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>

1 files changed, 10 insertions, 10 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index dae50d0d8e4b..4936d8886b4f 100644
--- 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. */