From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751349AbeAYSxk (ORCPT ); Thu, 25 Jan 2018 13:53:40 -0500 Received: from mail-wm0-f67.google.com ([74.125.82.67]:40332 "EHLO mail-wm0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751011AbeAYSxi (ORCPT ); Thu, 25 Jan 2018 13:53:38 -0500 X-Google-Smtp-Source: AH8x2271T7ex/S9HI+Hu3p93TNScE1whYijJ1Y3sBJVVDecHhvXKQFtftz7gxwhzjthntZZj190m6g== Date: Thu, 25 Jan 2018 11:53:30 -0700 From: Jason Gunthorpe To: Doug Ledford Cc: Davidlohr Bueso , roland@purestorage.com, linux-rdma@vger.kernel.org, linux-kernel@vger.kernel.org, Davidlohr Bueso Subject: Re: [PATCH] IB/mthca: Fix how mthca_map_user_db() calls gup Message-ID: <20180125185330.GH10706@ziepe.ca> References: <20180123205459.432-1-dave@stgolabs.net> <1516898063.27592.136.camel@redhat.com> <20180125175048.GG10706@ziepe.ca> <1516903584.27592.183.camel@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1516903584.27592.183.camel@redhat.com> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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