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Revert "cgroup: remove redundant variable in cgroup_mount()"

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This reverts commit 0c7bf3e8ca.

If there are child cgroups in the cgroupfs and then we umount it,
the superblock will be destroyed but the cgroup_root will be kept
around. When we mount it again, cgroup_mount() will find this
cgroup_root and allocate a new sb for it.

So with this commit we will be trapped in a dead loop in the case
described above, because kernfs_pin_sb() keeps returning NULL.

Currently I don't see how we can avoid using both pinned_sb and
new_sb, so just revert it.

Cc: Al Viro <viro@ZenIV.linux.org.uk>
Reported-by: Andrey Wagin <avagin@gmail.com>
Signed-off-by: Zefan Li <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Zefan Li 2014-09-26 12:03:25 +08:00 committed by Tejun Heo
parent 0c7bf3e8ca
commit e756c7b698
1 changed files with 7 additions and 4 deletions
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11
kernel/cgroup.c
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@ -1694,6 +1694,7 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
struct dentry *dentry;
int ret;
int i;
bool new_sb;
/*
* The first time anyone tries to mount a cgroup, enable the list
@ -1784,7 +1785,7 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
* path is super cold. Let's just sleep a bit and retry.
*/
pinned_sb = kernfs_pin_sb(root->kf_root, NULL);
if (IS_ERR_OR_NULL(pinned_sb) ||
if (IS_ERR(pinned_sb) ||
!percpu_ref_tryget_live(&root->cgrp.self.refcnt)) {
mutex_unlock(&cgroup_mutex);
if (!IS_ERR_OR_NULL(pinned_sb))
@ -1830,16 +1831,18 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
return ERR_PTR(ret);
dentry = kernfs_mount(fs_type, flags, root->kf_root,
CGROUP_SUPER_MAGIC, NULL);
if (IS_ERR(dentry) || pinned_sb)
CGROUP_SUPER_MAGIC, &new_sb);
if (IS_ERR(dentry) || !new_sb)
cgroup_put(&root->cgrp);
/*
* If @pinned_sb, we're reusing an existing root and holding an
* extra ref on its sb. Mount is complete. Put the extra ref.
*/
if (pinned_sb)
if (pinned_sb) {
WARN_ON(new_sb);
deactivate_super(pinned_sb);
}
return dentry;
}