cgroup: Remove unnecessary task_lock before fetching css_set on migration

When we fetch the css_set of the tasks on cgroup migration, we don't need
anymore to synchronize against cgroup_exit() that could swap the old one
with init_css_set. Now that we are using threadgroup_lock() during
the migrations, we don't need to worry about it anymore.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Reviewed-by: Mandeep Singh Baines <msb@chromium.org>
Reviewed-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Containers <containers@lists.linux-foundation.org>
Cc: Cgroups <cgroups@vger.kernel.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Paul Menage <paul@paulmenage.org>

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index dae50d0d8e4bc89af0027530d19409354ec38ee7..4936d8886b4f49b6127d50f12744737ecfc40523 100644 (file)
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1850,14 +1850,14 @@ static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
        struct css_set *newcg;
 
        /*
-        * get old css_set. we need to take task_lock and refcount it, because
-        * an exiting task can change its css_set to init_css_set and drop its
-        * old one without taking cgroup_mutex.
+        * get old css_set. We are synchronized through threadgroup_lock()
+        * against PF_EXITING setting such that we can't race against
+        * cgroup_exit() changing the css_set to init_css_set and dropping the
+        * old one.
         */
-       task_lock(tsk);
+       WARN_ON_ONCE(tsk->flags & PF_EXITING);
        oldcg = tsk->cgroups;
        get_css_set(oldcg);
-       task_unlock(tsk);
 
        /* locate or allocate a new css_set for this task. */
        if (guarantee) {
@@ -1879,9 +1879,7 @@ static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
        }
        put_css_set(oldcg);
 
-       /* @tsk can't exit as its threadgroup is locked */
        task_lock(tsk);
-       WARN_ON_ONCE(tsk->flags & PF_EXITING);
        rcu_assign_pointer(tsk->cgroups, newcg);
        task_unlock(tsk);
 
@@ -2182,11 +2180,13 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
                /* nothing to do if this task is already in the cgroup */
                if (tc->cgrp == cgrp)
                        continue;
-               /* get old css_set pointer */
-               task_lock(tc->task);
+               /*
+                * 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;
                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. */