Messages in this thread | | | Date | Sun, 10 Jan 2016 22:39:06 +0200 | From | "Kirill A. Shutemov" <> | Subject | Re: mm: possible deadlock in mm_take_all_locks |
| |
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@shutemov.name> 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.
-- Kirill A. Shutemov
| |