[PATCH v2] cgroup: remove redundate get/put of old css_set from migrate

[Mandeep Singh Baines <msb@chromium.org>]

Changes in V2:
* Updated commit message as per Tejun's feedback:
  * https://lkml.org/lkml/2011/12/19/289

-- >8 -- (snip)

We can now assume that the css_set reference held by the task
will not go away for an exiting task. PF_EXITING state can be
trusted throughout migration by checking it after locking
threadgroup.

While at it, renamed css_set_check_fetched to css_set_fetched.
!css_set_fetched() seems to read better than
!css_set_check_fetched().

This patch depends on:

commit cd3d095275374220921fcf0d4e0c16584b26ddbc
Author: Tejun Heo <tj@kernel.org>
Date:   Mon Dec 12 18:12:21 2011 -0800

    cgroup: always lock threadgroup during migration

Signed-off-by: Mandeep Singh Baines <msb@chromium.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: containers@lists.linux-foundation.org
Cc: cgroups@vger.kernel.org
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Paul Menage <paul@paulmenage.org>
---
 kernel/cgroup.c |   25 ++++++-------------------
 1 files changed, 6 insertions(+), 19 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 1b3b841..eb95e32 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1856,7 +1856,6 @@ static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
 	 */
 	task_lock(tsk);
 	oldcg = tsk->cgroups;
-	get_css_set(oldcg);
 	task_unlock(tsk);
 
 	/* locate or allocate a new css_set for this task. */
@@ -1872,12 +1871,9 @@ static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
 		might_sleep();
 		/* find_css_set will give us newcg already referenced. */
 		newcg = find_css_set(oldcg, cgrp);
-		if (!newcg) {
-			put_css_set(oldcg);
+		if (!newcg)
 			return -ENOMEM;
-		}
 	}
-	put_css_set(oldcg);
 
 	/* @tsk can't exit as its threadgroup is locked */
 	task_lock(tsk);
@@ -2015,9 +2011,8 @@ struct cg_list_entry {
 	struct list_head links;
 };
 
-static bool css_set_check_fetched(struct cgroup *cgrp,
-				  struct task_struct *tsk, struct css_set *cg,
-				  struct list_head *newcg_list)
+static bool css_set_fetched(struct cgroup *cgrp, struct task_struct *tsk,
+			    struct css_set *cg, struct list_head *newcg_list)
 {
 	struct css_set *newcg;
 	struct cg_list_entry *cg_entry;
@@ -2191,19 +2186,11 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
 		/* get old css_set pointer */
 		task_lock(tc->task);
 		oldcg = tc->task->cgroups;
-		get_css_set(oldcg);
 		task_unlock(tc->task);
-		/* see if the new one for us is already in the list? */
-		if (css_set_check_fetched(cgrp, tc->task, oldcg, &newcg_list)) {
-			/* was already there, nothing to do. */
-			put_css_set(oldcg);
-		} else {
-			/* we don't already have it. get new one. */
-			retval = css_set_prefetch(cgrp, oldcg, &newcg_list);
-			put_css_set(oldcg);
-			if (retval)
+		/* if we don't already have it in the list get a new one */
+		if (!css_set_fetched(cgrp, tc->task, oldcg, &newcg_list))
+			if (css_set_prefetch(cgrp, oldcg, &newcg_list))
 				goto out_list_teardown;
-		}
 	}
 
 	/*
-- 
1.7.3.1