netdev - Re: [PATCH v1 3/5] cgroup: Protect access to task_cls

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Date:	Mon, 20 Aug 2012 07:08:19 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Li Zefan <lizefan@...wei.com>
Cc:	Daniel Wagner <wagi@...om.org>, netdev@...r.kernel.org,
	cgroups@...r.kernel.org,
	Daniel Wagner <daniel.wagner@...-carit.de>,
	"David S. Miller" <davem@...emloft.net>,
	Gao feng <gaofeng@...fujitsu.com>,
	Jamal Hadi Salim <jhs@...atatu.com>,
	John Fastabend <john.r.fastabend@...el.com>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH v1 3/5] cgroup: Protect access to task_cls_classid() when
 built as module

On Mon, Aug 20, 2012 at 08:59:37AM +0800, Li Zefan wrote:
> 于 2012/8/18 2:28, Neil Horman 写道:
> > On Fri, Aug 17, 2012 at 04:58:12PM +0200, Daniel Wagner wrote:
> >> From: Daniel Wagner <daniel.wagner@...-carit.de>
> >>
> >> The module version of task_cls_classid() checks if net_cls_sbusys_id
> >> is valid to indentify when it is okay to access the controller.
> >>
> >> Instead relying on the subusys_id to be set, make it explicit
> >> with a jump label.
> >>
> >> Signed-off-by: Daniel Wagner <daniel.wagner@...-carit.de>
> >> Cc: "David S. Miller" <davem@...emloft.net>
> >> Cc: Gao feng <gaofeng@...fujitsu.com>
> >> Cc: Jamal Hadi Salim <jhs@...atatu.com>
> >> Cc: John Fastabend <john.r.fastabend@...el.com>
> >> Cc: Li Zefan <lizefan@...wei.com>
> >> Cc: Neil Horman <nhorman@...driver.com>
> >> Cc: Tejun Heo <tj@...nel.org>
> >> Cc: netdev@...r.kernel.org
> >> Cc: cgroups@...r.kernel.org
> >> ---
> >>  include/net/cls_cgroup.h | 5 ++++-
> >>  net/core/sock.c          | 5 +++++
> >>  net/sched/cls_cgroup.c   | 9 +++++++++
> >>  3 files changed, 18 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/include/net/cls_cgroup.h b/include/net/cls_cgroup.h
> >> index 401672c..bbbd957 100644
> >> --- a/include/net/cls_cgroup.h
> >> +++ b/include/net/cls_cgroup.h
> >> @@ -16,6 +16,7 @@
> >>  #include <linux/cgroup.h>
> >>  #include <linux/hardirq.h>
> >>  #include <linux/rcupdate.h>
> >> +#include <linux/jump_label.h>
> >>  
> >>  #ifdef CONFIG_CGROUPS
> >>  struct cgroup_cls_state
> >> @@ -44,6 +45,8 @@ static inline u32 task_cls_classid(struct task_struct *p)
> >>  }
> >>  
> >>  #elif IS_MODULE(CONFIG_NET_CLS_CGROUP)
> >> +extern struct static_key cgroup_cls_enabled;
> >> +#define clscg_enabled static_key_false(&cgroup_cls_enabled)
> >>  
> >>  extern int net_cls_subsys_id;
> >>  
> >> @@ -52,7 +55,7 @@ static inline u32 task_cls_classid(struct task_struct *p)
> >>  	int id;
> >>  	u32 classid = 0;
> >>  
> >> -	if (in_interrupt())
> >> +	if (!clscg_enabled || in_interrupt())
> >>  		return 0;
> >>  
> >>  	rcu_read_lock();
> >> diff --git a/net/core/sock.c b/net/core/sock.c
> >> index 8f67ced..8106e77 100644
> >> --- a/net/core/sock.c
> >> +++ b/net/core/sock.c
> >> @@ -327,6 +327,11 @@ int __sk_backlog_rcv(struct sock *sk, struct sk_buff *skb)
> >>  EXPORT_SYMBOL(__sk_backlog_rcv);
> >>  
> >>  #if defined(CONFIG_CGROUPS)
> >> +#if IS_MODULE(CONFIG_NET_CLS_CGROUP)
> >> +struct static_key cgroup_cls_enabled = STATIC_KEY_INIT_FALSE;
> >> +EXPORT_SYMBOL_GPL(cgroup_cls_enabled);
> >> +#endif
> >> +
> >>  #if !defined(CONFIG_NET_CLS_CGROUP)
> >>  int net_cls_subsys_id = -1;
> >>  EXPORT_SYMBOL_GPL(net_cls_subsys_id);
> >> diff --git a/net/sched/cls_cgroup.c b/net/sched/cls_cgroup.c
> >> index 7743ea8..0635894 100644
> >> --- a/net/sched/cls_cgroup.c
> >> +++ b/net/sched/cls_cgroup.c
> >> @@ -44,12 +44,21 @@ static struct cgroup_subsys_state *cgrp_create(struct cgroup *cgrp)
> >>  
> >>  	if (cgrp->parent)
> >>  		cs->classid = cgrp_cls_state(cgrp->parent)->classid;
> >> +#if IS_MODULE(CONFIG_NET_CLS_CGROUP)
> >> +	else if (!clscg_enabled)
> >> +                static_key_slow_inc(&cgroup_cls_enabled);
> > This is racy I think.  The read of the static key is atomic with other reads,
> > but the entire conditional is not atomic.  If two cpus were creating cgroups in
> > parallel, it would be possible for both to read the static key as being zero
> > (the second cpu would read the key before the first cpu could increment it).
> 
> static_key_slow_inc() is called only when we're creating the root cgroup, and that's
> module loading.
> 
> so it's safe.
> 
What makes you say that?  It appears to me that we call ss->create (and
potential ss->destroy, depending on failure conditions) from cgroup_create,
which in turn is called from cgroup_mkdir, which is called for every cgroup
instance created, not just the root cgroup.

Neil

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