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Tue, 25 Feb 2020 01:24:47 -0800 (PST) Received: from myrica ([2001:171b:c9a8:fbc0:116c:c27a:3e7f:5eaf]) by smtp.gmail.com with ESMTPSA id k16sm23442823wru.0.2020.02.25.01.24.46 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 25 Feb 2020 01:24:46 -0800 (PST) Date: Tue, 25 Feb 2020 10:24:39 +0100 From: Jean-Philippe Brucker To: Jason Gunthorpe Cc: iommu@lists.linux-foundation.org, devicetree@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-pci@vger.kernel.org, linux-mm@kvack.org, joro@8bytes.org, robh+dt@kernel.org, mark.rutland@arm.com, catalin.marinas@arm.com, will@kernel.org, robin.murphy@arm.com, kevin.tian@intel.com, baolu.lu@linux.intel.com, Jonathan.Cameron@huawei.com, jacob.jun.pan@linux.intel.com, christian.koenig@amd.com, yi.l.liu@intel.com, zhangfei.gao@linaro.org, Andrew Morton , Arnd Bergmann , Dimitri Sivanich , Greg Kroah-Hartman Subject: Re: [PATCH v4 01/26] mm/mmu_notifiers: pass private data down to alloc_notifier() Message-ID: <20200225092439.GB375953@myrica> References: <20200224182401.353359-1-jean-philippe@linaro.org> <20200224182401.353359-2-jean-philippe@linaro.org> <20200224190056.GT31668@ziepe.ca> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200224190056.GT31668@ziepe.ca> X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon, Feb 24, 2020 at 03:00:56PM -0400, Jason Gunthorpe wrote: > On Mon, Feb 24, 2020 at 07:23:36PM +0100, Jean-Philippe Brucker wrote: > > The new allocation scheme introduced by 2c7933f53f6b ("mm/mmu_notifiers: > > add a get/put scheme for the registration") provides a convenient way > > for users to attach notifier data to an mm. However, it would be even > > better to create this notifier data atomically. > > > > Since the alloc_notifier() callback only takes an mm argument at the > > moment, some users have to perform the allocation in two times. > > alloc_notifier() initially creates an incomplete structure, which is > > then finalized using more context once mmu_notifier_get() returns. This > > second step requires carrying an initialization lock in the notifier > > data and playing dirty tricks to order memory accesses against live > > invalidation. > > This was the intended pattern. Tthere shouldn't be an real issue as > there shouldn't be any data on which to invalidate, ie the later patch > does: > > + list_for_each_entry_rcu(bond, &io_mm->devices, mm_head) > > And that list is empty post-allocation, so no 'dirty tricks' required. Before introducing this patch I had the following code: + 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); + io_mm->ops->invalidate(bond->sva.dev, io_mm->pasid, io_mm->ctx, + start, end - start); + } (1) io_mm_get() would obtain an empty io_mm from iommu_notifier_get(). (2) then io_mm_finalize() would initialize io_mm->ops, io_mm->ctx, etc. (3) finally io_mm_attach() would add the bond to io_mm->devices. Since the above code can run before (2) it needs to observe valid io_mm->ctx, io_mm->ops initialized by (2) after obtaining the bond initialized by (3). Which I believe requires the address dependency from the rcu_dereference() above or some stronger barrier to pair with the list_add_rcu(). If io_mm->ctx and io_mm->ops are already valid before the mmu notifier is published, then we don't need that stuff. That's the main reason I would have liked moving everything to alloc_notifier(), the locking below isn't a big deal. > The other op callback is release, which also cannot be called as the > caller must hold a mmget to establish the notifier. > > So just use the locking that already exists. There is one function > that calls io_mm_get() which immediately calls io_mm_attach, which > immediately grabs the global iommu_sva_lock. > > Thus init the pasid for the first time under that lock and everything > is fine. I agree with this, can't remember why I used a separate lock for initialization rather than reusing iommu_sva_lock. Thanks, Jean > > There is nothing inherently wrong with the approach in this patch, but > it seems unneeded in this case.. > > Jason