cgroup: fix mount failure in a corner case

  # cat test.sh
  #! /bin/bash

  mount -t cgroup -o cpu xxx /cgroup
  umount /cgroup

  mount -t cgroup -o cpu,cpuacct xxx /cgroup
  umount /cgroup
  # ./test.sh
  mount: xxx already mounted or /cgroup busy
  mount: according to mtab, xxx is already mounted on /cgroup

It's because the cgroupfs_root of the first mount was under destruction
asynchronously.

Fix this by delaying and then retrying mount for this case.

v3:
- put the refcnt immediately after getting it. (Tejun)

v2:
- use percpu_ref_tryget_live() rather that introducing
  percpu_ref_alive(). (Tejun)
- adjust comment.

tj: Updated the comment a bit.

Cc: <stable@vger.kernel.org> # 3.15
Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index d9a8be9..6406866 100644 (file)
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1648,10 +1648,12 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
                         int flags, const char *unused_dev_name,
                         void *data)
 {
+       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 +1679,27 @@ 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;