[PATCH v3 13/17] iommufd: Make destroy_rwsem use a lock class per object type

From: Jason Gunthorpe <jgg@nvidia.com>
To: iommu@lists.linux.dev, Kevin Tian <kevin.tian@intel.com>,
	linux-kselftest@vger.kernel.org
Cc: Lu Baolu <baolu.lu@linux.intel.com>,
	kvm@vger.kernel.org, Nicolin Chen <nicolinc@nvidia.com>,
	Yi Liu <yi.l.liu@intel.com>
Subject: [PATCH v3 13/17] iommufd: Make destroy_rwsem use a lock class per object type
Date: Tue, 21 Mar 2023 16:14:47 -0300	[thread overview]
Message-ID: <13-v3-61d41fd9e13e+1f5-iommufd_alloc_jgg@nvidia.com> (raw)
In-Reply-To: <0-v3-61d41fd9e13e+1f5-iommufd_alloc_jgg@nvidia.com>

The selftest invokes things like replace under the object lock of its
idev which protects the idev in a similar way to a real user.
Unfortunately this triggers lockdep. A lock class per type will solve the
problem.

Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
---
 drivers/iommu/iommufd/iommufd_private.h |  1 +
 drivers/iommu/iommufd/main.c            | 11 ++++++++++-
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/drivers/iommu/iommufd/iommufd_private.h b/drivers/iommu/iommufd/iommufd_private.h
index 471a3fdff1e0b6..f80b012e1bc200 100644
--- a/drivers/iommu/iommufd/iommufd_private.h
+++ b/drivers/iommu/iommufd/iommufd_private.h
@@ -119,6 +119,7 @@ enum iommufd_object_type {
 #ifdef CONFIG_IOMMUFD_TEST
 	IOMMUFD_OBJ_SELFTEST,
 #endif
+	IOMMUFD_OBJ_MAX,
 };
 
 /* Base struct for all objects with a userspace ID handle. */
diff --git a/drivers/iommu/iommufd/main.c b/drivers/iommu/iommufd/main.c
index 8597f2fb88da3a..9cba592d0482e7 100644
--- a/drivers/iommu/iommufd/main.c
+++ b/drivers/iommu/iommufd/main.c
@@ -32,6 +32,7 @@ struct iommufd_object *_iommufd_object_alloc(struct iommufd_ctx *ictx,
 					     size_t size,
 					     enum iommufd_object_type type)
 {
+	static struct lock_class_key obj_keys[IOMMUFD_OBJ_MAX];
 	struct iommufd_object *obj;
 	int rc;
 
@@ -39,7 +40,15 @@ struct iommufd_object *_iommufd_object_alloc(struct iommufd_ctx *ictx,
 	if (!obj)
 		return ERR_PTR(-ENOMEM);
 	obj->type = type;
-	init_rwsem(&obj->destroy_rwsem);
+	/*
+	 * In most cases the destroy_rwsem is obtained with try so it doesn't
+	 * interact with lockdep, however on destroy we have to sleep. This
+	 * means if we have to destroy an object while holding a get on another
+	 * object it triggers lockdep. Using one locking class per object type
+	 * is a simple and reasonable way to avoid this.
+	 */
+	__init_rwsem(&obj->destroy_rwsem, "iommufd_object::destroy_rwsem",
+		     &obj_keys[type]);
 	refcount_set(&obj->users, 1);
 
 	/*
-- 
2.40.0

