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

[Michal Hocko <mhocko@suse.cz>]

On Tue 12-01-16 16:52:19, Kirill A. Shutemov wrote:
> 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.

You are right! Then it really makes a differencec.
Feel free to add
Reviewed-by: Michal Hocko <mhocko@suse.com>
-- 
Michal Hocko
SUSE Labs

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