linux-kernel - Re: [PATCH v2 1/3] cgroup: fix mount failure in a corner case

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Date:	Fri, 27 Jun 2014 16:16:18 +0800
From:	Li Zefan <lizefan@...wei.com>
To:	Tejun Heo <tj@...nel.org>
CC:	LKML <linux-kernel@...r.kernel.org>,
	Cgroups <cgroups@...r.kernel.org>
Subject: Re: [PATCH v2 1/3] cgroup: fix mount failure in a corner case

Oh sorry the cut&paste was incomplete. Here's the complete one:

================

From: Li Zefan <lizefan@...wei.com>
Date: Thu, 12 Jun 2014 09:11:00 +0800
Subject: [PATCH v2 1/3] 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.

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

Signed-off-by: Li Zefan <lizefan@...wei.com>
---
 kernel/cgroup.c | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 1c65f24..6826227 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, j = -1;
 	bool new_sb;
 
 	/*
@@ -1677,6 +1679,25 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
 		goto out_unlock;
 	}
 
+	/*
+	 * Destruction of cgroup root is asynchronous, so we may fail to
+	 * mount a cgroupfs if it immediately follows a umount. Let's wait
+	 * a little bit and retry.
+	 */
+	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;
+		}
+		j = i;
+	}
+
 	for_each_root(root) {
 		bool name_match = false;
 
@@ -1763,6 +1784,14 @@ out_free:
 	kfree(opts.release_agent);
 	kfree(opts.name);
 
+	for_each_subsys(ss, i) {
+		if (i > j)
+			break;
+		if (!(opts.subsys_mask & (1 << i)))
+			continue;
+		cgroup_put(&ss->root->cgrp);
+	}
+
 	if (ret)
 		return ERR_PTR(ret);
 
-- 
1.8.0.2


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