From: "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
We dropped cgroup mutex, because of a deadlock between memcg and cpuset.
cpuset took hotplug lock followed by cgroup_mutex, where as memcg pre_destroy
did lru_add_drain_all() which took hotplug lock while already holding
cgroup_mutex. The deadlock is explained in 3fa59dfbc3b223f02c26593be69ce6fc9a940405
But dropping cgroup_mutex in cgroup_rmdir also means tasks could get
added to cgroup while we are in pre_destroy. This makes error handling in
pre_destroy complex. So move the unlock/lock to memcg pre_destroy callback.
Core cgroup will now call pre_destroy with cgroup_mutex held.
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
---
kernel/cgroup.c | 3 +--
mm/memcontrol.c | 11 ++++++++++-
2 files changed, 11 insertions(+), 3 deletions(-)
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 7981850..01c67f4 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4151,7 +4151,6 @@ again:
mutex_unlock(&cgroup_mutex);
return -EBUSY;
}
- mutex_unlock(&cgroup_mutex);
/*
* In general, subsystem has no css->refcnt after pre_destroy(). But
@@ -4171,10 +4170,10 @@ again:
ret = cgroup_call_pre_destroy(cgrp);
if (ret) {
clear_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags);
+ mutex_unlock(&cgroup_mutex);
return ret;
}
- mutex_lock(&cgroup_mutex);
parent = cgrp->parent;
if (atomic_read(&cgrp->count) || !list_empty(&cgrp->children)) {
clear_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags);
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index e8ddc00..9bd56ee 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4993,9 +4993,18 @@ free_out:
static int mem_cgroup_pre_destroy(struct cgroup *cont)
{
+ int ret;
struct mem_cgroup *memcg = mem_cgroup_from_cont(cont);
- return mem_cgroup_force_empty(memcg, false);
+ cgroup_unlock();
+ /*
+ * we call lru_add_drain_all, which end up taking
+ * mutex_lock(&cpu_hotplug.lock), But cpuset have
+ * the reverse order. So drop the cgroup lock
+ */
+ ret = mem_cgroup_force_empty(memcg, false);
+ cgroup_lock();
+ return ret;
}
static void mem_cgroup_destroy(struct cgroup *cont)
--
1.7.10
--
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