Re: mm: possible deadlock in mm_take_all_locks

[Dmitry Vyukov]

On Sun, Jan 10, 2016 at 9:39 PM, Kirill A. Shutemov
<kirill@xxxxxxxxxxxxx> wrote:
> On Sun, Jan 10, 2016 at 09:05:32AM +0100, Dmitry Vyukov wrote:
>> On Sat, Jan 9, 2016 at 12:23 AM, Kirill A. Shutemov
>> <kirill@xxxxxxxxxxxxx> wrote:
>> > On Fri, Jan 08, 2016 at 05:58:33PM +0100, Dmitry Vyukov wrote:
>> >> Hello,
>> >>
>> >> I've hit the following deadlock warning while running syzkaller fuzzer
>> >> on commit b06f3a168cdcd80026276898fd1fee443ef25743. As far as I
>> >> understand this is a false positive, because both call stacks are
>> >> protected by mm_all_locks_mutex.
>> >
>> > +Michal
>> >
>> > I don't think it's false positive.
>> >
>> > The reason we don't care about order of taking i_mmap_rwsem is that we
>> > never takes i_mmap_rwsem under other i_mmap_rwsem, but that's not true for
>> > i_mmap_rwsem vs. hugetlbfs_i_mmap_rwsem_key. That's why we have the
>> > annotation in the first place.
>> >
>> > See commit b610ded71918 ("hugetlb: fix lockdep splat caused by pmd
>> > sharing").
>>
>> Description of b610ded71918 suggests that that code takes hugetlb
>> mutex first and them normal page mutex. In this patch you take them in
>> the opposite order: normal mutex, then hugetlb mutex. Won't this patch
>> only increase probability of deadlocks? Shouldn't you take them in the
>> opposite order?
>
> You are right. I got it wrong. Conditions should be reversed.
>
> The comment around hugetlbfs_i_mmap_rwsem_key definition is somewhat
> confusing:
>
> "This needs an annotation because huge_pmd_share() does an allocation
> under i_mmap_rwsem."
>
> I read this as we do hugetlb allocation when i_mmap_rwsem already taken
> and made locking order respectively. I guess i_mmap_rwsem should be
> replaced with hugetlbfs_i_mmap_rwsem_key in the comment.


Comment on mm_take_all_locks probably also needs updating.