On Thu, Jan 09, 2020 at 09:36:41AM +0100, Michal Hocko wrote: >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! Glad you like it~ Will prepare v2 with your suggestion :-) >-- >Michal Hocko >SUSE Labs -- Wei Yang Help you, Help me
