Re: mm: possible deadlock in mm_take_all_locks

[Dmitry Vyukov]

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?


> Consider totally untested patch below.
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 2ce04a649f6b..63aefcf409e1 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -3203,7 +3203,16 @@ int mm_take_all_locks(struct mm_struct *mm)
>         for (vma = mm->mmap; vma; vma = vma->vm_next) {
>                 if (signal_pending(current))
>                         goto out_unlock;
> -               if (vma->vm_file && vma->vm_file->f_mapping)
> +               if (vma->vm_file && vma->vm_file->f_mapping &&
> +                               !is_vm_hugetlb_page(vma))
> +                       vm_lock_mapping(mm, vma->vm_file->f_mapping);
> +       }
> +
> +       for (vma = mm->mmap; vma; vma = vma->vm_next) {
> +               if (signal_pending(current))
> +                       goto out_unlock;
> +               if (vma->vm_file && vma->vm_file->f_mapping &&
> +                               is_vm_hugetlb_page(vma))
>                         vm_lock_mapping(mm, vma->vm_file->f_mapping);
>         }
>
> --
>  Kirill A. Shutemov