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

[Wei Yang]

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().


-- 
Wei Yang
Help you, Help me