netdev - [PATCH net-next 2/4] net: rps: annotate data-races around (struct sd_flow

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Message-ID: <20250407163602.170356-3-edumazet@google.com>
Date: Mon,  7 Apr 2025 16:36:00 +0000
From: Eric Dumazet <edumazet@...gle.com>
To: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>
Cc: Simon Horman <horms@...nel.org>, Willem de Bruijn <willemb@...gle.com>, netdev@...r.kernel.org, 
	eric.dumazet@...il.com, Eric Dumazet <edumazet@...gle.com>
Subject: [PATCH net-next 2/4] net: rps: annotate data-races around (struct sd_flow_limit)->count

softnet_seq_show() can read fl->count while another cpu
updates this field from skb_flow_limit().

Make this field an 'unsigned int', as its only consumer
only deals with 32 bit.

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 net/core/dev.c        | 3 ++-
 net/core/dev.h        | 2 +-
 net/core/net-procfs.c | 3 ++-
 3 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index f674236f34be..969883173182 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -5049,7 +5049,8 @@ static bool skb_flow_limit(struct sk_buff *skb, unsigned int qlen)
 			fl->buckets[old_flow]--;
 
 		if (++fl->buckets[new_flow] > (FLOW_LIMIT_HISTORY >> 1)) {
-			fl->count++;
+			/* Pairs with READ_ONCE() in softnet_seq_show() */
+			WRITE_ONCE(fl->count, fl->count + 1);
 			rcu_read_unlock();
 			return true;
 		}
diff --git a/net/core/dev.h b/net/core/dev.h
index b1cd44b5f009..e855e1cb43fd 100644
--- a/net/core/dev.h
+++ b/net/core/dev.h
@@ -15,7 +15,7 @@ struct cpumask;
 /* Random bits of netdevice that don't need to be exposed */
 #define FLOW_LIMIT_HISTORY	(1 << 7)  /* must be ^2 and !overflow buckets */
 struct sd_flow_limit {
-	u64			count;
+	unsigned int		count;
 	u8			log_buckets;
 	unsigned int		history_head;
 	u16			history[FLOW_LIMIT_HISTORY];
diff --git a/net/core/net-procfs.c b/net/core/net-procfs.c
index 3e92bf0f9060..69782d62fbe1 100644
--- a/net/core/net-procfs.c
+++ b/net/core/net-procfs.c
@@ -132,8 +132,9 @@ static int softnet_seq_show(struct seq_file *seq, void *v)
 
 	rcu_read_lock();
 	fl = rcu_dereference(sd->flow_limit);
+	/* Pairs with WRITE_ONCE() in skb_flow_limit() */
 	if (fl)
-		flow_limit_count = fl->count;
+		flow_limit_count = READ_ONCE(fl->count);
 	rcu_read_unlock();
 #endif
 
-- 
2.49.0.504.g3bcea36a83-goog