Merge tag 'staging-3.16-rc8' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh...

[karo-tx-linux.git] / kernel / cgroup.c

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 7868fc3c0bc59b3223490114ca8e27f83b4c864c..70776aec2562b7180ddc2a39bef55603f584ea15 100644 (file)
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1648,10 +1648,13 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
                         int flags, const char *unused_dev_name,
                         void *data)
 {
+       struct super_block *pinned_sb = NULL;
+       struct cgroup_subsys *ss;
        struct cgroup_root *root;
        struct cgroup_sb_opts opts;
        struct dentry *dentry;
        int ret;
+       int i;
        bool new_sb;
 
        /*
@@ -1677,6 +1680,27 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
                goto out_unlock;
        }
 
+       /*
+        * Destruction of cgroup root is asynchronous, so subsystems may
+        * still be dying after the previous unmount.  Let's drain the
+        * dying subsystems.  We just need to ensure that the ones
+        * unmounted previously finish dying and don't care about new ones
+        * starting.  Testing ref liveliness is good enough.
+        */
+       for_each_subsys(ss, i) {
+               if (!(opts.subsys_mask & (1 << i)) ||
+                   ss->root == &cgrp_dfl_root)
+                       continue;
+
+               if (!percpu_ref_tryget_live(&ss->root->cgrp.self.refcnt)) {
+                       mutex_unlock(&cgroup_mutex);
+                       msleep(10);
+                       ret = restart_syscall();
+                       goto out_free;
+               }
+               cgroup_put(&ss->root->cgrp);
+       }
+
        for_each_root(root) {
                bool name_match = false;
 
@@ -1717,15 +1741,23 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
                }
 
                /*
-                * A root's lifetime is governed by its root cgroup.
-                * tryget_live failure indicate that the root is being
-                * destroyed.  Wait for destruction to complete so that the
-                * subsystems are free.  We can use wait_queue for the wait
-                * but this path is super cold.  Let's just sleep for a bit
-                * and retry.
+                * We want to reuse @root whose lifetime is governed by its
+                * ->cgrp.  Let's check whether @root is alive and keep it
+                * that way.  As cgroup_kill_sb() can happen anytime, we
+                * want to block it by pinning the sb so that @root doesn't
+                * get killed before mount is complete.
+                *
+                * With the sb pinned, tryget_live can reliably indicate
+                * whether @root can be reused.  If it's being killed,
+                * drain it.  We can use wait_queue for the wait but this
+                * path is super cold.  Let's just sleep a bit and retry.
                 */
-               if (!percpu_ref_tryget_live(&root->cgrp.self.refcnt)) {
+               pinned_sb = kernfs_pin_sb(root->kf_root, NULL);
+               if (IS_ERR(pinned_sb) ||
+                   !percpu_ref_tryget_live(&root->cgrp.self.refcnt)) {
                        mutex_unlock(&cgroup_mutex);
+                       if (!IS_ERR_OR_NULL(pinned_sb))
+                               deactivate_super(pinned_sb);
                        msleep(10);
                        ret = restart_syscall();
                        goto out_free;
@@ -1770,6 +1802,16 @@ out_free:
                                CGROUP_SUPER_MAGIC, &new_sb);
        if (IS_ERR(dentry) || !new_sb)
                cgroup_put(&root->cgrp);
+
+       /*
+        * If @pinned_sb, we're reusing an existing root and holding an
+        * extra ref on its sb.  Mount is complete.  Put the extra ref.
+        */
+       if (pinned_sb) {
+               WARN_ON(new_sb);
+               deactivate_super(pinned_sb);
+       }
+
        return dentry;
 }
 
@@ -3328,7 +3370,7 @@ bool css_has_online_children(struct cgroup_subsys_state *css)
 
        rcu_read_lock();
        css_for_each_child(child, css) {
-               if (css->flags & CSS_ONLINE) {
+               if (child->flags & CSS_ONLINE) {
                        ret = true;
                        break;
                }