[RFC][PATCH 7/9 v2] cgroup: avoid attaching task to a cgroup underrmdir()

[KAMEZAWA Hiroyuki]

attach_task() is done under cgroup_mutex() but ->pre_destroy() callback
in rmdir() isn't called under cgroup_mutex().

It's better to avoid attaching a task to a cgroup which
is under pre_destroy(). Considering memcg, the attached task may
increase resource usage after memcg's pre_destroy() confirms that
memcg is empty. This is not good.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
---
 kernel/cgroup.c |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index ad8eae5..7a3076b 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1953,6 +1953,9 @@ int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
 	if (cgrp == oldcgrp)
 		return 0;
 
+	if (test_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags))
+		return -EBUSY;
+
 	tset.single.task = tsk;
 	tset.single.cgrp = oldcgrp;
 
@@ -4181,7 +4184,6 @@ again:
 		mutex_unlock(&cgroup_mutex);
 		return -EBUSY;
 	}
-	mutex_unlock(&cgroup_mutex);
 
 	/*
 	 * In general, subsystem has no css->refcnt after pre_destroy(). But
@@ -4193,6 +4195,7 @@ again:
 	 * and css_tryget() and cgroup_wakeup_rmdir_waiter() implementation.
 	 */
 	set_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags);
+	mutex_unlock(&cgroup_mutex);
 
 	/*
 	 * Call pre_destroy handlers of subsys. Notify subsystems
-- 
1.7.4.1


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