From: Jacob Pan <jacob.jun.pan@linux.intel.com>
To: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Cc: mark.rutland@arm.com, devicetree@vger.kernel.org,
jacob.jun.pan@linux.intel.com, will.deacon@arm.com,
linux-kernel@vger.kernel.org, iommu@lists.linux-foundation.org,
robh+dt@kernel.org, robin.murphy@arm.com,
linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 1/8] iommu: Add I/O ASID allocator
Date: Tue, 11 Jun 2019 10:10:52 -0700 [thread overview]
Message-ID: <20190611101052.35af46df@jacob-builder> (raw)
In-Reply-To: <95292b47-4cf4-5fd9-b096-1cb016e2264f@arm.com>
On Tue, 11 Jun 2019 15:37:42 +0100
Jean-Philippe Brucker <jean-philippe.brucker@arm.com> wrote:
> On 11/06/2019 13:26, Jacob Pan wrote:
> >> +/**
> >> + * ioasid_set_data - Set private data for an allocated ioasid
> >> + * @ioasid: the ID to set data
> >> + * @data: the private data
> >> + *
> >> + * For IOASID that is already allocated, private data can be set
> >> + * via this API. Future lookup can be done via ioasid_find.
> >> + */
> >> +int ioasid_set_data(ioasid_t ioasid, void *data)
> >> +{
> >> + struct ioasid_data *ioasid_data;
> >> + int ret = 0;
> >> +
> >> + xa_lock(&ioasid_xa);
> > Just wondering if this is necessary, since xa_load is under
> > rcu_read_lock and we are not changing anything internal to xa. For
> > custom allocator I still need to have the mutex against allocator
> > removal.
>
> I think we do need this because of a possible race with ioasid_free():
>
> CPU1 CPU2
> ioasid_free(ioasid) ioasid_set_data(ioasid, foo)
> data = xa_load(...)
> xa_erase(...)
> kfree_rcu(data) (no RCU lock held)
> ...free(data)
> data->private = foo;
>
make sense, thanks for explaining.
> 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.
> >> + 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. No issues here. I will
integrate this in the next version.
> Thanks,
> Jean
[Jacob Pan]
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next prev parent reply other threads:[~2019-06-11 17:07 UTC|newest]
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 [this message]
2019-06-12 11:30 ` Jean-Philippe Brucker
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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