From: Wei Yang <richardw.yang@linux.intel.com>
To: Michal Hocko <mhocko@kernel.org>
Cc: Wei Yang <richardw.yang@linux.intel.com>,
hannes@cmpxchg.org, vdavydov.dev@gmail.com,
akpm@linux-foundation.org, kirill.shutemov@linux.intel.com,
cgroups@vger.kernel.org, linux-mm@kvack.org,
linux-kernel@vger.kernel.org, yang.shi@linux.alibaba.com
Subject: Re: [RFC PATCH] mm: thp: grab the lock before manipulation defer list
Date: Thu, 9 Jan 2020 11:18:21 +0800 [thread overview]
Message-ID: <20200109031821.GA5206@richard> (raw)
In-Reply-To: <20200108094041.GQ32178@dhcp22.suse.cz>
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
next prev parent reply other threads:[~2020-01-09 3:18 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-03 14:34 [RFC PATCH] mm: thp: grab the lock before manipulation defer list Wei Yang
2020-01-03 19:29 ` David Rientjes
2020-01-03 23:39 ` Wei Yang
2020-01-04 0:44 ` David Rientjes
2020-01-06 1:20 ` Wei Yang
2020-01-06 10:23 ` Michal Hocko
2020-01-07 1:22 ` Wei Yang
2020-01-07 8:38 ` Michal Hocko
2020-01-08 0:35 ` Wei Yang
2020-01-08 9:40 ` Michal Hocko
2020-01-09 2:03 ` Wei Yang
2020-01-09 2:03 ` Wei Yang
2020-01-09 8:34 ` Michal Hocko
2020-01-09 3:18 ` Wei Yang [this message]
2020-01-09 8:36 ` Michal Hocko
2020-01-09 8:52 ` Wei Yang
2020-01-06 16:18 ` Alexander Duyck
2020-01-07 1:26 ` Wei Yang
2020-01-07 2:07 ` David Rientjes
2020-01-07 2:33 ` Wei Yang
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