FAILED: Patch "netfilter: x_tables: Use correct memory barriers." failed to apply to 4.4-stable tree

The patch below does not apply to the 4.4-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From 175e476b8cdf2a4de7432583b49c871345e4f8a1 Mon Sep 17 00:00:00 2001
From: Mark Tomlinson <[email protected]>
Date: Mon, 8 Mar 2021 14:24:13 +1300
Subject: [PATCH] netfilter: x_tables: Use correct memory barriers.

When a new table value was assigned, it was followed by a write memory barrier. This ensured that all writes before this point would complete before any writes after this point. However, to determine whether the rules are unused, the sequence counter is read. To ensure that all writes have been done before these reads, a full memory barrier is needed, not just a write memory barrier. The same argument applies when incrementing the counter, before the rules are read.

Changing to using smp_mb() instead of smp_wmb() fixes the kernel panic reported in cc00bcaa5899 (which is still present), while still maintaining the same speed of replacing tables.

The smb_mb() barriers potentially slow the packet path, however testing has shown no measurable change in performance on a 4-core MIPS64 platform.

---
 include/linux/netfilter/x_tables.h | 2 +-
 net/netfilter/x_tables.c           | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

index 5deb099d156d..8ec48466410a 100644
@@ -376,7 +376,7 @@ static inline unsigned int xt_write_recseq_begin(void)
 	 * since addend is most likely 1
 	 */
 	__this_cpu_add(xt_recseq.sequence, addend);
-	smp_wmb();
+	smp_mb();
 
 	return addend;
 }

index 7df3aef39c5c..6bd31a7a27fc 100644
@@ -1389,7 +1389,7 @@ xt_replace_table(struct xt_table *table,
 	table->private = newinfo;
 
 	/* make sure all cpus see new ->private value */
-	smp_wmb();
+	smp_mb();
 
 	/*
 	 * Even though table entries have now been swapped, other CPU's

2.30.1