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SubjectRe: [PATCH] cgroup: Fix root_count when mount fails due to busy subsystem

* Paul Menage <menage@google.com> wrote:

> cgroup: Fix root_count when mount fails due to busy subsystem
>
> root_count was being incremented in cgroup_get_sb() after all error
> checking was complete, but decremented in cgroup_kill_sb(), which can be
> called on a superblock that we gave up on due to an error. This patch
> changes cgroup_kill_sb() to only decrement root_count if the root was
> previously linked into the list of roots.
>
> Signed-off-by: Paul Menage <menage@google.com>
>
> ---
>
> I was actually surprised to find that list_del() doesn't crash when
> run on an unattached list_head structure.
>
> kernel/cgroup.c | 6 ++++--
> 1 files changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index adcd0bb..9ce27e8 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -1115,8 +1115,10 @@ static void cgroup_kill_sb(struct super_block *sb) {
> }
> write_unlock(&css_set_lock);
>
> - list_del(&root->root_list);
> - root_count--;
> + if (!list_empty(&root->root_list)) {
> + list_del(&root->root_list);
> + root_count--;
> + }

That's ugly. It is _much_ cleaner to always keep the link head consistent
- i.e. initialize it with INIT_LIST_HEAD() and then remove from it via
list_del_init().

That way the error path will do the right thing automatically, and there's
no need for that ugly "if !list_empty" construct either.

Ingo


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