Messages in this thread | | | Subject | Re: [PATCH] IB/mthca: Fix how mthca_map_user_db() calls gup | From | Doug Ledford <> | Date | Thu, 25 Jan 2018 13:06:24 -0500 |
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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> GPG KeyID: B826A3330E572FDD Key fingerprint = AE6B 1BDA 122B 23B4 265B 1274 B826 A333 0E57 2FDD[unhandled content-type:application/pgp-signature] | |