linux-kernel - Re: [PATCH net-next 2/2] net sched act_vlan: VLAN action rewrite to use RCU lock/unlock and update

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Date:   Wed, 11 Oct 2017 06:30:03 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Manish Kurup <kurup.manish@...il.com>
Cc:     jhs@...atatu.com, xiyou.wangcong@...il.com, jiri@...nulli.us,
        davem@...emloft.net, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, aring@...atatu.com, mrv@...atatu.com,
        manish.kurup@...izon.com
Subject: Re: [PATCH net-next 2/2] net sched act_vlan: VLAN action rewrite to
 use RCU lock/unlock and update

On Wed, 2017-10-11 at 06:10 -0700, Eric Dumazet wrote:
> On Tue, 2017-10-10 at 22:33 -0400, Manish Kurup wrote:
> > Using a spinlock in the VLAN action causes performance issues when the VLAN
> > action is used on multiple cores. Rewrote the VLAN action to use RCU read
> > locking for reads and updates instead.
> > 
> > Signed-off-by: Manish Kurup <manish.kurup@...izon.com>
> > ---
> >  include/net/tc_act/tc_vlan.h | 21 ++++++++-----
> >  net/sched/act_vlan.c         | 73 ++++++++++++++++++++++++++++++--------------
> >  2 files changed, 63 insertions(+), 31 deletions(-)
> > 
> > diff --git a/include/net/tc_act/tc_vlan.h b/include/net/tc_act/tc_vlan.h
> > index c2090df..67fd355 100644
> > --- a/include/net/tc_act/tc_vlan.h
> > +++ b/include/net/tc_act/tc_vlan.h
> > @@ -13,12 +13,17 @@
> >  #include <net/act_api.h>
> >  #include <linux/tc_act/tc_vlan.h>
> >  
> > +struct tcf_vlan_params {
> > +	struct rcu_head   rcu;
> > +	int               tcfv_action;
> > +	u16               tcfv_push_vid;
> > +	__be16            tcfv_push_proto;
> > +	u8                tcfv_push_prio;
> > +};
> > +
> >  struct tcf_vlan {
> >  	struct tc_action	common;
> > -	int			tcfv_action;
> > -	u16			tcfv_push_vid;
> > -	__be16			tcfv_push_proto;
> > -	u8			tcfv_push_prio;
> > +	struct tcf_vlan_params __rcu *vlan_p;
> >  };
> >  #define to_vlan(a) ((struct tcf_vlan *)a)
> >  
> > @@ -33,22 +38,22 @@ static inline bool is_tcf_vlan(const struct tc_action *a)
> >  
> >  static inline u32 tcf_vlan_action(const struct tc_action *a)
> >  {
> > -	return to_vlan(a)->tcfv_action;
> > +	return to_vlan(a)->vlan_p->tcfv_action;
> 
> This is not proper RCU : ->vlan_p should be read once, and using
> rcu_dereference()
> 
> So the caller should provide to this helper the 'p' pointer instead of
> 'a'
> 
> 
> CONFIG_SPARSE_RCU_POINTER=y
> 
> and make C=2 would probably give you warnings about that.
> 


BTW no need to change your .config after :


commit 41a2901e7d220875752a8c870e0b53288a578c20
Author: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Date:   Fri May 12 15:56:35 2017 -0700

    rcu: Remove SPARSE_RCU_POINTER Kconfig option
    
    The sparse-based checking for non-RCU accesses to RCU-protected pointers
    has been around for a very long time, and it is now the only type of
    sparse-based checking that is optional.  This commit therefore makes
    it unconditional.
    
    Reported-by: Ingo Molnar <mingo@...nel.org>
    Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
    Cc: Fengguang Wu <fengguang.wu@...el.com>