linux-kernel - [PATCH 1/7] netconsole: Split UDP message building and sending operations

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Message-Id: <20250902-netpoll_untangle_v3-v1-1-51a03d6411be@debian.org>
Date: Tue, 02 Sep 2025 07:36:23 -0700
From: Breno Leitao <leitao@...ian.org>
To: Andrew Lunn <andrew+netdev@...n.ch>, 
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
 Simon Horman <horms@...nel.org>, 
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>, 
 Clark Williams <clrkwllms@...nel.org>, Steven Rostedt <rostedt@...dmis.org>
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
 linux-rt-devel@...ts.linux.dev, kernel-team@...a.com, efault@....de, 
 calvin@...nvd.org, Breno Leitao <leitao@...ian.org>
Subject: [PATCH 1/7] netconsole: Split UDP message building and sending
 operations

Split the netpoll_send_udp() function into two separate operations:
netpoll_prepare_skb() for message preparation and netpoll_send_skb()
for transmission.

This improves separation of concerns. SKB building logic is now isolated
from the actual network transmission, improving code modularity and
testability.

Why?

The separation of SKB preparation and transmission operations enables
more granular locking strategies. The netconsole buffer requires lock
protection during packet construction, but the transmission phase can
proceed without holding the same lock.

Also, this makes netpoll only reponsible for handling SKB.

netpoll_prepare_skb() is now exported, but, in the upcoming change, it
will be moved to netconsole, and become static.

Signed-off-by: Breno Leitao <leitao@...ian.org>
---
 drivers/net/netconsole.c | 21 ++++++++++++++-------
 include/linux/netpoll.h  |  2 ++
 net/core/netpoll.c       |  9 +++++----
 3 files changed, 21 insertions(+), 11 deletions(-)

diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c
index 194570443493b..5b8af2de719a2 100644
--- a/drivers/net/netconsole.c
+++ b/drivers/net/netconsole.c
@@ -1492,18 +1492,25 @@ static struct notifier_block netconsole_netdev_notifier = {
  */
 static void send_udp(struct netconsole_target *nt, const char *msg, int len)
 {
-	int result = netpoll_send_udp(&nt->np, msg, len);
+	struct sk_buff *skb;
+	netdev_tx_t result;

-	if (IS_ENABLED(CONFIG_NETCONSOLE_DYNAMIC)) {
-		if (result == NET_XMIT_DROP) {
-			u64_stats_update_begin(&nt->stats.syncp);
-			u64_stats_inc(&nt->stats.xmit_drop_count);
-			u64_stats_update_end(&nt->stats.syncp);
-		} else if (result == -ENOMEM) {
+	skb = netpoll_prepare_skb(&nt->np, msg, len);
+	if (!skb) {
+		if (IS_ENABLED(CONFIG_NETCONSOLE_DYNAMIC)) {
 			u64_stats_update_begin(&nt->stats.syncp);
 			u64_stats_inc(&nt->stats.enomem_count);
 			u64_stats_update_end(&nt->stats.syncp);
 		}
+		return;
+	}
+
+	result = netpoll_send_skb(&nt->np, skb);
+
+	if (IS_ENABLED(CONFIG_NETCONSOLE_DYNAMIC) && result == NET_XMIT_DROP) {
+		u64_stats_update_begin(&nt->stats.syncp);
+		u64_stats_inc(&nt->stats.xmit_drop_count);
+		u64_stats_update_end(&nt->stats.syncp);
 	}
 }

diff --git a/include/linux/netpoll.h b/include/linux/netpoll.h
index b5ea9882eda8b..ed74889e126c7 100644
--- a/include/linux/netpoll.h
+++ b/include/linux/netpoll.h
@@ -69,6 +69,8 @@ static inline void netpoll_poll_enable(struct net_device *dev) { return; }
 #endif

 int netpoll_send_udp(struct netpoll *np, const char *msg, int len);
+struct sk_buff *netpoll_prepare_skb(struct netpoll *np, const char *msg,
+				    int len);
 int __netpoll_setup(struct netpoll *np, struct net_device *ndev);
 int netpoll_setup(struct netpoll *np);
 void __netpoll_free(struct netpoll *np);
diff --git a/net/core/netpoll.c b/net/core/netpoll.c
index 5f65b62346d4e..e2098c19987f4 100644
--- a/net/core/netpoll.c
+++ b/net/core/netpoll.c
@@ -496,7 +496,8 @@ static void push_eth(struct netpoll *np, struct sk_buff *skb)
 		eth->h_proto = htons(ETH_P_IP);
 }

-int netpoll_send_udp(struct netpoll *np, const char *msg, int len)
+struct sk_buff *netpoll_prepare_skb(struct netpoll *np, const char *msg,
+				    int len)
 {
 	int total_len, ip_len, udp_len;
 	struct sk_buff *skb;
@@ -515,7 +516,7 @@ int netpoll_send_udp(struct netpoll *np, const char *msg, int len)
 	skb = find_skb(np, total_len + np->dev->needed_tailroom,
 		       total_len - len);
 	if (!skb)
-		return -ENOMEM;
+		return NULL;

 	skb_copy_to_linear_data(skb, msg, len);
 	skb_put(skb, len);
@@ -528,9 +529,9 @@ int netpoll_send_udp(struct netpoll *np, const char *msg, int len)
 	push_eth(np, skb);
 	skb->dev = np->dev;

-	return (int)netpoll_send_skb(np, skb);
+	return skb;
 }
-EXPORT_SYMBOL(netpoll_send_udp);
+EXPORT_SYMBOL(netpoll_prepare_skb);

 static void skb_pool_flush(struct netpoll *np)

-- 
2.47.3