Subject: net: netfilter: Serialize xt_write_recseq sections on RT
From: Thomas Gleixner <tglx@linutronix.de>
Date: Sun, 28 Oct 2012 11:18:08 +0100

The netfilter code relies only on the implicit semantics of
local_bh_disable() for serializing wt_write_recseq sections. RT breaks
that and needs explicit serialization here.

Reported-by: Peter LaDow <petela@gocougs.wsu.edu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

---
 include/linux/netfilter/x_tables.h |    7 +++++++
 net/netfilter/core.c               |    6 ++++++
 2 files changed, 13 insertions(+)

--- a/include/linux/netfilter/x_tables.h
+++ b/include/linux/netfilter/x_tables.h
@@ -3,6 +3,7 @@
 
 
 #include <linux/netdevice.h>
+#include <linux/locallock.h>
 #include <uapi/linux/netfilter/x_tables.h>
 
 /**
@@ -282,6 +283,8 @@ void xt_free_table_info(struct xt_table_
  */
 DECLARE_PER_CPU(seqcount_t, xt_recseq);
 
+DECLARE_LOCAL_IRQ_LOCK(xt_write_lock);
+
 /**
  * xt_write_recseq_begin - start of a write section
  *
@@ -296,6 +299,9 @@ static inline unsigned int xt_write_recs
 {
 	unsigned int addend;
 
+	/* RT protection */
+	local_lock(xt_write_lock);
+
 	/*
 	 * Low order bit of sequence is set if we already
 	 * called xt_write_recseq_begin().
@@ -326,6 +332,7 @@ static inline void xt_write_recseq_end(u
 	/* this is kind of a write_seqcount_end(), but addend is 0 or 1 */
 	smp_wmb();
 	__this_cpu_add(xt_recseq.sequence, addend);
+	local_unlock(xt_write_lock);
 }
 
 /*
--- a/net/netfilter/core.c
+++ b/net/netfilter/core.c
@@ -22,11 +22,17 @@
 #include <linux/proc_fs.h>
 #include <linux/mutex.h>
 #include <linux/slab.h>
+#include <linux/locallock.h>
 #include <net/net_namespace.h>
 #include <net/sock.h>
 
 #include "nf_internals.h"
 
+#ifdef CONFIG_PREEMPT_RT_BASE
+DEFINE_LOCAL_IRQ_LOCK(xt_write_lock);
+EXPORT_PER_CPU_SYMBOL(xt_write_lock);
+#endif
+
 static DEFINE_MUTEX(afinfo_mutex);
 
 const struct nf_afinfo __rcu *nf_afinfo[NFPROTO_NUMPROTO] __read_mostly;