[PATCH 7/7] cgroup: fix lockdep warning in debug controller

Tejun Heo
2017-06-14T20:09:34Z

The debug controller grabs cgroup_mutex from interface file show
functions which can deadlock and triggers lockdep warnings. Fix it by
using cgroup_kn_lock_live()/cgroup_kn_unlock() instead.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Waiman Long <longman@redhat.com>
---
kernel/cgroup/debug.c | 29 +++++++++++++++++++++--------
1 file changed, 21 insertions(+), 8 deletions(-)

diff --git a/kernel/cgroup/debug.c b/kernel/cgroup/debug.c
index 163fdbd7adf6..dac46af22782 100644
--- a/kernel/cgroup/debug.c
+++ b/kernel/cgroup/debug.c
@@ -38,12 +38,15 @@ static u64 debug_taskcount_read(struct cgroup_subsys_state *css,

static int current_css_set_read(struct seq_file *seq, void *v)
{
+ struct kernfs_open_file *of = seq->private;
struct css_set *cset;
struct cgroup_subsys *ss;
struct cgroup_subsys_state *css;
int i, refcnt;

- mutex_lock(&cgroup_mutex);
+ if (!cgroup_kn_lock_live(of->kn, false))
+ return -ENODEV;
+
spin_lock_irq(&css_set_lock);
rcu_read_lock();
cset = rcu_dereference(current->cgroups);
@@ -65,7 +68,7 @@ static int current_css_set_read(struct seq_file *seq, void *v)
}
rcu_read_unlock();
spin_unlock_irq(&css_set_lock);
- mutex_unlock(&cgroup_mutex);
+ cgroup_kn_unlock(of->kn);
return 0;
}

@@ -174,13 +177,17 @@ static int cgroup_css_links_read(struct seq_file *seq, void *v)

static int cgroup_subsys_states_read(struct seq_file *seq, void *v)
{
- struct cgroup *cgrp = seq_css(seq)->cgroup;
+ struct kernfs_open_file *of = seq->private;
+ struct cgroup *cgrp;
struct cgroup_subsys *ss;
struct cgroup_subsys_state *css;
char pbuf[16];
int i;

- mutex_lock(&cgroup_mutex);
+ cgrp = cgroup_kn_lock_live(of->kn, false);
+ if (!cgrp)
+ return -ENODEV;
+
for_each_subsys(ss, i) {
css = rcu_dereference_check(cgrp->subsys[ss->id], true);
if (!css)
@@ -196,7 +203,8 @@ static int cgroup_subsys_states_read(struct seq_file *seq, void *v)
(unsigned long)css, css->id,
atomic_read(&css->online_cnt), pbuf);
}
- mutex_unlock(&cgroup_mutex);
+
+ cgroup_kn_unlock(of->kn);
return 0;
}

@@ -221,12 +229,17 @@ static void cgroup_masks_read_one(struct seq_file *seq, const char *name,

static int cgroup_masks_read(struct seq_file *seq, void *v)
{
- struct cgroup *cgrp = seq_css(seq)->cgroup;
+ struct kernfs_open_file *of = seq->private;
+ struct cgroup *cgrp;
+
+ cgrp = cgroup_kn_lock_live(of->kn, false);
+ if (!cgrp)
+ return -ENODEV;

- mutex_lock(&cgroup_mutex);
cgroup_masks_read_one(seq, "subtree_control", cgrp->subtree_control);
cgroup_masks_read_one(seq, "subtree_ss_mask", cgrp->subtree_ss_mask);
- mutex_unlock(&cgroup_mutex);
+
+ cgroup_kn_unlock(of->kn);
return 0;
}

--
2.13.0