From: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
To: Jacob Pan <jacob.jun.pan@linux.intel.com>
Cc: mark.rutland@arm.com, devicetree@vger.kernel.org,
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 15:37:42 +0100 [thread overview]
Message-ID: <95292b47-4cf4-5fd9-b096-1cb016e2264f@arm.com> (raw)
In-Reply-To: <20190611052626.20bed59a@jacob-builder>
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;
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)
>> + 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
Thanks,
Jean
next prev parent reply other threads:[~2019-06-11 14:38 UTC|newest]
Thread overview: 44+ 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 [this message]
2019-06-11 17:10 ` Jacob Pan
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
2019-09-19 15:05 ` Jean-Philippe Brucker
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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