[PATCH 3.15 68/84] cgroup: fix mount failure in a corner case

Greg Kroah-Hartman
2014-07-15T23:32:00Z

From: Li Zefan <lizefan@huawei.com>

commit 970317aa48c6ef66cd023c039c2650c897bad927 upstream.

# 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.

Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
[lizf: Backported to 3.15:
- s/percpu_ref_tryget_live/atomic_inc_not_zero/
- Use goto instead of calling restart_syscall()
- Add cgroup_tree_mutex]
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
kernel/cgroup.c | 25 +++++++++++++++++++++++++
1 file changed, 25 insertions(+)

--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1484,10 +1484,12 @@ static struct dentry *cgroup_mount(struc
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;

/*
@@ -1514,6 +1516,29 @@ retry:
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 (!atomic_inc_not_zero(&ss->root->cgrp.refcnt)) {
+ mutex_unlock(&cgroup_mutex);
+ mutex_unlock(&cgroup_tree_mutex);
+ msleep(10);
+ mutex_lock(&cgroup_tree_mutex);
+ mutex_lock(&cgroup_mutex);
+ goto retry;
+ }
+ cgroup_put(&ss->root->cgrp);
+ }
+
for_each_root(root) {
bool name_match = false;