From: Jason Gunthorpe <jgg@ziepe.ca>
To: Jean-Philippe Brucker <jean-philippe@linaro.org>
Cc: mark.rutland@arm.com, linux-pci@vger.kernel.org,
linux-mm@kvack.org, will@kernel.org,
Dimitri Sivanich <sivanich@sgi.com>,
catalin.marinas@arm.com, zhangfei.gao@linaro.org,
devicetree@vger.kernel.org, kevin.tian@intel.com,
Arnd Bergmann <arnd@arndb.de>,
robh+dt@kernel.org, linux-arm-kernel@lists.infradead.org,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
iommu@lists.linux-foundation.org,
Andrew Morton <akpm@linux-foundation.org>,
robin.murphy@arm.com, christian.koenig@amd.com
Subject: Re: [PATCH v4 01/26] mm/mmu_notifiers: pass private data down to alloc_notifier()
Date: Tue, 25 Feb 2020 10:08:14 -0400 [thread overview]
Message-ID: <20200225140814.GW31668@ziepe.ca> (raw)
In-Reply-To: <20200225092439.GB375953@myrica>
On Tue, Feb 25, 2020 at 10:24:39AM +0100, Jean-Philippe Brucker wrote:
> 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);
A rcu_dereference isn't need here, just a normal derference is fine.
> + 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().
The list_for_each_entry_rcu() is an acquire that already pairs with
the release in list_add_rcu(), all you need is a data dependency chain
starting on bond to be correct on ordering.
But this is super tricky :\
> 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?
Jason
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next prev parent reply other threads:[~2020-02-25 14:08 UTC|newest]
Thread overview: 70+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-24 18:23 [PATCH v4 00/26] iommu: Shared Virtual Addressing and SMMUv3 support Jean-Philippe Brucker
2020-02-24 18:23 ` [PATCH v4 01/26] mm/mmu_notifiers: pass private data down to alloc_notifier() Jean-Philippe Brucker
2020-02-24 19:00 ` Jason Gunthorpe
2020-02-25 9:24 ` Jean-Philippe Brucker
2020-02-25 14:08 ` Jason Gunthorpe [this message]
2020-02-28 14:39 ` Jean-Philippe Brucker
2020-02-28 14:48 ` Jason Gunthorpe
2020-02-28 15:04 ` Jean-Philippe Brucker
2020-02-28 15:13 ` Jason Gunthorpe
2020-03-06 9:56 ` Jean-Philippe Brucker
2020-03-06 13:09 ` Jason Gunthorpe
2020-03-06 14:35 ` Jean-Philippe Brucker
2020-03-06 14:52 ` Jason Gunthorpe
2020-03-06 16:15 ` Jean-Philippe Brucker
2020-03-06 17:42 ` Jason Gunthorpe
2020-03-13 18:49 ` Jean-Philippe Brucker
2020-03-13 19:13 ` Jason Gunthorpe
2020-03-16 15:46 ` Christoph Hellwig
2020-03-17 18:40 ` Jason Gunthorpe
2020-03-05 16:36 ` Christoph Hellwig
2020-02-24 18:23 ` [PATCH v4 02/26] iommu/sva: Manage process address spaces Jean-Philippe Brucker
2020-02-26 12:35 ` Jonathan Cameron
2020-02-28 14:43 ` Jean-Philippe Brucker
2020-02-28 16:26 ` Jonathan Cameron
2020-02-26 19:13 ` Jacob Pan
2020-02-28 14:40 ` Jean-Philippe Brucker
2020-02-28 14:57 ` Jason Gunthorpe
2020-02-24 18:23 ` [PATCH v4 03/26] iommu: Add a page fault handler Jean-Philippe Brucker
2020-02-25 3:30 ` Xu Zaibo
2020-02-25 9:25 ` Jean-Philippe Brucker
2020-02-26 3:05 ` Xu Zaibo
2020-02-26 13:59 ` Jonathan Cameron
2020-02-28 14:44 ` Jean-Philippe Brucker
2020-02-24 18:23 ` [PATCH v4 04/26] iommu/sva: Search mm by PASID Jean-Philippe Brucker
2020-02-24 18:23 ` [PATCH v4 05/26] iommu/iopf: Handle mm faults Jean-Philippe Brucker
2020-02-24 18:23 ` [PATCH v4 06/26] iommu/sva: Register page fault handler Jean-Philippe Brucker
2020-02-26 19:39 ` Jacob Pan
2020-02-28 14:44 ` Jean-Philippe Brucker
2020-02-24 18:23 ` [PATCH v4 07/26] arm64: mm: Pin down ASIDs for sharing mm with devices Jean-Philippe Brucker
2020-02-27 17:43 ` Jonathan Cameron
2020-03-04 14:10 ` Jean-Philippe Brucker
2020-02-24 18:23 ` [PATCH v4 08/26] iommu/io-pgtable-arm: Move some definitions to a header Jean-Philippe Brucker
2020-02-24 18:23 ` [PATCH v4 09/26] iommu/arm-smmu-v3: Manage ASIDs with xarray Jean-Philippe Brucker
2020-02-24 18:23 ` [PATCH v4 10/26] arm64: cpufeature: Export symbol read_sanitised_ftr_reg() Jean-Philippe Brucker
2020-02-24 18:23 ` [PATCH v4 11/26] iommu/arm-smmu-v3: Share process page tables Jean-Philippe Brucker
2020-02-24 18:23 ` [PATCH v4 12/26] iommu/arm-smmu-v3: Seize private ASID Jean-Philippe Brucker
2020-02-24 18:23 ` [PATCH v4 13/26] iommu/arm-smmu-v3: Add support for VHE Jean-Philippe Brucker
2020-02-24 18:23 ` [PATCH v4 14/26] iommu/arm-smmu-v3: Enable broadcast TLB maintenance Jean-Philippe Brucker
2020-02-24 18:23 ` [PATCH v4 15/26] iommu/arm-smmu-v3: Add SVA feature checking Jean-Philippe Brucker
2020-02-24 18:23 ` [PATCH v4 16/26] iommu/arm-smmu-v3: Add dev_to_master() helper Jean-Philippe Brucker
2020-02-24 18:23 ` [PATCH v4 17/26] iommu/arm-smmu-v3: Implement mm operations Jean-Philippe Brucker
2020-02-24 18:23 ` [PATCH v4 18/26] iommu/arm-smmu-v3: Hook up ATC invalidation to mm ops Jean-Philippe Brucker
2020-02-24 18:23 ` [PATCH v4 19/26] iommu/arm-smmu-v3: Add support for Hardware Translation Table Update Jean-Philippe Brucker
2020-02-24 18:23 ` [PATCH v4 20/26] iommu/arm-smmu-v3: Maintain a SID->device structure Jean-Philippe Brucker
2020-02-24 18:23 ` [PATCH v4 21/26] iommu/arm-smmu-v3: Ratelimit event dump Jean-Philippe Brucker
2021-05-28 8:09 ` Aaro Koskinen
2021-05-28 16:25 ` Jean-Philippe Brucker
2020-02-24 18:23 ` [PATCH v4 22/26] dt-bindings: document stall property for IOMMU masters Jean-Philippe Brucker
2020-02-24 18:23 ` [PATCH v4 23/26] iommu/arm-smmu-v3: Add stall support for platform devices Jean-Philippe Brucker
2020-02-26 8:44 ` Xu Zaibo
2020-03-04 14:09 ` Jean-Philippe Brucker
2020-02-27 18:17 ` Jonathan Cameron
2020-03-04 14:08 ` Jean-Philippe Brucker
2020-03-09 10:48 ` Jonathan Cameron
2020-02-24 18:23 ` [PATCH v4 24/26] PCI/ATS: Add PRI stubs Jean-Philippe Brucker
2020-02-27 20:55 ` Bjorn Helgaas
2020-02-24 18:24 ` [PATCH v4 25/26] PCI/ATS: Export symbols of PRI functions Jean-Philippe Brucker
2020-02-27 20:55 ` Bjorn Helgaas
2020-02-24 18:24 ` [PATCH v4 26/26] iommu/arm-smmu-v3: Add support for PRI Jean-Philippe Brucker
2020-02-27 18:22 ` [PATCH v4 00/26] iommu: Shared Virtual Addressing and SMMUv3 support Jonathan Cameron
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