[PATCH cgroup/for-3.10] cgroup: make cgroup_mutex outer to threadgroup_lock

[Tejun Heo <tj@kernel.org>]

It doesn't make sense to nest cgroup_mutex inside threadgroup_lock
when it should be outer to most all locks used by all cgroup
controllers.  It was nested inside threadgroup_lock only because some
controllers were abusing cgroup_mutex inside controllers leading to
locking order inversion.

cgroup_mutex is no longer abused by controllers and can be put outer
to threadgroup_lock.  Reverse the locking order in
attach_task_by_pid().

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Li Zefan <lizefan@huawei.com>
---
Li, can you please ack this?

Thanks!

 kernel/cgroup.c |   21 ++++++++-------------
 1 file changed, 8 insertions(+), 13 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 04fa2ab..24106b8 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2134,17 +2134,13 @@ static int attach_task_by_pid(struct cgroup *cgrp, u64 pid, bool threadgroup)
 	const struct cred *cred = current_cred(), *tcred;
 	int ret;
 
-	if (!cgroup_lock_live_group(cgrp))
-		return -ENODEV;
-
 retry_find_task:
 	rcu_read_lock();
 	if (pid) {
 		tsk = find_task_by_vpid(pid);
 		if (!tsk) {
 			rcu_read_unlock();
-			ret= -ESRCH;
-			goto out_unlock_cgroup;
+			return -ESRCH;
 		}
 		/*
 		 * even if we're attaching all tasks in the thread group, we
@@ -2155,8 +2151,7 @@ retry_find_task:
 		    !uid_eq(cred->euid, tcred->uid) &&
 		    !uid_eq(cred->euid, tcred->suid)) {
 			rcu_read_unlock();
-			ret = -EACCES;
-			goto out_unlock_cgroup;
+			return -EACCES;
 		}
 	} else
 		tsk = current;
@@ -2170,9 +2165,8 @@ retry_find_task:
 	 * with no rt_runtime allocated.  Just say no.
 	 */
 	if (tsk == kthreadd_task || (tsk->flags & PF_THREAD_BOUND)) {
-		ret = -EINVAL;
 		rcu_read_unlock();
-		goto out_unlock_cgroup;
+		return -EINVAL;
 	}
 
 	get_task_struct(tsk);
@@ -2194,13 +2188,14 @@ retry_find_task:
 		}
 	}
 
-	ret = cgroup_attach_task(cgrp, tsk, threadgroup);
+	ret = -ENODEV;
+	if (cgroup_lock_live_group(cgrp)) {
+		ret = cgroup_attach_task(cgrp, tsk, threadgroup);
+		cgroup_unlock();
+	}
 
 	threadgroup_unlock(tsk);
-
 	put_task_struct(tsk);
-out_unlock_cgroup:
-	cgroup_unlock();
 	return ret;
 }