From: Jean-Philippe Brucker <jean-philippe@linaro.org>
To: Jason Gunthorpe <jgg@ziepe.ca>
Cc: mark.rutland@arm.com, linux-pci@vger.kernel.org,
linux-mm@kvack.org, yi.l.liu@intel.com, will@kernel.org,
Dimitri Sivanich <sivanich@sgi.com>,
joro@8bytes.org, catalin.marinas@arm.com,
zhangfei.gao@linaro.org, devicetree@vger.kernel.org,
kevin.tian@intel.com, jacob.jun.pan@linux.intel.com,
Arnd Bergmann <arnd@arndb.de>,
robh+dt@kernel.org, Jonathan.Cameron@huawei.com,
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,
baolu.lu@linux.intel.com
Subject: Re: [PATCH v4 01/26] mm/mmu_notifiers: pass private data down to alloc_notifier()
Date: Tue, 25 Feb 2020 10:24:39 +0100 [thread overview]
Message-ID: <20200225092439.GB375953@myrica> (raw)
In-Reply-To: <20200224190056.GT31668@ziepe.ca>
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
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next prev parent reply other threads:[~2020-02-25 9:24 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 [this message]
2020-02-25 14:08 ` Jason Gunthorpe
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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