block: avoid infinite loop in get_task_io_context()

Calling get_task_io_context() on a exiting task which isn't %current can
loop forever. This triggers at boot time on my dev machine.

BUG: soft lockup - CPU#3 stuck for 22s ! [mountall.1603]

Fix this by making create_task_io_context() returns -EBUSY in this case
to break the loop.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Alan Cox <alan@linux.intel.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-ioc.c

@@ -235,6 +235,7 @@ void ioc_clear_queue(struct request_queue *q)
 int create_task_io_context(struct task_struct *task, gfp_t gfp_flags, int node)
 {
 	struct io_context *ioc;
+	int ret;
 
 	ioc = kmem_cache_alloc_node(iocontext_cachep, gfp_flags | __GFP_ZERO,
 				    node);
@@ -262,9 +263,12 @@ int create_task_io_context(struct task_struct *task, gfp_t gfp_flags, int node)
 		task->io_context = ioc;
 	else
 		kmem_cache_free(iocontext_cachep, ioc);
+
+	ret = task->io_context ? 0 : -EBUSY;
+
 	task_unlock(task);
 
-	return 0;
+	return ret;
 }
 
 /**