Re: [PATCH v8 2/4] iommufd: Add iommufd_access_replace() API

[Jason Gunthorpe]

On Mon, Jul 24, 2023 at 12:47:05PM -0700, Nicolin Chen wrote:
> -int iommufd_access_attach(struct iommufd_access *access, u32 ioas_id)
> +static int iommufd_access_change_pt(struct iommufd_access *access, u32 ioas_id)
>  {
> +	struct iommufd_ioas *cur_ioas = access->ioas;
>  	struct iommufd_ioas *new_ioas;
> -	int rc = 0;
> +	int rc;
>  
> -	mutex_lock(&access->ioas_lock);
> -	if (WARN_ON(access->ioas || access->ioas_unpin)) {
> -		mutex_unlock(&access->ioas_lock);
> -		return -EINVAL;
> -	}
> +	lockdep_assert_held(&access->ioas_lock);
>  
>  	new_ioas = iommufd_get_ioas(access->ictx, ioas_id);
> -	if (IS_ERR(new_ioas)) {
> -		mutex_unlock(&access->ioas_lock);
> +	if (IS_ERR(new_ioas))
>  		return PTR_ERR(new_ioas);
> -	}
> +
> +	if (cur_ioas)
> +		__iommufd_access_detach(access);

The drop of the mutex while this function runs is racey with the rest
of this, we can mitigate it by blocking concurrent change while
detaching which is if access->ioas_unpin is set
  
>  	rc = iopt_add_access(&new_ioas->iopt, access);
>  	if (rc) {
> -		mutex_unlock(&access->ioas_lock);
>  		iommufd_put_object(&new_ioas->obj);
> +		if (cur_ioas)
> +			WARN_ON(iommufd_access_change_pt(access,
> +							 cur_ioas->obj.id));

We've already dropped our ref to cur_ioas, so this is also racy with
destroy.

This is what I came up with:

diff --git a/drivers/iommu/iommufd/device.c b/drivers/iommu/iommufd/device.c
index 57c0e81f5073b2..e55d6e902edb98 100644
--- a/drivers/iommu/iommufd/device.c
+++ b/drivers/iommu/iommufd/device.c
@@ -758,64 +758,101 @@ void iommufd_access_destroy(struct iommufd_access *access)
 }
 EXPORT_SYMBOL_NS_GPL(iommufd_access_destroy, IOMMUFD);
 
-void iommufd_access_detach(struct iommufd_access *access)
+static int iommufd_access_change_ioas(struct iommufd_access *access,
+				      struct iommufd_ioas *new_ioas)
 {
 	struct iommufd_ioas *cur_ioas = access->ioas;
+	int rc;
+
+	lockdep_assert_held(&access->ioas_lock);
+
+	/* We are racing with a concurrent detach, bail */
+	if (access->ioas_unpin)
+		return -EBUSY;
+
+	if (IS_ERR(new_ioas))
+		return PTR_ERR(new_ioas);
+
+	if (cur_ioas == new_ioas)
+		return 0;
 
-	mutex_lock(&access->ioas_lock);
-	if (WARN_ON(!access->ioas))
-		goto out;
 	/*
 	 * Set ioas to NULL to block any further iommufd_access_pin_pages().
 	 * iommufd_access_unpin_pages() can continue using access->ioas_unpin.
 	 */
 	access->ioas = NULL;
-
-	if (access->ops->unmap) {
+	if (cur_ioas && access->ops->unmap) {
 		mutex_unlock(&access->ioas_lock);
 		access->ops->unmap(access->data, 0, ULONG_MAX);
 		mutex_lock(&access->ioas_lock);
 	}
+
+	if (new_ioas) {
+		rc = iopt_add_access(&new_ioas->iopt, access);
+		if (rc) {
+			iommufd_put_object(&new_ioas->obj);
+			access->ioas = cur_ioas;
+			return rc;
+		}
+		iommufd_ref_to_users(&new_ioas->obj);
+	}
+
+	access->ioas = new_ioas;
+	access->ioas_unpin = new_ioas;
 	iopt_remove_access(&cur_ioas->iopt, access);
 	refcount_dec(&cur_ioas->obj.users);
-out:
-	access->ioas_unpin = NULL;
+
+	return 0;
+}
+
+void iommufd_access_detach(struct iommufd_access *access)
+{
+	int rc;
+
+	mutex_lock(&access->ioas_lock);
+	if (WARN_ON(!access->ioas)) {
+		mutex_unlock(&access->ioas_lock);
+		return;
+	}
+	rc = iommufd_access_change_ioas(access, NULL);
+	WARN_ON(rc);
 	mutex_unlock(&access->ioas_lock);
 }
 EXPORT_SYMBOL_NS_GPL(iommufd_access_detach, IOMMUFD);
 
 int iommufd_access_attach(struct iommufd_access *access, u32 ioas_id)
 {
-	struct iommufd_ioas *new_ioas;
-	int rc = 0;
+	int rc;
 
 	mutex_lock(&access->ioas_lock);
-	if (WARN_ON(access->ioas || access->ioas_unpin)) {
+	if (WARN_ON(access->ioas)) {
 		mutex_unlock(&access->ioas_lock);
 		return -EINVAL;
 	}
 
-	new_ioas = iommufd_get_ioas(access->ictx, ioas_id);
-	if (IS_ERR(new_ioas)) {
-		mutex_unlock(&access->ioas_lock);
-		return PTR_ERR(new_ioas);
-	}
-
-	rc = iopt_add_access(&new_ioas->iopt, access);
-	if (rc) {
-		mutex_unlock(&access->ioas_lock);
-		iommufd_put_object(&new_ioas->obj);
-		return rc;
-	}
-	iommufd_ref_to_users(&new_ioas->obj);
-
-	access->ioas = new_ioas;
-	access->ioas_unpin = new_ioas;
+	rc = iommufd_access_change_ioas(access,
+				      iommufd_get_ioas(access->ictx, ioas_id));
 	mutex_unlock(&access->ioas_lock);
-	return 0;
+	return rc;
 }
 EXPORT_SYMBOL_NS_GPL(iommufd_access_attach, IOMMUFD);
 
+int iommufd_access_replace(struct iommufd_access *access, u32 ioas_id)
+{
+	int rc;
+
+	mutex_lock(&access->ioas_lock);
+	if (!access->ioas) {
+		mutex_unlock(&access->ioas_lock);
+		return -ENOENT;
+	}
+	rc = iommufd_access_change_ioas(access,
+				      iommufd_get_ioas(access->ictx, ioas_id));
+	mutex_unlock(&access->ioas_lock);
+	return rc;
+}
+EXPORT_SYMBOL_NS_GPL(iommufd_access_replace, IOMMUFD);
+
 /**
  * iommufd_access_notify_unmap - Notify users of an iopt to stop using it
  * @iopt: iopt to work on
diff --git a/include/linux/iommufd.h b/include/linux/iommufd.h
index 0ac60256b65929..ffc3a949f8374f 100644
--- a/include/linux/iommufd.h
+++ b/include/linux/iommufd.h
@@ -49,6 +49,7 @@ iommufd_access_create(struct iommufd_ctx *ictx,
 		      const struct iommufd_access_ops *ops, void *data, u32 *id);
 void iommufd_access_destroy(struct iommufd_access *access);
 int iommufd_access_attach(struct iommufd_access *access, u32 ioas_id);
+int iommufd_access_replace(struct iommufd_access *access, u32 ioas_id);
 void iommufd_access_detach(struct iommufd_access *access);
 
 void iommufd_ctx_get(struct iommufd_ctx *ictx);


Jason