cgroup: only need to check oldcgrp==newgrp once

In cgroup_attach_proc it is now sufficient to only check that
oldcgrp==newcgrp once. Now that we are using threadgroup_lock()
during the migrations, oldcgrp will not change.

Signed-off-by: Mandeep Singh Baines <msb@chromium.org>
Acked-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
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

@@ -2067,7 +2067,7 @@ static int css_set_prefetch(struct cgroup *cgrp, struct css_set *cg,
  */
 int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
 {
-	int retval, i, group_size, nr_migrating_tasks;
+	int retval, i, group_size;
 	struct cgroup_subsys *ss, *failed_ss = NULL;
 	/* guaranteed to be initialized later, but the compiler needs this */
 	struct css_set *oldcg;
@@ -2118,7 +2118,7 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
 	}
 
 	tsk = leader;
-	i = nr_migrating_tasks = 0;
+	i = 0;
 	do {
 		struct task_and_cgroup ent;
 
@@ -2134,11 +2134,12 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
 		 */
 		ent.task = tsk;
 		ent.cgrp = task_cgroup_from_root(tsk, root);
+		/* nothing to do if this task is already in the cgroup */
+		if (ent.cgrp == cgrp)
+			continue;
 		retval = flex_array_put(group, i, &ent, GFP_ATOMIC);
 		BUG_ON(retval != 0);
 		i++;
-		if (ent.cgrp != cgrp)
-			nr_migrating_tasks++;
 	} while_each_thread(leader, tsk);
 	/* remember the number of threads in the array for later. */
 	group_size = i;
@@ -2148,7 +2149,7 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
 
 	/* methods shouldn't be called if no task is actually migrating */
 	retval = 0;
-	if (!nr_migrating_tasks)
+	if (!group_size)
 		goto out_free_group_list;
 
 	/*
@@ -2171,14 +2172,6 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
 	INIT_LIST_HEAD(&newcg_list);
 	for (i = 0; i < group_size; i++) {
 		tc = flex_array_get(group, i);
-		/* nothing to do if this task is already in the cgroup */
-		if (tc->cgrp == cgrp)
-			continue;
-		/*
-		 * get old css_set pointer. threadgroup is locked so this is
-		 * safe against concurrent cgroup_exit() changing this to
-		 * init_css_set.
-		 */
 		oldcg = tc->task->cgroups;
 
 		/* if we don't already have it in the list get a new one */
@@ -2194,9 +2187,6 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
 	 */
 	for (i = 0; i < group_size; i++) {
 		tc = flex_array_get(group, i);
-		/* leave current thread as it is if it's already there */
-		if (tc->cgrp == cgrp)
-			continue;
 		retval = cgroup_task_migrate(cgrp, tc->cgrp, tc->task, true);
 		BUG_ON(retval);
 	}