On Thu, Jan 25, 2018 at 01:06:24PM -0500, Doug Ledford wrote: > 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? I had understood that was only a concern because Davidlohr was having trouble proving the callchain didn't include mmap_sem already.. I can see the call chain all ends on verbs ops, and I know verbs ops with ucontext's are never called under mmap_sem by the core code.. Jason -- To unsubscribe from this list: send the line "unsubscribe linux-rdma" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
