IB/uverbs: Revise and clarify the rwsem and uobjects_lock

Rename 'cleanup_rwsem' to 'hw_destroy_rwsem' which is held across any call
to the type destroy function (aka 'hw' destroy). The main purpose of this
lock is to prevent normal add and destroy from running concurrently with
uverbs_cleanup_ufile()

Since the uobjects list is always manipulated under the 'hw_destroy_rwsem'
we can eliminate the uobjects_lock in the cleanup function. This allows
converting that lock to a very simple spinlock with a narrow critical
section.
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/core/rdma_core.c

@@ -461,9 +461,9 @@ static int __must_check _rdma_remove_commit_uobject(struct ib_uobject *uobj,
 
 	uobj->object = NULL;
 
-	mutex_lock(&ufile->uobjects_lock);
+	spin_lock_irq(&ufile->uobjects_lock);
 	list_del(&uobj->list);
-	mutex_unlock(&ufile->uobjects_lock);
+	spin_unlock_irq(&ufile->uobjects_lock);
 	/* Pairs with the get in rdma_alloc_commit_uobject() */
 	uverbs_uobject_put(uobj);
 
@@ -491,14 +491,14 @@ int rdma_explicit_destroy(struct ib_uobject *uobject)
 	struct ib_uverbs_file *ufile = uobject->ufile;
 
 	/* Cleanup is running. Calling this should have been impossible */
-	if (!down_read_trylock(&ufile->cleanup_rwsem)) {
+	if (!down_read_trylock(&ufile->hw_destroy_rwsem)) {
 		WARN(true, "ib_uverbs: Cleanup is running while removing an uobject\n");
 		return 0;
 	}
 	assert_uverbs_usecnt(uobject, true);
 	ret = _rdma_remove_commit_uobject(uobject, RDMA_REMOVE_DESTROY);
 
-	up_read(&ufile->cleanup_rwsem);
+	up_read(&ufile->hw_destroy_rwsem);
 	return ret;
 }
 
@@ -541,7 +541,7 @@ int rdma_alloc_commit_uobject(struct ib_uobject *uobj)
 	struct ib_uverbs_file *ufile = uobj->ufile;
 
 	/* Cleanup is running. Calling this should have been impossible */
-	if (!down_read_trylock(&ufile->cleanup_rwsem)) {
+	if (!down_read_trylock(&ufile->hw_destroy_rwsem)) {
 		int ret;
 
 		WARN(true, "ib_uverbs: Cleanup is running while allocating an uobject\n");
@@ -559,13 +559,13 @@ int rdma_alloc_commit_uobject(struct ib_uobject *uobj)
 
 	/* kref is held so long as the uobj is on the uobj list. */
 	uverbs_uobject_get(uobj);
-	mutex_lock(&ufile->uobjects_lock);
+	spin_lock_irq(&ufile->uobjects_lock);
 	list_add(&uobj->list, &ufile->uobjects);
-	mutex_unlock(&ufile->uobjects_lock);
+	spin_unlock_irq(&ufile->uobjects_lock);
 
 	/* alloc_commit consumes the uobj kref */
 	uobj->type->type_class->alloc_commit(uobj);
-	up_read(&ufile->cleanup_rwsem);
+	up_read(&ufile->hw_destroy_rwsem);
 
 	return 0;
 }
@@ -681,9 +681,9 @@ void uverbs_close_fd(struct file *f)
 	struct ib_uobject *uobj = f->private_data;
 	struct ib_uverbs_file *ufile = uobj->ufile;
 
-	if (down_read_trylock(&ufile->cleanup_rwsem)) {
+	if (down_read_trylock(&ufile->hw_destroy_rwsem)) {
 		_uverbs_close_fd(uobj);
-		up_read(&ufile->cleanup_rwsem);
+		up_read(&ufile->hw_destroy_rwsem);
 	}
 
 	uobj->object = NULL;
@@ -710,7 +710,6 @@ static int __uverbs_cleanup_ufile(struct ib_uverbs_file *ufile,
 	 * We take and release the lock per traversal in order to let
 	 * other threads (which might still use the FDs) chance to run.
 	 */
-	mutex_lock(&ufile->uobjects_lock);
 	ufile->cleanup_reason = reason;
 	list_for_each_entry_safe(obj, next_obj, &ufile->uobjects, list) {
 		/*
@@ -736,7 +735,6 @@ static int __uverbs_cleanup_ufile(struct ib_uverbs_file *ufile,
 		uverbs_uobject_put(obj);
 		ret = 0;
 	}
-	mutex_unlock(&ufile->uobjects_lock);
 	return ret;
 }
 
@@ -751,7 +749,7 @@ void uverbs_cleanup_ufile(struct ib_uverbs_file *ufile, bool device_removed)
 	 * want to hold this forever as the context is going to be destroyed,
 	 * but we'll release it since it causes a "held lock freed" BUG message.
 	 */
-	down_write(&ufile->cleanup_rwsem);
+	down_write(&ufile->hw_destroy_rwsem);
 	ufile->ucontext->cleanup_retryable = true;
 	while (!list_empty(&ufile->uobjects))
 		if (__uverbs_cleanup_ufile(ufile, reason)) {
@@ -766,7 +764,7 @@ void uverbs_cleanup_ufile(struct ib_uverbs_file *ufile, bool device_removed)
 	if (!list_empty(&ufile->uobjects))
 		__uverbs_cleanup_ufile(ufile, reason);
 
-	up_write(&ufile->cleanup_rwsem);
+	up_write(&ufile->hw_destroy_rwsem);
 }
 
 const struct uverbs_obj_type_class uverbs_fd_class = {

drivers/infiniband/core/uverbs.h

@@ -145,12 +145,16 @@ struct ib_uverbs_file {
 	struct list_head			list;
 	int					is_closed;
 
-	/* locking the uobjects_list */
-	struct mutex		uobjects_lock;
+	/*
+	 * To access the uobjects list hw_destroy_rwsem must be held for write
+	 * OR hw_destroy_rwsem held for read AND uobjects_lock held.
+	 * hw_destroy_rwsem should be called across any destruction of the HW
+	 * object of an associated uobject.
+	 */
+	struct rw_semaphore	hw_destroy_rwsem;
+	spinlock_t		uobjects_lock;
 	struct list_head	uobjects;
 
-	/* protects cleanup process from other actions */
-	struct rw_semaphore	cleanup_rwsem;
 	enum rdma_remove_reason cleanup_reason;
 
 	struct idr		idr;

drivers/infiniband/core/uverbs_main.c

@@ -889,9 +889,9 @@ static int ib_uverbs_open(struct inode *inode, struct file *filp)
 	mutex_init(&file->mutex);
 	mutex_init(&file->cleanup_mutex);
 
-	mutex_init(&file->uobjects_lock);
+	spin_lock_init(&file->uobjects_lock);
 	INIT_LIST_HEAD(&file->uobjects);
-	init_rwsem(&file->cleanup_rwsem);
+	init_rwsem(&file->hw_destroy_rwsem);
 
 	filp->private_data = file;
 	kobject_get(&dev->kobj);