IB/uverbs: Allow RDMA_REMOVE_DESTROY to work concurrently with disassociate

After all the recent structural changes this is now straightfoward, hoist
the hw_destroy_rwsem up out of rdma_destroy_explicit and wrap it around
the uobject write lock as well as the destroy.

This is necessary as obtaining a write lock concurrently with
uverbs_destroy_ufile_hw() will cause malfunction.

After this change none of the destroy callbacks require the
disassociate_srcu lock to be correct.

This requires introducing a new lookup mode, UVERBS_LOOKUP_DESTROY as the
IOCTL interface needs to hold an unlocked kref until all command
verification is completed.
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/core/rdma_core.c

@@ -127,8 +127,10 @@ static int uverbs_try_lock_object(struct ib_uobject *uobj,
 		return __atomic_add_unless(&uobj->usecnt, 1, -1) == -1 ?
 			-EBUSY : 0;
 	case UVERBS_LOOKUP_WRITE:
-		/* lock is either WRITE or DESTROY - should be exclusive */
+		/* lock is exclusive */
 		return atomic_cmpxchg(&uobj->usecnt, 0, -1) == 0 ? 0 : -EBUSY;
+	case UVERBS_LOOKUP_DESTROY:
+		return 0;
 	}
 	return 0;
 }
@@ -144,6 +146,8 @@ static void assert_uverbs_usecnt(struct ib_uobject *uobj,
 	case UVERBS_LOOKUP_WRITE:
 		WARN_ON(atomic_read(&uobj->usecnt) != -1);
 		break;
+	case UVERBS_LOOKUP_DESTROY:
+		break;
 	}
 #endif
 }
@@ -227,6 +231,35 @@ static int uverbs_destroy_uobject(struct ib_uobject *uobj,
 	return 0;
 }
 
+/*
+ * This calls uverbs_destroy_uobject() using the RDMA_REMOVE_DESTROY
+ * sequence. It should only be used from command callbacks. On success the
+ * caller must pair this with rdma_lookup_put_uobject(LOOKUP_WRITE). This
+ * version requires the caller to have already obtained an
+ * LOOKUP_DESTROY uobject kref.
+ */
+int uobj_destroy(struct ib_uobject *uobj)
+{
+	struct ib_uverbs_file *ufile = uobj->ufile;
+	int ret;
+
+	down_read(&ufile->hw_destroy_rwsem);
+
+	ret = uverbs_try_lock_object(uobj, UVERBS_LOOKUP_WRITE);
+	if (ret)
+		goto out_unlock;
+
+	ret = uverbs_destroy_uobject(uobj, RDMA_REMOVE_DESTROY);
+	if (ret) {
+		atomic_set(&uobj->usecnt, 0);
+		goto out_unlock;
+	}
+
+out_unlock:
+	up_read(&ufile->hw_destroy_rwsem);
+	return ret;
+}
+
 /*
  * uobj_get_destroy destroys the HW object and returns a handle to the uobj
  * with a NULL object pointer. The caller must pair this with
@@ -238,13 +271,13 @@ struct ib_uobject *__uobj_get_destroy(const struct uverbs_obj_type *type,
 	struct ib_uobject *uobj;
 	int ret;
 
-	uobj = rdma_lookup_get_uobject(type, ufile, id, UVERBS_LOOKUP_WRITE);
+	uobj = rdma_lookup_get_uobject(type, ufile, id, UVERBS_LOOKUP_DESTROY);
 	if (IS_ERR(uobj))
 		return uobj;
 
-	ret = rdma_explicit_destroy(uobj);
+	ret = uobj_destroy(uobj);
 	if (ret) {
-		rdma_lookup_put_uobject(uobj, UVERBS_LOOKUP_WRITE);
+		rdma_lookup_put_uobject(uobj, UVERBS_LOOKUP_DESTROY);
 		return ERR_PTR(ret);
 	}
 
@@ -265,6 +298,11 @@ int __uobj_perform_destroy(const struct uverbs_obj_type *type, u32 id,
 	if (IS_ERR(uobj))
 		return PTR_ERR(uobj);
 
+	/*
+	 * FIXME: After destroy this is not safe. We no longer hold the rwsem
+	 * so disassociation could have completed and unloaded the module that
+	 * backs the uobj->type pointer.
+	 */
 	rdma_lookup_put_uobject(uobj, UVERBS_LOOKUP_WRITE);
 	return success_res;
 }
@@ -534,23 +572,6 @@ static int __must_check remove_commit_fd_uobject(struct ib_uobject *uobj,
 	return 0;
 }
 
-int rdma_explicit_destroy(struct ib_uobject *uobject)
-{
-	int ret;
-	struct ib_uverbs_file *ufile = uobject->ufile;
-
-	/* Cleanup is running. Calling this should have been impossible */
-	if (!down_read_trylock(&ufile->hw_destroy_rwsem)) {
-		WARN(true, "ib_uverbs: Cleanup is running while removing an uobject\n");
-		return 0;
-	}
-
-	ret = uverbs_destroy_uobject(uobject, RDMA_REMOVE_DESTROY);
-
-	up_read(&ufile->hw_destroy_rwsem);
-	return ret;
-}
-
 static int alloc_commit_idr_uobject(struct ib_uobject *uobj)
 {
 	struct ib_uverbs_file *ufile = uobj->ufile;
@@ -686,6 +707,8 @@ void rdma_lookup_put_uobject(struct ib_uobject *uobj,
 	case UVERBS_LOOKUP_WRITE:
 		atomic_set(&uobj->usecnt, 0);
 		break;
+	case UVERBS_LOOKUP_DESTROY:
+		break;
 	}
 
 	/* Pairs with the kref obtained by type->lookup_get */
@@ -911,6 +934,9 @@ uverbs_get_uobject_from_file(const struct uverbs_obj_type *type_attrs,
 		return rdma_lookup_get_uobject(type_attrs, ufile, id,
 					       UVERBS_LOOKUP_READ);
 	case UVERBS_ACCESS_DESTROY:
+		/* Actual destruction is done inside uverbs_handle_method */
+		return rdma_lookup_get_uobject(type_attrs, ufile, id,
+					       UVERBS_LOOKUP_DESTROY);
 	case UVERBS_ACCESS_WRITE:
 		return rdma_lookup_get_uobject(type_attrs, ufile, id,
 					       UVERBS_LOOKUP_WRITE);
@@ -942,7 +968,8 @@ int uverbs_finalize_object(struct ib_uobject *uobj,
 		rdma_lookup_put_uobject(uobj, UVERBS_LOOKUP_WRITE);
 		break;
 	case UVERBS_ACCESS_DESTROY:
-		rdma_lookup_put_uobject(uobj, UVERBS_LOOKUP_WRITE);
+		if (uobj)
+			rdma_lookup_put_uobject(uobj, UVERBS_LOOKUP_DESTROY);
 		break;
 	case UVERBS_ACCESS_NEW:
 		if (commit)

drivers/infiniband/core/rdma_core.h

@@ -52,6 +52,8 @@ const struct uverbs_method_spec *uverbs_get_method(const struct uverbs_object_sp
 void uverbs_destroy_ufile_hw(struct ib_uverbs_file *ufile,
 			     enum rdma_remove_reason reason);
 
+int uobj_destroy(struct ib_uobject *uobj);
+
 /*
  * uverbs_uobject_get is called in order to increase the reference count on
  * an uobject. This is useful when a handler wants to keep the uobject's memory

drivers/infiniband/core/uverbs_ioctl.c

@@ -349,13 +349,18 @@ static int uverbs_handle_method(struct ib_uverbs_attr __user *uattr_ptr,
 	 * not get to manipulate the HW objects.
 	 */
 	if (destroy_attr) {
-		ret = rdma_explicit_destroy(destroy_attr->uobject);
+		ret = uobj_destroy(destroy_attr->uobject);
 		if (ret)
 			goto cleanup;
 	}
 
 	ret = method_spec->handler(ibdev, ufile, attr_bundle);
 
+	if (destroy_attr) {
+		uobj_put_destroy(destroy_attr->uobject);
+		destroy_attr->uobject = NULL;
+	}
+
 cleanup:
 	finalize_ret = uverbs_finalize_attrs(attr_bundle,
 					     method_spec->attr_buckets,

include/rdma/uverbs_types.h

@@ -41,6 +41,12 @@ struct uverbs_obj_type;
 enum rdma_lookup_mode {
 	UVERBS_LOOKUP_READ,
 	UVERBS_LOOKUP_WRITE,
+	/*
+	 * Destroy is like LOOKUP_WRITE, except that the uobject is not
+	 * locked.  uobj_destroy is used to convert a LOOKUP_DESTROY lock into
+	 * a LOOKUP_WRITE lock.
+	 */
+	UVERBS_LOOKUP_DESTROY,
 };
 
 /*
@@ -129,7 +135,6 @@ struct ib_uobject *rdma_alloc_begin_uobject(const struct uverbs_obj_type *type,
 					    struct ib_uverbs_file *ufile);
 void rdma_alloc_abort_uobject(struct ib_uobject *uobj);
 int __must_check rdma_alloc_commit_uobject(struct ib_uobject *uobj);
-int rdma_explicit_destroy(struct ib_uobject *uobject);
 
 struct uverbs_obj_fd_type {
 	/*