1 files changed, 80 insertions, 16 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index cace83ddbcdc..c92fb9549358 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -57,10 +57,14 @@
 
 static DEFINE_MUTEX(cgroup_mutex);
 
-/* Generate an array of cgroup subsystem pointers */
+/*
+ * Generate an array of cgroup subsystem pointers. At boot time, this is
+ * populated up to CGROUP_BUILTIN_SUBSYS_COUNT, and modular subsystems are
+ * registered after that. The mutable section of this array is protected by
+ * cgroup_mutex.
+ */
 #define SUBSYS(_x) &_x ## _subsys,
-
-static struct cgroup_subsys *subsys[] = {
+static struct cgroup_subsys *subsys[CGROUP_SUBSYS_COUNT] = {
 #include <linux/cgroup_subsys.h>
 };
 
@@ -448,8 +452,11 @@ static struct css_set *find_existing_css_set(
         struct hlist_node *node;
         struct css_set *cg;
 
-        /* Built the set of subsystem state objects that we want to
-         * see in the new css_set */
+        /*
+         * Build the set of subsystem state objects that we want to see in the
+         * new css_set. while subsystems can change globally, the entries here
+         * won't change, so no need for locking.
+         */
         for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
                 if (root->subsys_bits & (1UL << i)) {
                         /* Subsystem is in this hierarchy. So we want
@@ -884,7 +891,9 @@ void cgroup_release_and_wakeup_rmdir(struct cgroup_subsys_state *css)
         css_put(css);
 }
 
-
+/*
+ * Call with cgroup_mutex held.
+ */
 static int rebind_subsystems(struct cgroupfs_root *root,
                               unsigned long final_bits)
 {
@@ -892,6 +901,8 @@ static int rebind_subsystems(struct cgroupfs_root *root,
         struct cgroup *cgrp = &root->top_cgroup;
         int i;
 
+        BUG_ON(!mutex_is_locked(&cgroup_mutex));
+
         removed_bits = root->actual_subsys_bits & ~final_bits;
         added_bits = final_bits & ~root->actual_subsys_bits;
         /* Check that any added subsystems are currently free */
@@ -900,6 +911,12 @@ static int rebind_subsystems(struct cgroupfs_root *root,
                 struct cgroup_subsys *ss = subsys[i];
                 if (!(bit & added_bits))
                         continue;
+                /*
+                 * Nobody should tell us to do a subsys that doesn't exist:
+                 * parse_cgroupfs_options should catch that case and refcounts
+                 * ensure that subsystems won't disappear once selected.
+                 */
+                BUG_ON(ss == NULL);
                 if (ss->root != &rootnode) {
                         /* Subsystem isn't free */
                         return -EBUSY;
@@ -919,6 +936,7 @@ static int rebind_subsystems(struct cgroupfs_root *root,
                 unsigned long bit = 1UL << i;
                 if (bit & added_bits) {
                         /* We're binding this subsystem to this hierarchy */
+                        BUG_ON(ss == NULL);
                         BUG_ON(cgrp->subsys[i]);
                         BUG_ON(!dummytop->subsys[i]);
                         BUG_ON(dummytop->subsys[i]->cgroup != dummytop);
@@ -932,6 +950,7 @@ static int rebind_subsystems(struct cgroupfs_root *root,
                         mutex_unlock(&ss->hierarchy_mutex);
                 } else if (bit & removed_bits) {
                         /* We're removing this subsystem */
+                        BUG_ON(ss == NULL);
                         BUG_ON(cgrp->subsys[i] != dummytop->subsys[i]);
                         BUG_ON(cgrp->subsys[i]->cgroup != cgrp);
                         mutex_lock(&ss->hierarchy_mutex);
@@ -944,6 +963,7 @@ static int rebind_subsystems(struct cgroupfs_root *root,
                         mutex_unlock(&ss->hierarchy_mutex);
                 } else if (bit & final_bits) {
                         /* Subsystem state should already exist */
+                        BUG_ON(ss == NULL);
                         BUG_ON(!cgrp->subsys[i]);
                 } else {
                         /* Subsystem state shouldn't exist */
@@ -986,14 +1006,18 @@ struct cgroup_sb_opts {
 
 };
 
-/* Convert a hierarchy specifier into a bitmask of subsystems and
- * flags. */
+/*
+ * Convert a hierarchy specifier into a bitmask of subsystems and flags. Call
+ * with cgroup_mutex held to protect the subsys[] array.
+ */
 static int parse_cgroupfs_options(char *data,
                                      struct cgroup_sb_opts *opts)
 {
         char *token, *o = data ?: "all";
         unsigned long mask = (unsigned long)-1;
 
+        BUG_ON(!mutex_is_locked(&cgroup_mutex));
+
 #ifdef CONFIG_CPUSETS
         mask = ~(1UL << cpuset_subsys_id);
 #endif
@@ -1009,6 +1033,8 @@ static int parse_cgroupfs_options(char *data,
                         opts->subsys_bits = 0;
                         for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
                                 struct cgroup_subsys *ss = subsys[i];
+                                if (ss == NULL)
+                                        continue;
                                 if (!ss->disabled)
                                         opts->subsys_bits |= 1ul << i;
                         }
@@ -1053,6 +1079,8 @@ static int parse_cgroupfs_options(char *data,
                         int i;
                         for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
                                 ss = subsys[i];
+                                if (ss == NULL)
+                                        continue;
                                 if (!strcmp(token, ss->name)) {
                                         if (!ss->disabled)
                                                 set_bit(i, &opts->subsys_bits);
@@ -1306,7 +1334,9 @@ static int cgroup_get_sb(struct file_system_type *fs_type,
         struct cgroupfs_root *new_root;
 
         /* First find the desired set of subsystems */
+        mutex_lock(&cgroup_mutex);
         ret = parse_cgroupfs_options(data, &opts);
+        mutex_unlock(&cgroup_mutex);
         if (ret)
                 goto out_err;
 
@@ -2918,8 +2948,14 @@ static void cgroup_lock_hierarchy(struct cgroupfs_root *root)
         /* We need to take each hierarchy_mutex in a consistent order */
         int i;
 
+        /*
+         * No worry about a race with rebind_subsystems that might mess up the
+         * locking order, since both parties are under cgroup_mutex.
+         */
         for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
                 struct cgroup_subsys *ss = subsys[i];
+                if (ss == NULL)
+                        continue;
                 if (ss->root == root)
                         mutex_lock(&ss->hierarchy_mutex);
         }
@@ -2931,6 +2967,8 @@ static void cgroup_unlock_hierarchy(struct cgroupfs_root *root)
 
         for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
                 struct cgroup_subsys *ss = subsys[i];
+                if (ss == NULL)
+                        continue;
                 if (ss->root == root)
                         mutex_unlock(&ss->hierarchy_mutex);
         }
@@ -3054,11 +3092,16 @@ static int cgroup_has_css_refs(struct cgroup *cgrp)
          * synchronization other than RCU, and the subsystem linked
          * list isn't RCU-safe */
         int i;
+        /*
+         * We won't need to lock the subsys array, because the subsystems
+         * we're concerned about aren't going anywhere since our cgroup root
+         * has a reference on them.
+         */
         for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
                 struct cgroup_subsys *ss = subsys[i];
                 struct cgroup_subsys_state *css;
-                /* Skip subsystems not in this hierarchy */
-                if (ss->root != cgrp->root)
+                /* Skip subsystems not present or not in this hierarchy */
+                if (ss == NULL || ss->root != cgrp->root)
                         continue;
                 css = cgrp->subsys[ss->subsys_id];
                 /* When called from check_for_release() it's possible
@@ -3279,7 +3322,8 @@ int __init cgroup_init_early(void)
         for (i = 0; i < CSS_SET_TABLE_SIZE; i++)
                 INIT_HLIST_HEAD(&css_set_table[i]);
 
-        for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
+        /* at bootup time, we don't worry about modular subsystems */
+        for (i = 0; i < CGROUP_BUILTIN_SUBSYS_COUNT; i++) {
                 struct cgroup_subsys *ss = subsys[i];
 
                 BUG_ON(!ss->name);
@@ -3314,7 +3358,8 @@ int __init cgroup_init(void)
         if (err)
                 return err;
 
-        for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
+        /* at bootup time, we don't worry about modular subsystems */
+        for (i = 0; i < CGROUP_BUILTIN_SUBSYS_COUNT; i++) {
                 struct cgroup_subsys *ss = subsys[i];
                 if (!ss->early_init)
                         cgroup_init_subsys(ss);
@@ -3423,9 +3468,16 @@ static int proc_cgroupstats_show(struct seq_file *m, void *v)
         int i;
 
         seq_puts(m, "#subsys_name\thierarchy\tnum_cgroups\tenabled\n");
+        /*
+         * ideally we don't want subsystems moving around while we do this.
+         * cgroup_mutex is also necessary to guarantee an atomic snapshot of
+         * subsys/hierarchy state.
+         */
         mutex_lock(&cgroup_mutex);
         for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
                 struct cgroup_subsys *ss = subsys[i];
+                if (ss == NULL)
+                        continue;
                 seq_printf(m, "%s\t%d\t%d\t%d\n",
                            ss->name, ss->root->hierarchy_id,
                            ss->root->number_of_cgroups, !ss->disabled);
@@ -3483,7 +3535,12 @@ void cgroup_fork_callbacks(struct task_struct *child)
 {
         if (need_forkexit_callback) {
                 int i;
-                for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
+                /*
+                 * forkexit callbacks are only supported for builtin
+                 * subsystems, and the builtin section of the subsys array is
+                 * immutable, so we don't need to lock the subsys array here.
+                 */
+                for (i = 0; i < CGROUP_BUILTIN_SUBSYS_COUNT; i++) {
                         struct cgroup_subsys *ss = subsys[i];
                         if (ss->fork)
                                 ss->fork(ss, child);
@@ -3552,7 +3609,11 @@ void cgroup_exit(struct task_struct *tsk, int run_callbacks)
         struct css_set *cg;
 
         if (run_callbacks && need_forkexit_callback) {
-                for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
+                /*
+                 * modular subsystems can't use callbacks, so no need to lock
+                 * the subsys array
+                 */
+                for (i = 0; i < CGROUP_BUILTIN_SUBSYS_COUNT; i++) {
                         struct cgroup_subsys *ss = subsys[i];
                         if (ss->exit)
                                 ss->exit(ss, tsk);
@@ -3844,8 +3905,11 @@ static int __init cgroup_disable(char *str)
         while ((token = strsep(&str, ",")) != NULL) {
                 if (!*token)
                         continue;
-
-                for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
+                /*
+                 * cgroup_disable, being at boot time, can't know about module
+                 * subsystems, so we don't worry about them.
+                 */
+                for (i = 0; i < CGROUP_BUILTIN_SUBSYS_COUNT; i++) {
                         struct cgroup_subsys *ss = subsys[i];
 
                         if (!strcmp(token, ss->name)) {