From e333fe3f51830ff44a56236eec3c8c05bd50d3ea Mon Sep 17 00:00:00 2001
From: Mandeep Singh Baines <msb@chromium.org>
Date: Tue, 3 Jan 2012 21:18:30 -0800
Subject: [PATCH] --- yaml --- r: 289717 b: refs/heads/master c:
 b78949ebfb563c29808a9d0a772e3adb5561bc80 h: refs/heads/master i:   289715:
 0faa665b4b51bf5ed8190082b5427fb6691e75ae v: v3

---
 [refs]                |  2 +-
 trunk/kernel/cgroup.c | 79 ++++++++++++++++---------------------------
 2 files changed, 30 insertions(+), 51 deletions(-)

diff --git a/[refs] b/[refs]
index efbd0eaf82dd..718f8dd5bde3 100644
--- a/[refs]
+++ b/[refs]
@@ -1,2 +1,2 @@
 ---
-refs/heads/master: 245282557c49754af3dbcc732316e814340d6bce
+refs/heads/master: b78949ebfb563c29808a9d0a772e3adb5561bc80
diff --git a/trunk/kernel/cgroup.c b/trunk/kernel/cgroup.c
index 6ca7acad7c55..12c07e8fd69c 100644
--- a/trunk/kernel/cgroup.c
+++ b/trunk/kernel/cgroup.c
@@ -2104,19 +2104,6 @@ static int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
 
 	/* prevent changes to the threadgroup list while we take a snapshot. */
 	read_lock(&tasklist_lock);
-	if (!thread_group_leader(leader)) {
-		/*
-		 * a race with de_thread from another thread's exec() may strip
-		 * us of our leadership, making while_each_thread unsafe to use
-		 * on this task. if this happens, there is no choice but to
-		 * throw this task away and try again (from cgroup_procs_write);
-		 * this is "double-double-toil-and-trouble-check locking".
-		 */
-		read_unlock(&tasklist_lock);
-		retval = -EAGAIN;
-		goto out_free_group_list;
-	}
-
 	tsk = leader;
 	i = 0;
 	do {
@@ -2245,22 +2232,14 @@ static int attach_task_by_pid(struct cgroup *cgrp, u64 pid, bool threadgroup)
 	if (!cgroup_lock_live_group(cgrp))
 		return -ENODEV;
 
+retry_find_task:
+	rcu_read_lock();
 	if (pid) {
-		rcu_read_lock();
 		tsk = find_task_by_vpid(pid);
 		if (!tsk) {
 			rcu_read_unlock();
-			cgroup_unlock();
-			return -ESRCH;
-		}
-		if (threadgroup) {
-			/*
-			 * RCU protects this access, since tsk was found in the
-			 * tid map. a race with de_thread may cause group_leader
-			 * to stop being the leader, but cgroup_attach_proc will
-			 * detect it later.
-			 */
-			tsk = tsk->group_leader;
+			ret= -ESRCH;
+			goto out_unlock_cgroup;
 		}
 		/*
 		 * even if we're attaching all tasks in the thread group, we
@@ -2271,29 +2250,38 @@ 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;
+			ret = -EACCES;
+			goto out_unlock_cgroup;
 		}
-		get_task_struct(tsk);
-		rcu_read_unlock();
-	} else {
-		if (threadgroup)
-			tsk = current->group_leader;
-		else
-			tsk = current;
-		get_task_struct(tsk);
-	}
-
-	threadgroup_lock(tsk);
+	} else
+		tsk = current;
 
 	if (threadgroup)
+		tsk = tsk->group_leader;
+	get_task_struct(tsk);
+	rcu_read_unlock();
+
+	threadgroup_lock(tsk);
+	if (threadgroup) {
+		if (!thread_group_leader(tsk)) {
+			/*
+			 * a race with de_thread from another thread's exec()
+			 * may strip us of our leadership, if this happens,
+			 * there is no choice but to throw this task away and
+			 * try again; this is
+			 * "double-double-toil-and-trouble-check locking".
+			 */
+			threadgroup_unlock(tsk);
+			put_task_struct(tsk);
+			goto retry_find_task;
+		}
 		ret = cgroup_attach_proc(cgrp, tsk);
-	else
+	} else
 		ret = cgroup_attach_task(cgrp, tsk);
-
 	threadgroup_unlock(tsk);
 
 	put_task_struct(tsk);
+out_unlock_cgroup:
 	cgroup_unlock();
 	return ret;
 }
@@ -2305,16 +2293,7 @@ static int cgroup_tasks_write(struct cgroup *cgrp, struct cftype *cft, u64 pid)
 
 static int cgroup_procs_write(struct cgroup *cgrp, struct cftype *cft, u64 tgid)
 {
-	int ret;
-	do {
-		/*
-		 * attach_proc fails with -EAGAIN if threadgroup leadership
-		 * changes in the middle of the operation, in which case we need
-		 * to find the task_struct for the new leader and start over.
-		 */
-		ret = attach_task_by_pid(cgrp, tgid, true);
-	} while (ret == -EAGAIN);
-	return ret;
+	return attach_task_by_pid(cgrp, tgid, true);
 }
 
 /**