Re: [PATCH] device_cgroup: don't grab mutex in rcu callback

From: Tejun Heo
Date: Thu Feb 14 2013 - 11:48:52 EST


(cc'ing Andrew)

Andrew, can you please pick up this one?

http://article.gmane.org/gmane.linux.kernel.cgroups/6158/raw

Thank you.

On Thu, Feb 14, 2013 at 09:46:03AM -0700, Jerry Snitselaar wrote:
> commit 103a197 "security/device_cgroup: lock assert fails in
> dev_exception_clean()" grabs devcgroup_mutex to fix assert failure,
> but mutex can't be grabbed in rcu callback. Since there shouldn't be
> any other references when css_free is called, mutex isn't needed for
> list cleanup in devcgroup_css_free().
>
> Signed-off-by: Jerry Snitselaar <jerry.snitselaar@xxxxxxxxxx>
> Acked-by: Tejun Heo <tj@xxxxxxxxxx>
> Acked-by: Aristeu Rozanski <aris@xxxxxxxxxx>
> ---
> security/device_cgroup.c | 21 ++++++++++++---------
> 1 file changed, 12 insertions(+), 9 deletions(-)
>
> diff --git a/security/device_cgroup.c b/security/device_cgroup.c
> index d794abc..1c69e38 100644
> --- a/security/device_cgroup.c
> +++ b/security/device_cgroup.c
> @@ -159,6 +159,16 @@ static void dev_exception_rm(struct dev_cgroup *dev_cgroup,
> }
> }
>
> +static void __dev_exception_clean(struct dev_cgroup *dev_cgroup)
> +{
> + struct dev_exception_item *ex, *tmp;
> +
> + list_for_each_entry_safe(ex, tmp, &dev_cgroup->exceptions, list) {
> + list_del_rcu(&ex->list);
> + kfree_rcu(ex, rcu);
> + }
> +}
> +
> /**
> * dev_exception_clean - frees all entries of the exception list
> * @dev_cgroup: dev_cgroup with the exception list to be cleaned
> @@ -167,14 +177,9 @@ static void dev_exception_rm(struct dev_cgroup *dev_cgroup,
> */
> static void dev_exception_clean(struct dev_cgroup *dev_cgroup)
> {
> - struct dev_exception_item *ex, *tmp;
> -
> lockdep_assert_held(&devcgroup_mutex);
>
> - list_for_each_entry_safe(ex, tmp, &dev_cgroup->exceptions, list) {
> - list_del_rcu(&ex->list);
> - kfree_rcu(ex, rcu);
> - }
> + __dev_exception_clean(dev_cgroup);
> }
>
> /*
> @@ -215,9 +220,7 @@ static void devcgroup_css_free(struct cgroup *cgroup)
> struct dev_cgroup *dev_cgroup;
>
> dev_cgroup = cgroup_to_devcgroup(cgroup);
> - mutex_lock(&devcgroup_mutex);
> - dev_exception_clean(dev_cgroup);
> - mutex_unlock(&devcgroup_mutex);
> + __dev_exception_clean(dev_cgroup);
> kfree(dev_cgroup);
> }
>
> --
> 1.8.1.1.293.gfe73786
>

--
tejun
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