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>

1 files 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;
 }