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Fri, 28 Feb 2020 07:04:35 -0800 (PST) Received: from myrica ([2001:171b:c9a8:fbc0:116c:c27a:3e7f:5eaf]) by smtp.gmail.com with ESMTPSA id w19sm2377341wmc.22.2020.02.28.07.04.32 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 28 Feb 2020 07:04:34 -0800 (PST) Date: Fri, 28 Feb 2020 16:04:27 +0100 From: Jean-Philippe Brucker To: Jason Gunthorpe Cc: mark.rutland@arm.com, linux-pci@vger.kernel.org, linux-mm@kvack.org, will@kernel.org, Dimitri Sivanich , catalin.marinas@arm.com, zhangfei.gao@linaro.org, devicetree@vger.kernel.org, kevin.tian@intel.com, Arnd Bergmann , robh+dt@kernel.org, linux-arm-kernel@lists.infradead.org, Greg Kroah-Hartman , iommu@lists.linux-foundation.org, Andrew Morton , robin.murphy@arm.com, christian.koenig@amd.com Subject: Re: [PATCH v4 01/26] mm/mmu_notifiers: pass private data down to alloc_notifier() Message-ID: <20200228150427.GF2156@myrica> References: <20200224182401.353359-1-jean-philippe@linaro.org> <20200224182401.353359-2-jean-philippe@linaro.org> <20200224190056.GT31668@ziepe.ca> <20200225092439.GB375953@myrica> <20200225140814.GW31668@ziepe.ca> <20200228143935.GA2156@myrica> <20200228144844.GQ31668@ziepe.ca> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200228144844.GQ31668@ziepe.ca> X-Bogosity: Ham, tests=bogofilter, spamicity=0.000003, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Fri, Feb 28, 2020 at 10:48:44AM -0400, Jason Gunthorpe wrote: > On Fri, Feb 28, 2020 at 03:39:35PM +0100, Jean-Philippe Brucker wrote: > > > > + list_for_each_entry_rcu(bond, &io_mm->devices, mm_head) { > > > > + /* > > > > + * To ensure that we observe the initialization of io_mm fields > > > > + * by io_mm_finalize() before the registration of this bond to > > > > + * the list by io_mm_attach(), introduce an address dependency > > > > + * between bond and io_mm. It pairs with the smp_store_release() > > > > + * from list_add_rcu(). > > > > + */ > > > > + io_mm = rcu_dereference(bond->io_mm); > > > > > > A rcu_dereference isn't need here, just a normal derference is fine. > > > > bond->io_mm is annotated with __rcu (for iommu_sva_get_pasid_generic(), > > which does bond->io_mm under rcu_read_lock()) > > I'm surprised the bond->io_mm can change over the lifetime of the > bond memory.. The normal lifetime of the bond is between device driver calls to bind() and unbind(). If the mm exits early, though, we clear bond->io_mm. The bond is then stale but can only be freed when the device driver releases it with unbind(). > > > > > If io_mm->ctx and io_mm->ops are already valid before the > > > > mmu notifier is published, then we don't need that stuff. > > > > > > So, this trickyness with RCU is not a bad reason to introduce the priv > > > scheme, maybe explain it in the commit message? > > > > Ok, I've added this to the commit message: > > > > The IOMMU SVA module, which attaches an mm to multiple devices, > > exemplifies this situation. In essence it does: > > > > mmu_notifier_get() > > alloc_notifier() > > A = kzalloc() > > /* MMU notifier is published */ > > A->ctx = ctx; // (1) > > device->A = A; > > list_add_rcu(device, A->devices); // (2) > > > > The invalidate notifier, which may start running before A is fully > > initialized at (1), does the following: > > > > io_mm_invalidate(A) > > list_for_each_entry_rcu(device, A->devices) > > A = device->A; // (3) > > I would drop the work around from the decription, it is enough to say > that the line below needs to observe (1) after (2) and this is > trivially achieved by moving (1) to before publishing the notifier so > the core MM locking can be used. Ok, will do Thanks, Jean