Re: [RT PATCH 2/2] net: add a lock around icmp_sk()

On Wed, 31 Aug 2016 09:37:43 -0700
Eric Dumazet [off-list ref] wrote:

On Wed, 2016-08-31 at 18:00 +0200, Sebastian Andrzej Siewior wrote:

quoted

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...

quoted

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.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

quoted

 	sk = icmp_sk(net);
 
 	if (unlikely(!spin_trylock(&sk->sk_lock.slock))) {  

...

quoted

 		/* This can happen if the output path signals a
 		 * dst_link_failure() for an outgoing ICMP packet.
 		 */  

quoted

+		local_unlock(icmp_sk_lock);
 		local_bh_enable();
 		return NULL;