[PATCH v3.4-rc5] block: iocontext->nr_tasks should be initialized to one

[Tejun Heo <tj@kernel.org>]

create_task_io_context() left ioc->nr_tasks at zero; however, a newly
created ioc should have its nr_tasks initialized to one as it begins
attached to the task creating it.

This affects only CLONE_IO which currently doesn't seem to have any
actual user.  Sasha triggered WARN_ON_ONCE() in ioc_task_link() using
syscall fuzzer.  Even when it happens, the failure mode isn't critical
(blk-cgroup may allow attaching a CLONE_IO'd task to a cgroup when it
shouldn't and blkcg limits may behave weirdly).

Fix it by initializing it to one in create_task_io_context().

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Sasha Levin <levinsasha928@gmail.com>
LKML-Reference: <1335873936.16988.148.camel@lappy>
Cc: stable@vger.kernel.org
---
 block/blk-ioc.c |    1 +
 1 file changed, 1 insertion(+)

diff --git a/block/blk-ioc.c b/block/blk-ioc.c
index 1e2d53b..c942409 100644
--- a/block/blk-ioc.c
+++ b/block/blk-ioc.c
@@ -244,6 +244,7 @@ int create_task_io_context(struct task_struct *task, gfp_t gfp_flags, int node)
 	/* initialize */
 	atomic_long_set(&ioc->refcount, 1);
 	atomic_set(&ioc->active_ref, 1);
+	atomic_set(&ioc->nr_tasks, 1);
 	spin_lock_init(&ioc->lock);
 	INIT_RADIX_TREE(&ioc->icq_tree, GFP_ATOMIC | __GFP_HIGH);
 	INIT_HLIST_HEAD(&ioc->icq_list);