[PATCH 1/4] cgroup: change locking order in attach_task_by_pid()

[Tejun Heo]

From: Tejun Heo <tj at kernel.org>

cgroup_mutex is updated to nest inside threadgroup_fork_lock instead
of the other way around.  threadgroup locking is scheduled to be
updated to cover all threadgroup altering operations and nesting it
inside cgroup_mutex complicates locking dependency unnecessarily.
This also simplifies code a bit.

The ugly "if (threadgroup)" conditionals for threadgroup locking will
soon be removed.

Signed-off-by: Tejun Heo <tj at kernel.org>
Cc: Oleg Nesterov <oleg at redhat.com>
Cc: Andrew Morton <akpm at linux-foundation.org>
Cc: Paul Menage <menage at google.com>
Cc: Li Zefan <lizf at cn.fujitsu.com>
---
 kernel/cgroup.c |   22 ++++++++++------------
 1 files changed, 10 insertions(+), 12 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 1d2b6ce..bd1fb5f 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2203,15 +2203,11 @@ 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;
-
 	if (pid) {
 		rcu_read_lock();
 		tsk = find_task_by_vpid(pid);
 		if (!tsk) {
 			rcu_read_unlock();
-			cgroup_unlock();
 			return -ESRCH;
 		}
 		if (threadgroup) {
@@ -2225,7 +2221,6 @@ static int attach_task_by_pid(struct cgroup *cgrp, u64 pid, bool threadgroup)
 		} else if (tsk->flags & PF_EXITING) {
 			/* optimization for the single-task-only case */
 			rcu_read_unlock();
-			cgroup_unlock();
 			return -ESRCH;
 		}
 
@@ -2238,7 +2233,6 @@ static int attach_task_by_pid(struct cgroup *cgrp, u64 pid, bool threadgroup)
 		    cred->euid != tcred->uid &&
 		    cred->euid != tcred->suid) {
 			rcu_read_unlock();
-			cgroup_unlock();
 			return -EACCES;
 		}
 		get_task_struct(tsk);
@@ -2251,15 +2245,19 @@ static int attach_task_by_pid(struct cgroup *cgrp, u64 pid, bool threadgroup)
 		get_task_struct(tsk);
 	}
 
-	if (threadgroup) {
+	if (threadgroup)
 		threadgroup_fork_write_lock(tsk);
-		ret = cgroup_attach_proc(cgrp, tsk);
-		threadgroup_fork_write_unlock(tsk);
-	} else {
-		ret = cgroup_attach_task(cgrp, tsk);
+	ret = -ENODEV;
+	if (cgroup_lock_live_group(cgrp)) {
+		if (threadgroup)
+			ret = cgroup_attach_proc(cgrp, tsk);
+		else
+			ret = cgroup_attach_task(cgrp, tsk);
+		cgroup_unlock();
 	}
+	if (threadgroup)
+		threadgroup_fork_write_unlock(tsk);
 	put_task_struct(tsk);
-	cgroup_unlock();
 	return ret;
 }
 
-- 
1.7.6