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Commit 970317aa authored by Li Zefan's avatar Li Zefan Committed by Tejun Heo
Browse files

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: default avatarLi Zefan <lizefan@huawei.com>
Signed-off-by: default avatarTejun Heo <tj@kernel.org>
parent 391acf97

kernel/cgroup.c

+23 −0
Original line number Diff line number Diff line
@@ -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;