user visible effect
> >etc.
>
> Hi, Michal, here is my attempt for an example. Hope this one looks good to
> you.
>
>
> For example, the potential race would be:
>
> CPU1 CPU2
> mem_cgroup_move_account split_huge_page_to_list
> !list_empty
> lock
> !list_empty
> list_del
> unlock
> lock
> # !list_empty might not hold anymore
> list_del_init
> unlock
>
> When this sequence happens, the list_del_init() in
> mem_cgroup_move_account() would crash since the page is already been
> removed by list_del in split_huge_page_to_list().

Yes this looks much more informative. I would just add that this will
crash if CONFIG_DEBUG_LIST.

Thanks!
--
Michal Hocko
SUSE Labs