[PATCH 3/5] cgroup: fix mount failure in a corner case

[Li Zefan]

  # 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 in this case.

Signed-off-by: Li Zefan <lizefan@xxxxxxxxxx>
---
 kernel/cgroup.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 1c65f24..bd37e8d 100644
--- 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 +1679,22 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
 		goto out_unlock;
 	}
 
+	/*
+	 * If some subsystems have been bound to existing cgroup hierarchies,
+	 * but those hierachies are being destroyed, let's wait a little bit
+	 * and retry.
+	 */
+	for_each_subsys(ss, i) {
+		if (!(opts.subsys_mask & (1 << i)))
+			continue;
+		if (!percpu_ref_alive(&ss->root->cgrp.self.refcnt)) {
+			mutex_unlock(&cgroup_mutex);
+			msleep(10);
+			ret = restart_syscall();
+			goto out_free;
+		}
+	}
+
 	for_each_root(root) {
 		bool name_match = false;
 
-- 
1.8.0.2

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