[RFC][PATCH 3/8] memcg: move memcg_tasklist mutex

[Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>]

memcg_tasklist was introduced to serialize mem_cgroup_out_of_memory() and
mem_cgroup_move_task() to ensure tasks cannot be moved to another cgroup
during select_bad_process().

task_in_mem_cgroup(), which can be called by select_bad_process(), will check
whether a task is in the mem_cgroup or not by dereferencing task->cgroups
->subsys[]. So, it would be desirable to change task->cgroups
(rcu_assign_pointer() in cgroup_attach_task() does it) with memcg_tasklist held.

Now that we can define cancel_attach(), we can safely release memcg_tasklist
on fail path even if we hold memcg_tasklist in can_attach(). So let's move
mutex_lock/unlock() of memcg_tasklist.

Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
---
 mm/memcontrol.c |   22 ++++++++++++++++++++--
 1 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index fb20564..7084cb1 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3395,18 +3395,34 @@ static int mem_cgroup_populate(struct cgroup_subsys *ss,
 	return ret;
 }
 
+static int mem_cgroup_can_attach(struct cgroup_subsys *ss,
+				struct cgroup *cgroup,
+				struct task_struct *p,
+				bool threadgroup)
+{
+	mutex_lock(&memcg_tasklist);
+	return 0;
+}
+
+static void mem_cgroup_cancel_attach(struct cgroup_subsys *ss,
+				struct cgroup *cgroup,
+				struct task_struct *p,
+				bool threadgroup)
+{
+	mutex_unlock(&memcg_tasklist);
+}
+
 static void mem_cgroup_move_task(struct cgroup_subsys *ss,
 				struct cgroup *cgroup,
 				struct cgroup *old_cgroup,
 				struct task_struct *p,
 				bool threadgroup)
 {
-	mutex_lock(&memcg_tasklist);
+	mutex_unlock(&memcg_tasklist);
 	/*
 	 * FIXME: It's better to move charges of this process from old
 	 * memcg to new memcg. But it's just on TODO-List now.
 	 */
-	mutex_unlock(&memcg_tasklist);
 }
 
 struct cgroup_subsys mem_cgroup_subsys = {
@@ -3416,6 +3432,8 @@ struct cgroup_subsys mem_cgroup_subsys = {
 	.pre_destroy = mem_cgroup_pre_destroy,
 	.destroy = mem_cgroup_destroy,
 	.populate = mem_cgroup_populate,
+	.can_attach = mem_cgroup_can_attach,
+	.cancel_attach = mem_cgroup_cancel_attach,
 	.attach = mem_cgroup_move_task,
 	.early_init = 0,
 	.use_id = 1,
-- 
1.5.6.1

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