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

[Wei Yang]

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