From: Jean-Philippe Brucker <jean-philippe.brucker@arm.com> To: Jacob Pan <jacob.jun.pan@linux.intel.com> Cc: Mark Rutland <Mark.Rutland@arm.com>, "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>, Will Deacon <Will.Deacon@arm.com>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, "iommu@lists.linux-foundation.org" <iommu@lists.linux-foundation.org>, "robh+dt@kernel.org" <robh+dt@kernel.org>, Robin Murphy <Robin.Murphy@arm.com>, "linux-arm-kernel@lists.infradead.org" <linux-arm-kernel@lists.infradead.org> Subject: Re: [PATCH 1/8] iommu: Add I/O ASID allocator Date: Wed, 12 Jun 2019 12:30:05 +0100 Message-ID: <f68f4ccb-1422-4f93-dc9c-2bcdf61c9ed4@arm.com> (raw) In-Reply-To: <20190611101052.35af46df@jacob-builder> On 11/06/2019 18:10, Jacob Pan wrote: >> The issue is theoretical at the moment because no users do this, but >> I'd be more comfortable taking the xa_lock, which prevents a >> concurrent xa_erase()+free(). (I commented on your v3 but you might >> have missed it) >> > Did you reply to my v3? I did not see it. I only saw your comments about > v3 in your commit message. My fault, I sneaked the comments in a random reply three levels down the thread: https://lore.kernel.org/linux-iommu/836caf0d-699e-33ba-5303-b1c9c949c9ca@arm.com/ (Great, linux-iommu is indexed by lore! I won't have to Cc lkml anymore) >>>> + ioasid_data = xa_load(&ioasid_xa, ioasid); >>>> + if (ioasid_data) >>>> + rcu_assign_pointer(ioasid_data->private, data); >>> it is good to publish and have barrier here. But I just wonder even >>> for weakly ordered machine, this pointer update is quite far away >>> from its data update. >> >> I don't know, it could be right before calling ioasid_set_data(): >> >> mydata = kzalloc(sizeof(*mydata)); >> mydata->ops = &my_ops; (1) >> ioasid_set_data(ioasid, mydata); >> ... /* no write barrier here */ >> data->private = mydata; (2) >> >> And then another thread calls ioasid_find(): >> >> mydata = ioasid_find(ioasid); >> if (mydata) >> mydata->ops->do_something(); >> >> On a weakly ordered machine, this thread could observe the pointer >> assignment (2) before the ops assignment (1), and dereference NULL. >> Using rcu_assign_pointer() should fix that >> > I agree it is better to have the barrier. Just thought there is already > a rcu_read_lock() in xa_load() in between. rcu_read_lock() may have > barrier in some case but better not count on it. Yes, and even if rcu_read_lock() provided a barrier I don't think it would be sufficient, because acquire semantics don't guarantee that prior writes appear to happen before the barrier, only the other way round. A lock operation with release semantics, for example spin_unlock(), should work. Thanks, Jean > No issues here. I will > integrate this in the next version. > >> Thanks, >> Jean > > [Jacob Pan] > _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
next prev parent reply index Thread overview: 43+ messages / expand[flat|nested] mbox.gz Atom feed top 2019-06-10 18:47 [PATCH 0/8] iommu: Add auxiliary domain and PASID support to Arm SMMUv3 Jean-Philippe Brucker 2019-06-10 18:47 ` [PATCH 1/8] iommu: Add I/O ASID allocator Jean-Philippe Brucker 2019-06-11 9:36 ` Jonathan Cameron 2019-06-11 14:35 ` Jean-Philippe Brucker 2019-06-11 18:13 ` Jacob Pan 2019-06-18 14:22 ` Jean-Philippe Brucker 2019-06-18 17:05 ` Jacob Pan 2019-06-19 14:26 ` Jean-Philippe Brucker 2019-06-11 12:26 ` Jacob Pan 2019-06-11 14:37 ` Jean-Philippe Brucker 2019-06-11 17:10 ` Jacob Pan 2019-06-12 11:30 ` Jean-Philippe Brucker [this message] 2019-06-10 18:47 ` [PATCH 2/8] dt-bindings: document PASID property for IOMMU masters Jean-Philippe Brucker 2019-07-08 7:58 ` Auger Eric 2019-06-10 18:47 ` [PATCH 3/8] iommu/arm-smmu-v3: Support platform SSID Jean-Philippe Brucker 2019-06-11 9:42 ` Jonathan Cameron 2019-06-11 14:35 ` Jean-Philippe Brucker 2019-06-18 18:08 ` Will Deacon 2019-06-19 11:53 ` Jean-Philippe Brucker 2019-07-08 7:58 ` Auger Eric 2019-09-19 14:51 ` Jean-Philippe Brucker 2019-06-10 18:47 ` [PATCH 4/8] iommu/arm-smmu-v3: Add support for Substream IDs Jean-Philippe Brucker 2019-06-11 10:19 ` Jonathan Cameron 2019-06-11 14:35 ` Jean-Philippe Brucker 2019-06-26 18:00 ` Will Deacon 2019-07-04 9:33 ` Jean-Philippe Brucker 2019-09-19 14:57 ` Jean-Philippe Brucker 2019-07-08 15:31 ` Auger Eric 2019-09-19 15:01 ` Jean-Philippe Brucker 2019-06-10 18:47 ` [PATCH 5/8] iommu/arm-smmu-v3: Add second level of context descriptor table Jean-Philippe Brucker 2019-06-11 10:24 ` Jonathan Cameron 2019-07-08 15:13 ` Auger Eric 2019-06-10 18:47 ` [PATCH 6/8] iommu/arm-smmu-v3: Support auxiliary domains Jean-Philippe Brucker 2019-06-26 17:59 ` Will Deacon 2019-07-05 16:29 ` Jean-Philippe Brucker 2019-09-19 15:06 ` Jean-Philippe Brucker 2019-06-10 18:47 ` [PATCH 7/8] iommu/arm-smmu-v3: Improve add_device() error handling Jean-Philippe Brucker 2019-07-08 7:58 ` Auger Eric 2019-06-10 18:47 ` [PATCH 8/8] iommu/arm-smmu-v3: Add support for PCI PASID Jean-Philippe Brucker 2019-06-11 10:45 ` Jonathan Cameron 2019-06-11 14:35 ` Jean-Philippe Brucker 2019-07-08 7:58 ` Auger Eric 2019-09-19 15:10 ` Jean-Philippe Brucker
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