Re: [PATCH] mm: fix locking order in mm_take_all_locks()

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Tue, Jan 12, 2016 at 03:45:21PM +0100, Michal Hocko wrote:
> On Mon 11-01-16 14:05:28, Kirill A. Shutemov wrote:
> > Dmitry Vyukov has reported[1] possible deadlock (triggered by his syzkaller
> > fuzzer):
> > 
> >  Possible unsafe locking scenario:
> > 
> >        CPU0                    CPU1
> >        ----                    ----
> >   lock(&hugetlbfs_i_mmap_rwsem_key);
> >                                lock(&mapping->i_mmap_rwsem);
> >                                lock(&hugetlbfs_i_mmap_rwsem_key);
> >   lock(&mapping->i_mmap_rwsem);
> > 
> > Both traces points to mm_take_all_locks() as a source of the problem.
> > It doesn't take care about ordering or hugetlbfs_i_mmap_rwsem_key (aka
> > mapping->i_mmap_rwsem for hugetlb mapping) vs. i_mmap_rwsem.
> 
> Hmm, but huge_pmd_share is called with mmap_sem held no?

Why does it matter?

Both mappings can be mapped to different processes, so mmap_sem is no good
here.

-- 
 Kirill A. Shutemov

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