block: ioc_task_link() can't fail

ioc_task_link() is used to share %current's ioc on clone.  If
%current->io_context is set, %current is guaranteed to have refcount
on the ioc and, thus, ioc_task_link() can't fail.

Replace error checking in ioc_task_link() with WARN_ON_ONCE() and make
it just increment refcount and nr_tasks.

-v2: Description typo fix (Vivek).
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/iocontext.h

@@ -120,18 +120,12 @@ struct io_context {
 	struct work_struct release_work;
 };
 
-static inline struct io_context *ioc_task_link(struct io_context *ioc)
+static inline void ioc_task_link(struct io_context *ioc)
 {
-	/*
-	 * if ref count is zero, don't allow sharing (ioc is going away, it's
-	 * a race).
-	 */
-	if (ioc && atomic_long_inc_not_zero(&ioc->refcount)) {
-		atomic_inc(&ioc->nr_tasks);
-		return ioc;
-	}
-
-	return NULL;
+	WARN_ON_ONCE(atomic_long_read(&ioc->refcount) <= 0);
+	WARN_ON_ONCE(atomic_read(&ioc->nr_tasks) <= 0);
+	atomic_long_inc(&ioc->refcount);
+	atomic_inc(&ioc->nr_tasks);
 }
 
 struct task_struct;

kernel/fork.c

@@ -901,9 +901,8 @@ static int copy_io(unsigned long clone_flags, struct task_struct *tsk)
 	 * Share io context with parent, if CLONE_IO is set
 	 */
 	if (clone_flags & CLONE_IO) {
-		tsk->io_context = ioc_task_link(ioc);
-		if (unlikely(!tsk->io_context))
-			return -ENOMEM;
+		ioc_task_link(ioc);
+		tsk->io_context = ioc;
 	} else if (ioprio_valid(ioc->ioprio)) {
 		new_ioc = get_task_io_context(tsk, GFP_KERNEL, NUMA_NO_NODE);
 		if (unlikely(!new_ioc))