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Subject[PATCH 3/5] 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 in this case.

Signed-off-by: Li Zefan <lizefan@huawei.com>
---
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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