linux-kernel - Re: [RT PATCH 2/2] net: add a lock around icmp

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Date:   Wed, 31 Aug 2016 13:03:46 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, netdev@...r.kernel.org
Subject: Re: [RT PATCH 2/2] net: add a lock around icmp_sk()

On Wed, 31 Aug 2016 09:37:43 -0700
Eric Dumazet <eric.dumazet@...il.com> wrote:

> On Wed, 2016-08-31 at 18:00 +0200, Sebastian Andrzej Siewior wrote:
> > It looks like the this_cpu_ptr() access in icmp_sk() is protected with
> > local_bh_disable(). To avoid missing serialization in -RT I am adding
> > here a local lock. No crash has been observed, this is just precaution.
> >   
> 
> 
> Hmm...
> 
> > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> > ---
> >  net/ipv4/icmp.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> > 
> > diff --git a/net/ipv4/icmp.c b/net/ipv4/icmp.c
> > index c1f1d5030d37..63731fd6af3e 100644
> > --- a/net/ipv4/icmp.c
> > +++ b/net/ipv4/icmp.c
> > @@ -78,6 +78,7 @@
> >  #include <linux/string.h>
> >  #include <linux/netfilter_ipv4.h>
> >  #include <linux/slab.h>
> > +#include <linux/locallock.h>
> >  #include <net/snmp.h>
> >  #include <net/ip.h>
> >  #include <net/route.h>
> > @@ -205,6 +206,8 @@ static const struct icmp_control icmp_pointers[NR_ICMP_TYPES+1];
> >   *
> >   *	On SMP we have one ICMP socket per-cpu.
> >   */
> > +static DEFINE_LOCAL_IRQ_LOCK(icmp_sk_lock);
> > +
> >  static struct sock *icmp_sk(struct net *net)
> >  {
> >  	return *this_cpu_ptr(net->ipv4.icmp_sk);
> > @@ -216,12 +219,14 @@ static inline struct sock *icmp_xmit_lock(struct net *net)
> >  
> >  	local_bh_disable();
> >  
> > +	local_lock(icmp_sk_lock);  
> 
> Deadlock alert ?
> Please read the comment few lines after, explaining why we have to use
> spin_trylock().
> Or maybe I should double check what is local_lock() in RT

And I don't know exactly what the deadlock scenario of the comment
below is. Is it racing with a softirq somehow?

Note, in RT softirqs can schedule out, and are preemptable. But I don't
know enough about this code to know if that is enough to not have a
deadlock here.

-- Steve


> 
> >  	sk = icmp_sk(net);
> >  
> >  	if (unlikely(!spin_trylock(&sk->sk_lock.slock))) {  
> 
> ...
> >  		/* This can happen if the output path signals a
> >  		 * dst_link_failure() for an outgoing ICMP packet.
> >  		 */  
> 
> 
> > +		local_unlock(icmp_sk_lock);
> >  		local_bh_enable();
> >  		return NULL;  
>