2 files changed, 135 insertions, 78 deletions

diff --git a/drivers/iommu/iommufd/iommufd_private.h b/drivers/iommu/iommufd/iommufd_private.h
index f918a22f0d48..abae041e256f 100644
--- a/drivers/iommu/iommufd/iommufd_private.h
+++ b/drivers/iommu/iommufd/iommufd_private.h
@@ -21,6 +21,7 @@ struct iommufd_ctx {
 	struct file *file;
 	struct xarray objects;
 	struct xarray groups;
+	wait_queue_head_t destroy_wait;
 
 	u8 account_mode;
 	/* Compatibility with VFIO no iommu */
@@ -135,7 +136,7 @@ enum iommufd_object_type {
 
 /* Base struct for all objects with a userspace ID handle. */
 struct iommufd_object {
-	struct rw_semaphore destroy_rwsem;
+	refcount_t shortterm_users;
 	refcount_t users;
 	enum iommufd_object_type type;
 	unsigned int id;
@@ -143,10 +144,15 @@ struct iommufd_object {
 
 static inline bool iommufd_lock_obj(struct iommufd_object *obj)
 {
-	if (!down_read_trylock(&obj->destroy_rwsem))
+	if (!refcount_inc_not_zero(&obj->users))
 		return false;
-	if (!refcount_inc_not_zero(&obj->users)) {
-		up_read(&obj->destroy_rwsem);
+	if (!refcount_inc_not_zero(&obj->shortterm_users)) {
+		/*
+		 * If the caller doesn't already have a ref on obj this must be
+		 * called under the xa_lock. Otherwise the caller is holding a
+		 * ref on users. Thus it cannot be one before this decrement.
+		 */
+		refcount_dec(&obj->users);
 		return false;
 	}
 	return true;
@@ -157,8 +163,13 @@ struct iommufd_object *iommufd_get_object(struct iommufd_ctx *ictx, u32 id,
 static inline void iommufd_put_object(struct iommufd_ctx *ictx,
 				      struct iommufd_object *obj)
 {
+	/*
+	 * Users first, then shortterm so that REMOVE_WAIT_SHORTTERM never sees
+	 * a spurious !0 users with a 0 shortterm_users.
+	 */
 	refcount_dec(&obj->users);
-	up_read(&obj->destroy_rwsem);
+	if (refcount_dec_and_test(&obj->shortterm_users))
+		wake_up_interruptible_all(&ictx->destroy_wait);
 }
 
 void iommufd_object_abort(struct iommufd_ctx *ictx, struct iommufd_object *obj);
@@ -166,17 +177,49 @@ void iommufd_object_abort_and_destroy(struct iommufd_ctx *ictx,
 				      struct iommufd_object *obj);
 void iommufd_object_finalize(struct iommufd_ctx *ictx,
 			     struct iommufd_object *obj);
-void __iommufd_object_destroy_user(struct iommufd_ctx *ictx,
-				   struct iommufd_object *obj, bool allow_fail);
+
+enum {
+	REMOVE_WAIT_SHORTTERM = 1,
+};
+int iommufd_object_remove(struct iommufd_ctx *ictx,
+			  struct iommufd_object *to_destroy, u32 id,
+			  unsigned int flags);
+
+/*
+ * The caller holds a users refcount and wants to destroy the object. At this
+ * point the caller has no shortterm_users reference and at least the xarray
+ * will be holding one.
+ */
 static inline void iommufd_object_destroy_user(struct iommufd_ctx *ictx,
 					       struct iommufd_object *obj)
 {
-	__iommufd_object_destroy_user(ictx, obj, false);
+	int ret;
+
+	ret = iommufd_object_remove(ictx, obj, obj->id, REMOVE_WAIT_SHORTTERM);
+
+	/*
+	 * If there is a bug and we couldn't destroy the object then we did put
+	 * back the caller's users refcount and will eventually try to free it
+	 * again during close.
+	 */
+	WARN_ON(ret);
 }
-static inline void iommufd_object_deref_user(struct iommufd_ctx *ictx,
-					     struct iommufd_object *obj)
+
+/*
+ * The HWPT allocated by autodomains is used in possibly many devices and
+ * is automatically destroyed when its refcount reaches zero.
+ *
+ * If userspace uses the HWPT manually, even for a short term, then it will
+ * disrupt this refcounting and the auto-free in the kernel will not work.
+ * Userspace that tries to use the automatically allocated HWPT must be careful
+ * to ensure that it is consistently destroyed, eg by not racing accesses
+ * and by not attaching an automatic HWPT to a device manually.
+ */
+static inline void
+iommufd_object_put_and_try_destroy(struct iommufd_ctx *ictx,
+				   struct iommufd_object *obj)
 {
-	__iommufd_object_destroy_user(ictx, obj, true);
+	iommufd_object_remove(ictx, obj, obj->id, 0);
 }
 
 struct iommufd_object *_iommufd_object_alloc(struct iommufd_ctx *ictx,
@@ -312,7 +355,7 @@ static inline void iommufd_hw_pagetable_put(struct iommufd_ctx *ictx,
 		lockdep_assert_not_held(&hwpt_paging->ioas->mutex);
 
 		if (hwpt_paging->auto_domain) {
-			iommufd_object_deref_user(ictx, &hwpt->obj);
+			iommufd_object_put_and_try_destroy(ictx, &hwpt->obj);
 			return;
 		}
 	}
diff --git a/drivers/iommu/iommufd/main.c b/drivers/iommu/iommufd/main.c
index 45b9d40773b1..c9091e46d208 100644
--- a/drivers/iommu/iommufd/main.c
+++ b/drivers/iommu/iommufd/main.c
@@ -33,7 +33,6 @@ 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;
 
@@ -41,15 +40,8 @@ struct iommufd_object *_iommufd_object_alloc(struct iommufd_ctx *ictx,
 	if (!obj)
 		return ERR_PTR(-ENOMEM);
 	obj->type = type;
-	/*
-	 * 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]);
+	/* Starts out bias'd by 1 until it is removed from the xarray */
+	refcount_set(&obj->shortterm_users, 1);
 	refcount_set(&obj->users, 1);
 
 	/*
@@ -129,92 +121,113 @@ struct iommufd_object *iommufd_get_object(struct iommufd_ctx *ictx, u32 id,
 	return obj;
 }
 
+static int iommufd_object_dec_wait_shortterm(struct iommufd_ctx *ictx,
+					     struct iommufd_object *to_destroy)
+{
+	if (refcount_dec_and_test(&to_destroy->shortterm_users))
+		return 0;
+
+	if (wait_event_timeout(ictx->destroy_wait,
+				refcount_read(&to_destroy->shortterm_users) ==
+					0,
+				msecs_to_jiffies(10000)))
+		return 0;
+
+	pr_crit("Time out waiting for iommufd object to become free\n");
+	refcount_inc(&to_destroy->shortterm_users);
+	return -EBUSY;
+}
+
 /*
  * Remove the given object id from the xarray if the only reference to the
- * object is held by the xarray. The caller must call ops destroy().
+ * object is held by the xarray.
  */
-static struct iommufd_object *iommufd_object_remove(struct iommufd_ctx *ictx,
-						    u32 id, bool extra_put)
+int iommufd_object_remove(struct iommufd_ctx *ictx,
+			  struct iommufd_object *to_destroy, u32 id,
+			  unsigned int flags)
 {
 	struct iommufd_object *obj;
 	XA_STATE(xas, &ictx->objects, id);
-
-	xa_lock(&ictx->objects);
-	obj = xas_load(&xas);
-	if (xa_is_zero(obj) || !obj) {
-		obj = ERR_PTR(-ENOENT);
-		goto out_xa;
-	}
+	bool zerod_shortterm = false;
+	int ret;
 
 	/*
-	 * If the caller is holding a ref on obj we put it here under the
-	 * spinlock.
+	 * The purpose of the shortterm_users is to ensure deterministic
+	 * destruction of objects used by external drivers and destroyed by this
+	 * function. Any temporary increment of the refcount must increment
+	 * shortterm_users, such as during ioctl execution.
 	 */
-	if (extra_put)
+	if (flags & REMOVE_WAIT_SHORTTERM) {
+		ret = iommufd_object_dec_wait_shortterm(ictx, to_destroy);
+		if (ret) {
+			/*
+			 * We have a bug. Put back the callers reference and
+			 * defer cleaning this object until close.
+			 */
+			refcount_dec(&to_destroy->users);
+			return ret;
+		}
+		zerod_shortterm = true;
+	}
+
+	xa_lock(&ictx->objects);
+	obj = xas_load(&xas);
+	if (to_destroy) {
+		/*
+		 * If the caller is holding a ref on obj we put it here under
+		 * the spinlock.
+		 */
 		refcount_dec(&obj->users);
 
+		if (WARN_ON(obj != to_destroy)) {
+			ret = -ENOENT;
+			goto err_xa;
+		}
+	} else if (xa_is_zero(obj) || !obj) {
+		ret = -ENOENT;
+		goto err_xa;
+	}
+
 	if (!refcount_dec_if_one(&obj->users)) {
-		obj = ERR_PTR(-EBUSY);
-		goto out_xa;
+		ret = -EBUSY;
+		goto err_xa;
 	}
 
 	xas_store(&xas, NULL);
 	if (ictx->vfio_ioas == container_of(obj, struct iommufd_ioas, obj))
 		ictx->vfio_ioas = NULL;
-
-out_xa:
 	xa_unlock(&ictx->objects);
 
-	/* The returned object reference count is zero */
-	return obj;
-}
-
-/*
- * The caller holds a users refcount and wants to destroy the object. Returns
- * true if the object was destroyed. In all cases the caller no longer has a
- * reference on obj.
- */
-void __iommufd_object_destroy_user(struct iommufd_ctx *ictx,
-				   struct iommufd_object *obj, bool allow_fail)
-{
-	struct iommufd_object *ret;
-
 	/*
-	 * The purpose of the destroy_rwsem is to ensure deterministic
-	 * destruction of objects used by external drivers and destroyed by this
-	 * function. Any temporary increment of the refcount must hold the read
-	 * side of this, such as during ioctl execution.
-	 */
-	down_write(&obj->destroy_rwsem);
-	ret = iommufd_object_remove(ictx, obj->id, true);
-	up_write(&obj->destroy_rwsem);
-
-	if (allow_fail && IS_ERR(ret))
-		return;
-
-	/*
-	 * If there is a bug and we couldn't destroy the object then we did put
-	 * back the caller's refcount and will eventually try to free it again
-	 * during close.
+	 * Since users is zero any positive users_shortterm must be racing
+	 * iommufd_put_object(), or we have a bug.
 	 */
-	if (WARN_ON(IS_ERR(ret)))
-		return;
+	if (!zerod_shortterm) {
+		ret = iommufd_object_dec_wait_shortterm(ictx, obj);
+		if (WARN_ON(ret))
+			return ret;
+	}
 
 	iommufd_object_ops[obj->type].destroy(obj);
 	kfree(obj);
+	return 0;
+
+err_xa:
+	if (zerod_shortterm) {
+		/* Restore the xarray owned reference */
+		refcount_set(&obj->shortterm_users, 1);
+	}
+	xa_unlock(&ictx->objects);
+
+	/* The returned object reference count is zero */
+	return ret;
 }
 
 static int iommufd_destroy(struct iommufd_ucmd *ucmd)
 {
 	struct iommu_destroy *cmd = ucmd->cmd;
-	struct iommufd_object *obj;
 
-	obj = iommufd_object_remove(ucmd->ictx, cmd->id, false);
-	if (IS_ERR(obj))
-		return PTR_ERR(obj);
-	iommufd_object_ops[obj->type].destroy(obj);
-	kfree(obj);
-	return 0;
+	return iommufd_object_remove(ucmd->ictx, NULL, cmd->id, 0);
 }
 
 static int iommufd_fops_open(struct inode *inode, struct file *filp)
@@ -238,6 +251,7 @@ static int iommufd_fops_open(struct inode *inode, struct file *filp)
 	xa_init_flags(&ictx->objects, XA_FLAGS_ALLOC1 | XA_FLAGS_ACCOUNT);
 	xa_init(&ictx->groups);
 	ictx->file = filp;
+	init_waitqueue_head(&ictx->destroy_wait);
 	filp->private_data = ictx;
 	return 0;
 }