cgroups: make cgroup_path() RCU-safe

Fix races between /proc/sched_debug by freeing cgroup objects via an RCU
callback.  Thus any cgroup reference obtained from an RCU-safe source will
remain valid during the RCU section.  Since dentries are also RCU-safe,
this allows us to traverse up the tree safely.

Additionally, make cgroup_path() check for a NULL cgrp->dentry to avoid
trying to report a path for a partially-created cgroup.

[lizf@cn.fujitsu.com: call deactive_super() in cgroup_diput()]
Signed-off-by: Paul Menage <menage@google.com>
Reviewed-by: Li Zefan <lizf@cn.fujitsu.com>
Tested-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 25 insertions, 10 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index f68dfd8dd53a..73d1c730c3c4 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -116,7 +116,7 @@ struct cgroup {
         struct list_head children;      /* my children */
 
         struct cgroup *parent;  /* my parent */
-        struct dentry *dentry;          /* cgroup fs entry */
+        struct dentry *dentry;          /* cgroup fs entry, RCU protected */
 
         /* Private pointers for each registered subsystem */
         struct cgroup_subsys_state *subsys[CGROUP_SUBSYS_COUNT];
@@ -145,6 +145,9 @@ struct cgroup {
         int pids_use_count;
         /* Length of the current tasks_pids array */
         int pids_length;
+
+        /* For RCU-protected deletion */
+        struct rcu_head rcu_head;
 };
 
 /* A css_set is a structure holding pointers to a set of
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index cb7c72b91f46..83ea4f524be5 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -271,7 +271,7 @@ static void __put_css_set(struct css_set *cg, int taskexit)
 
         rcu_read_lock();
         for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
-                struct cgroup *cgrp = cg->subsys[i]->cgroup;
+                struct cgroup *cgrp = rcu_dereference(cg->subsys[i]->cgroup);
                 if (atomic_dec_and_test(&cgrp->count) &&
                     notify_on_release(cgrp)) {
                         if (taskexit)
@@ -594,6 +594,13 @@ static void cgroup_call_pre_destroy(struct cgroup *cgrp)
         return;
 }
 
+static void free_cgroup_rcu(struct rcu_head *obj)
+{
+        struct cgroup *cgrp = container_of(obj, struct cgroup, rcu_head);
+
+        kfree(cgrp);
+}
+
 static void cgroup_diput(struct dentry *dentry, struct inode *inode)
 {
         /* is dentry a directory ? if so, kfree() associated cgroup */
@@ -619,11 +626,13 @@ static void cgroup_diput(struct dentry *dentry, struct inode *inode)
                 cgrp->root->number_of_cgroups--;
                 mutex_unlock(&cgroup_mutex);
 
-                /* Drop the active superblock reference that we took when we
-                 * created the cgroup */
+                /*
+                 * Drop the active superblock reference that we took when we
+                 * created the cgroup
+                 */
                 deactivate_super(cgrp->root->sb);
 
-                kfree(cgrp);
+                call_rcu(&cgrp->rcu_head, free_cgroup_rcu);
         }
         iput(inode);
 }
@@ -1134,14 +1143,16 @@ static inline struct cftype *__d_cft(struct dentry *dentry)
  * @buf: the buffer to write the path into
  * @buflen: the length of the buffer
  *
- * Called with cgroup_mutex held. Writes path of cgroup into buf.
- * Returns 0 on success, -errno on error.
+ * Called with cgroup_mutex held or else with an RCU-protected cgroup
+ * reference.  Writes path of cgroup into buf.  Returns 0 on success,
+ * -errno on error.
  */
 int cgroup_path(const struct cgroup *cgrp, char *buf, int buflen)
 {
         char *start;
+        struct dentry *dentry = rcu_dereference(cgrp->dentry);
 
-        if (cgrp == dummytop) {
+        if (!dentry || cgrp == dummytop) {
                 /*
                  * Inactive subsystems have no dentry for their root
                  * cgroup
@@ -1154,13 +1165,14 @@ int cgroup_path(const struct cgroup *cgrp, char *buf, int buflen)
 
         *--start = '\0';
         for (;;) {
-                int len = cgrp->dentry->d_name.len;
+                int len = dentry->d_name.len;
                 if ((start -= len) < buf)
                         return -ENAMETOOLONG;
                 memcpy(start, cgrp->dentry->d_name.name, len);
                 cgrp = cgrp->parent;
                 if (!cgrp)
                         break;
+                dentry = rcu_dereference(cgrp->dentry);
                 if (!cgrp->parent)
                         continue;
                 if (--start < buf)
@@ -1663,7 +1675,7 @@ static int cgroup_create_dir(struct cgroup *cgrp, struct dentry *dentry,
         if (!error) {
                 dentry->d_fsdata = cgrp;
                 inc_nlink(parent->d_inode);
-                cgrp->dentry = dentry;
+                rcu_assign_pointer(cgrp->dentry, dentry);
                 dget(dentry);
         }
         dput(dentry);