cgroup: fix lockdep warning in debug controller

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>

diff --git a/kernel/cgroup/debug.c b/kernel/cgroup/debug.c
index 163fdbd7adf616a24e6b43898788af09305f207b..dac46af2278285e0abc546abfaacc26a2c52361b 100644 (file)
--- 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;
 }