Re: [PATCH] IB/mthca: Fix how mthca_map_user_db() calls gup

[Doug Ledford <dledford@redhat.com>]

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On Thu, 2018-01-25 at 10:50 -0700, Jason Gunthorpe wrote:
> On Thu, Jan 25, 2018 at 11:34:23AM -0500, Doug Ledford wrote:
> > > - Should I be wrong about no callers already holding mmap_sem,
> > >   I still think calling gup without the mutex makes sense for
> > >   improved paralellism. Now, if callers can hold the mmap_sem,
> > >   it's wrong to do copy_from_user right before calling mthca_map_user_db.
> > 
> > So, if I understand you correctly, we (well, you and Al would be more
> > correct, we haven't looked into the situation yet, so Mellanox people
> > that worked on this in the day might now, or someone taking the time to
> > research it could find out) don't have a clear understanding of all the
> > conditions this function is called under, and so we actually don't know
> > what the best way forward is to fix it?
> 
> I looked at it enough to be confident that mthca_map_user_db is never
> called with mmap_sem held.
> 
> Also pretty confident that mthca_unmap_user_db is never called with
> mmap_sem.
> 
> So how about just grabbing mmap_sem around the call to get_user_pages
> and no other changes?

Since the original post was referred to an ABBA deadlock, wouldn't we
have to drop db_tab->mutex, then grab both in the proper order?

-- 
Doug Ledford <dledford@redhat.com>
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