[PATCH v2 0/2] iommu/tegra-smmu: Two followup changes

[PATCH v2 0/2] iommu/tegra-smmu: Two followup changes

[Nicolin Chen]

Two followup patches for tegra-smmu:
PATCH-1 is a clean-up patch for the recently applied SWGROUP change.
PATCH-2 fixes a potential race condition

Changelog
v1->v2:
 * Separated first two changs of V1 so they may get applied first,
   since the other three changes need some extra time to finalize.

Nicolin Chen (2):
  iommu/tegra-smmu: Unwrap tegra_smmu_group_get
  iommu/tegra-smmu: Expend mutex protection range

 drivers/iommu/tegra-smmu.c | 53 ++++++++++++++++++--------------------
 1 file changed, 25 insertions(+), 28 deletions(-)

-- 
2.17.1

[PATCH v2 1/2] iommu/tegra-smmu: Unwrap tegra_smmu_group_get

[Nicolin Chen]

The tegra_smmu_group_get was added to group devices in different
SWGROUPs and it'd return a NULL group pointer upon a mismatch at
tegra_smmu_find_group(), so for most of clients/devices, it very
likely would mismatch and need a fallback generic_device_group().

But now tegra_smmu_group_get handles devices in same SWGROUP too,
which means that it would allocate a group for every new SWGROUP
or would directly return an existing one upon matching a SWGROUP,
i.e. any device will go through this function.

So possibility of having a NULL group pointer in device_group()
is upon failure of either devm_kzalloc() or iommu_group_alloc().
In either case, calling generic_device_group() no longer makes a
sense. Especially for devm_kzalloc() failing case, it'd cause a
problem if it fails at devm_kzalloc() yet succeeds at a fallback
generic_device_group(), because it does not create a group->list
for other devices to match.

This patch simply unwraps the function to clean it up.

Signed-off-by: Nicolin Chen <nicoleotsuka@gmail.com>
---

Changelog
v1->v2:
 * Changed type of swgroup to "unsigned int", following Thierry's
   commnets.

 drivers/iommu/tegra-smmu.c | 19 ++++---------------
 1 file changed, 4 insertions(+), 15 deletions(-)

diff --git a/drivers/iommu/tegra-smmu.c b/drivers/iommu/tegra-smmu.c
index 0becdbfea306..ec4c9dafff95 100644
--- a/drivers/iommu/tegra-smmu.c
+++ b/drivers/iommu/tegra-smmu.c
@@ -903,10 +903,12 @@ static void tegra_smmu_group_release(void *iommu_data)
 	mutex_unlock(&smmu->lock);
 }
 
-static struct iommu_group *tegra_smmu_group_get(struct tegra_smmu *smmu,
-						unsigned int swgroup)
+static struct iommu_group *tegra_smmu_device_group(struct device *dev)
 {
+	struct iommu_fwspec *fwspec = dev_iommu_fwspec_get(dev);
+	struct tegra_smmu *smmu = dev_iommu_priv_get(dev);
 	const struct tegra_smmu_group_soc *soc;
+	unsigned int swgroup = fwspec->ids[0];
 	struct tegra_smmu_group *group;
 	struct iommu_group *grp;
 
@@ -950,19 +952,6 @@ static struct iommu_group *tegra_smmu_group_get(struct tegra_smmu *smmu,
 	return group->group;
 }
 
-static struct iommu_group *tegra_smmu_device_group(struct device *dev)
-{
-	struct iommu_fwspec *fwspec = dev_iommu_fwspec_get(dev);
-	struct tegra_smmu *smmu = dev_iommu_priv_get(dev);
-	struct iommu_group *group;
-
-	group = tegra_smmu_group_get(smmu, fwspec->ids[0]);
-	if (!group)
-		group = generic_device_group(dev);
-
-	return group;
-}
-
 static int tegra_smmu_of_xlate(struct device *dev,
 			       struct of_phandle_args *args)
 {
-- 
2.17.1

[PATCH v2 2/2] iommu/tegra-smmu: Expend mutex protection range

[Nicolin Chen]

This is used to protect potential race condition at use_count.
since probes of client drivers, calling attach_dev(), may run
concurrently.

Signed-off-by: Nicolin Chen <nicoleotsuka@gmail.com>
---

Changelog
v1->v2:
 * N/A

 drivers/iommu/tegra-smmu.c | 34 +++++++++++++++++++++-------------
 1 file changed, 21 insertions(+), 13 deletions(-)

diff --git a/drivers/iommu/tegra-smmu.c b/drivers/iommu/tegra-smmu.c
index ec4c9dafff95..acda5902e095 100644
--- a/drivers/iommu/tegra-smmu.c
+++ b/drivers/iommu/tegra-smmu.c
@@ -256,26 +256,19 @@ static int tegra_smmu_alloc_asid(struct tegra_smmu *smmu, unsigned int *idp)
 {
 	unsigned long id;
 
-	mutex_lock(&smmu->lock);
-
 	id = find_first_zero_bit(smmu->asids, smmu->soc->num_asids);
-	if (id >= smmu->soc->num_asids) {
-		mutex_unlock(&smmu->lock);
+	if (id >= smmu->soc->num_asids)
 		return -ENOSPC;
-	}
 
 	set_bit(id, smmu->asids);
 	*idp = id;
 
-	mutex_unlock(&smmu->lock);
 	return 0;
 }
 
 static void tegra_smmu_free_asid(struct tegra_smmu *smmu, unsigned int id)
 {
-	mutex_lock(&smmu->lock);
 	clear_bit(id, smmu->asids);
-	mutex_unlock(&smmu->lock);
 }
 
 static bool tegra_smmu_capable(enum iommu_cap cap)
@@ -420,17 +413,21 @@ static int tegra_smmu_as_prepare(struct tegra_smmu *smmu,
 				 struct tegra_smmu_as *as)
 {
 	u32 value;
-	int err;
+	int err = 0;
+
+	mutex_lock(&smmu->lock);
 
 	if (as->use_count > 0) {
 		as->use_count++;
-		return 0;
+		goto err_unlock;
 	}
 
 	as->pd_dma = dma_map_page(smmu->dev, as->pd, 0, SMMU_SIZE_PD,
 				  DMA_TO_DEVICE);
-	if (dma_mapping_error(smmu->dev, as->pd_dma))
-		return -ENOMEM;
+	if (dma_mapping_error(smmu->dev, as->pd_dma)) {
+		err = -ENOMEM;
+		goto err_unlock;
+	}
 
 	/* We can't handle 64-bit DMA addresses */
 	if (!smmu_dma_addr_valid(smmu, as->pd_dma)) {
@@ -453,24 +450,35 @@ static int tegra_smmu_as_prepare(struct tegra_smmu *smmu,
 	as->smmu = smmu;
 	as->use_count++;
 
+	mutex_unlock(&smmu->lock);
+
 	return 0;
 
 err_unmap:
 	dma_unmap_page(smmu->dev, as->pd_dma, SMMU_SIZE_PD, DMA_TO_DEVICE);
+err_unlock:
+	mutex_unlock(&smmu->lock);
+
 	return err;
 }
 
 static void tegra_smmu_as_unprepare(struct tegra_smmu *smmu,
 				    struct tegra_smmu_as *as)
 {
-	if (--as->use_count > 0)
+	mutex_lock(&smmu->lock);
+
+	if (--as->use_count > 0) {
+		mutex_unlock(&smmu->lock);
 		return;
+	}
 
 	tegra_smmu_free_asid(smmu, as->id);
 
 	dma_unmap_page(smmu->dev, as->pd_dma, SMMU_SIZE_PD, DMA_TO_DEVICE);
 
 	as->smmu = NULL;
+
+	mutex_unlock(&smmu->lock);
 }
 
 static int tegra_smmu_attach_dev(struct iommu_domain *domain,
-- 
2.17.1

Re: [PATCH v2 2/2] iommu/tegra-smmu: Expend mutex protection range

[Dmitry Osipenko]

...
>  static bool tegra_smmu_capable(enum iommu_cap cap)
> @@ -420,17 +413,21 @@ static int tegra_smmu_as_prepare(struct tegra_smmu *smmu,
>  				 struct tegra_smmu_as *as)
>  {
>  	u32 value;
> -	int err;
> +	int err = 0;
> +
> +	mutex_lock(&smmu->lock);
>  
>  	if (as->use_count > 0) {
>  		as->use_count++;
> -		return 0;
> +		goto err_unlock;

This looks a bit odd because it's not a error condition. Perhaps should
be better to "goto bump_usecount"?

Or make it similar to tegra_smmu_as_unprepare()?

>  	}
>  
>  	as->pd_dma = dma_map_page(smmu->dev, as->pd, 0, SMMU_SIZE_PD,
>  				  DMA_TO_DEVICE);
> -	if (dma_mapping_error(smmu->dev, as->pd_dma))
> -		return -ENOMEM;
> +	if (dma_mapping_error(smmu->dev, as->pd_dma)) {
> +		err = -ENOMEM;
> +		goto err_unlock;
> +	}
>  
>  	/* We can't handle 64-bit DMA addresses */
>  	if (!smmu_dma_addr_valid(smmu, as->pd_dma)) {
> @@ -453,24 +450,35 @@ static int tegra_smmu_as_prepare(struct tegra_smmu *smmu,
>  	as->smmu = smmu;

bump_usecount:

>  	as->use_count++;
>  
> +	mutex_unlock(&smmu->lock);
> +
>  	return 0;
>  
>  err_unmap:
>  	dma_unmap_page(smmu->dev, as->pd_dma, SMMU_SIZE_PD, DMA_TO_DEVICE);
> +err_unlock:
> +	mutex_unlock(&smmu->lock);
> +
>  	return err;
>  }
>  
>  static void tegra_smmu_as_unprepare(struct tegra_smmu *smmu,
>  				    struct tegra_smmu_as *as)
>  {
> -	if (--as->use_count > 0)
> +	mutex_lock(&smmu->lock);
> +
> +	if (--as->use_count > 0) {
> +		mutex_unlock(&smmu->lock);
>  		return;
> +	}
>  
>  	tegra_smmu_free_asid(smmu, as->id);
>  
>  	dma_unmap_page(smmu->dev, as->pd_dma, SMMU_SIZE_PD, DMA_TO_DEVICE);
>  
>  	as->smmu = NULL;
> +
> +	mutex_unlock(&smmu->lock);
>  }
>  
>  static int tegra_smmu_attach_dev(struct iommu_domain *domain,
> 

Re: [PATCH v2 2/2] iommu/tegra-smmu: Expend mutex protection range

[Nicolin Chen]

On Tue, Sep 29, 2020 at 03:17:58AM +0300, Dmitry Osipenko wrote:
> ...
> >  static bool tegra_smmu_capable(enum iommu_cap cap)
> > @@ -420,17 +413,21 @@ static int tegra_smmu_as_prepare(struct tegra_smmu *smmu,
> >  				 struct tegra_smmu_as *as)
> >  {
> >  	u32 value;
> > -	int err;
> > +	int err = 0;
> > +
> > +	mutex_lock(&smmu->lock);
> >  
> >  	if (as->use_count > 0) {
> >  		as->use_count++;
> > -		return 0;
> > +		goto err_unlock;
> 
> This looks a bit odd because it's not a error condition. Perhaps should
> be better to "goto bump_usecount"?
> 
> Or make it similar to tegra_smmu_as_unprepare()?

Hmm...I think it's simple to just make it "goto unlock" then.