Re: [PATCH v4 11/13] iommu/arm-smmu-v3: Improve add_device() error handling

[Will Deacon <will@kernel.org>]

On Thu, Dec 19, 2019 at 05:30:31PM +0100, Jean-Philippe Brucker wrote:
> Let add_device() clean up after itself. The iommu_bus_init() function
> does call remove_device() on error, but other sites (e.g. of_iommu) do
> not.
> 
> Don't free level-2 stream tables because we'd have to track if we
> allocated each of them or if they are used by other endpoints. It's not
> worth the hassle since they are managed resources.
> 
> Reviewed-by: Eric Auger <eric.auger@redhat.com>
> Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
> ---
>  drivers/iommu/arm-smmu-v3.c | 28 +++++++++++++++++++++-------
>  1 file changed, 21 insertions(+), 7 deletions(-)

I think this is alright, with one caveat relating to:


	/*
	 * We _can_ actually withstand dodgy bus code re-calling add_device()
	 * without an intervening remove_device()/of_xlate() sequence, but
	 * we're not going to do so quietly...
	 */
	if (WARN_ON_ONCE(fwspec->iommu_priv)) {
		master = fwspec->iommu_priv;
		smmu = master->smmu;
	} ...


which may be on shakey ground if the subsequent add_device() call can fail
and free stuff that the first one allocated. At least, I don't know what
we're trying to support with this, so it's hard to tell whether or not it
still works as intended after your change.

How is this supposed to work? I don't recall ever seeing that WARN fire,
so can we just remove this and bail instead? Robin?

Something like below before your changes...

Will

--->8

diff --git a/drivers/iommu/arm-smmu-v3.c b/drivers/iommu/arm-smmu-v3.c
index effe72eb89e7..6ae3df2f3495 100644
--- a/drivers/iommu/arm-smmu-v3.c
+++ b/drivers/iommu/arm-smmu-v3.c
@@ -2534,28 +2534,23 @@ static int arm_smmu_add_device(struct device *dev)
 
 	if (!fwspec || fwspec->ops != &arm_smmu_ops)
 		return -ENODEV;
-	/*
-	 * We _can_ actually withstand dodgy bus code re-calling add_device()
-	 * without an intervening remove_device()/of_xlate() sequence, but
-	 * we're not going to do so quietly...
-	 */
-	if (WARN_ON_ONCE(fwspec->iommu_priv)) {
-		master = fwspec->iommu_priv;
-		smmu = master->smmu;
-	} else {
-		smmu = arm_smmu_get_by_fwnode(fwspec->iommu_fwnode);
-		if (!smmu)
-			return -ENODEV;
-		master = kzalloc(sizeof(*master), GFP_KERNEL);
-		if (!master)
-			return -ENOMEM;
 
-		master->dev = dev;
-		master->smmu = smmu;
-		master->sids = fwspec->ids;
-		master->num_sids = fwspec->num_ids;
-		fwspec->iommu_priv = master;
-	}
+	if (WARN_ON_ONCE(fwspec->iommu_priv))
+		return -EBUSY;
+
+	smmu = arm_smmu_get_by_fwnode(fwspec->iommu_fwnode);
+	if (!smmu)
+		return -ENODEV;
+
+	master = kzalloc(sizeof(*master), GFP_KERNEL);
+	if (!master)
+		return -ENOMEM;
+
+	master->dev = dev;
+	master->smmu = smmu;
+	master->sids = fwspec->ids;
+	master->num_sids = fwspec->num_ids;
+	fwspec->iommu_priv = master;
 
 	/* Check the SIDs are in range of the SMMU and our stream table */
 	for (i = 0; i < master->num_sids; i++) {