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@xxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: Li Zefan <lizf@xxxxxxxxxxxxxx>
Cc: Containers <containers@xxxxxxxxxxxxxxxxxxxxxxxxxx>
Cc: Cgroups <cgroups@xxxxxxxxxxxxxxx>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Paul Menage <paul@xxxxxxxxxxxxxx>
Cc: Mandeep Singh Baines <msb@xxxxxxxxxxxx>
---
kernel/cgroup.c | 21 +++++++++++----------
1 files changed, 11 insertions(+), 10 deletions(-)
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 5fdd98c..6b5f693 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1850,13 +1850,13 @@ 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;
- task_unlock(tsk);
/* locate or allocate a new css_set for this task. */
if (guarantee) {
@@ -1875,9 +1875,7 @@ static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
return -ENOMEM;
}
- /* @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);
@@ -2177,10 +2175,13 @@ 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;
- task_unlock(tc->task);
+
/* 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))
--
1.7.5.4
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