linux-kernel - Re: [Patch v2] mm: thp: grab the lock before manipulation defer list

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Message-ID: <20200114103112.o6ozdbkfnzdsc2ke@box>
Date:   Tue, 14 Jan 2020 13:31:12 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Wei Yang <richardw.yang@...ux.intel.com>, hannes@...xchg.org,
        vdavydov.dev@...il.com, akpm@...ux-foundation.org,
        cgroups@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kirill.shutemov@...ux.intel.com,
        yang.shi@...ux.alibaba.com, alexander.duyck@...il.com,
        rientjes@...gle.com
Subject: Re: [Patch v2] mm: thp: grab the lock before manipulation defer list

On Tue, Jan 14, 2020 at 10:31:22AM +0100, Michal Hocko wrote:
> On Sat 11-01-20 03:03:52, Kirill A. Shutemov wrote:
> > On Thu, Jan 09, 2020 at 10:30:54PM +0800, Wei Yang wrote:
> > > As all the other places, we grab the lock before manipulate the defer list.
> > > Current implementation may face a race condition.
> > > 
> > > 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
> > 
> > I don't think this particular race is possible. Both parties take page
> > lock before messing with deferred queue, but anytway:
> > 
> > Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> 
> I am confused, if the above race is not possible then what would be a
> real race? We really do not want to have a patch with a misleading
> changelog, do we?

The alternative is to make sure that all page_deferred_list() called with
page lock taken.

I'll look into it.

-- 
 Kirill A. Shutemov