From mboxrd@z Thu Jan 1 00:00:00 1970 From: Alexander Duyck Subject: Re: [driver-core PATCH v6 4/9] driver core: Move async_synchronize_full call Date: Tue, 27 Nov 2018 09:38:01 -0800 Message-ID: References: <154170028986.12967.2108024712555179678.stgit@ahduyck-desk1.jf.intel.com> <154170042103.12967.5841784115552956171.stgit@ahduyck-desk1.jf.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org To: Dan Williams Cc: Linux Kernel Mailing List , Greg KH , linux-nvdimm , Tejun Heo , Andrew Morton , Linux-pm mailing list , jiangshanlai@gmail.com, "Rafael J. Wysocki" , "Brown, Len" , Pavel Machek , zwisler@kernel.org, Dave Jiang , bvanassche@acm.org List-Id: linux-pm@vger.kernel.org On Mon, 2018-11-26 at 18:11 -0800, Dan Williams wrote: > On Thu, Nov 8, 2018 at 10:07 AM Alexander Duyck > wrote: > > > > Move the async_synchronize_full call out of __device_release_driver and > > into driver_detach. > > > > The idea behind this is that the async_synchronize_full call will only > > guarantee that any existing async operations are flushed. This doesn't do > > anything to guarantee that a hotplug event that may occur while we are > > doing the release of the driver will not be asynchronously scheduled. > > > > By moving this into the driver_detach path we can avoid potential deadlocks > > as we aren't holding the device lock at this point and we should not have > > the driver we want to flush loaded so the flush will take care of any > > asynchronous events the driver we are detaching might have scheduled. > > > > What problem is this patch solving in practice, because if there were > drivers issuing async work from probe they would need to be > responsible for flushing it themselves. That said it seems broken that > the async probing infrastructure takes the device_lock inside > async_schedule and then holds the lock when calling > async_syncrhonize_full. Is it just luck that this hasn't caused > deadlocks in practice? My understanding is that it has caused some deadlocks. There was another patch set that Bart Van Assche had submitted that was addressing this. I just tweaked my patch set to address both the issues he had seen as well as the performance improvements included in my original patch set. > Given that the device_lock is hidden from lockdep I think it would be > helpful to have a custom lock_map_acquire() setup, similar to the > workqueue core, to try to keep the locking rules enforced / > documented. > > The only documentation I can find for async-probe deadlock avoidance > is the comment block in do_init_module() for async_probe_requested. Would it make sense to just add any lockdep or deadlock documentation as a seperate patch? I can work on it but I am not sure it makes sense to add to this patch since there is a chance this one will need to be backported to stable at some point. > Stepping back a bit, does this patch have anything to do with the > performance improvement, or is it a separate "by the way I also found > this" kind of patch? This is more of a seperate "by the way" type of patch based on the discussion Bart and I had about how to best address the issue. There may be some improvement since we only call async_synchronize_full once and only when we are removing the driver, but I don't think it would be very noticable.