[PATCH net-next] net: add in_softirq() debug checking in napi_consume_skb()

[Yunsheng Lin]

The current semantic for napi_consume_skb() is that caller need
to provide non-zero budget when calling from NAPI context, and
breaking this semantic will cause hard to debug problem, because
_kfree_skb_defer() need to run in atomic context in order to push
the skb to the particular cpu' napi_alloc_cache atomically.

So add a in_softirq() debug checking in napi_consume_skb() to catch
this kind of error.

Suggested-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Signed-off-by: Yunsheng Lin <linyunsheng@xxxxxxxxxx>
---
v1: drop RFC in the title
---
 include/linux/netdevice.h | 6 ++++++
 net/Kconfig               | 7 +++++++
 net/core/skbuff.c         | 4 ++++
 3 files changed, 17 insertions(+)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 964b494..8042bf1 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -5158,6 +5158,12 @@ do {								\
 })
 #endif
 
+#if defined(CONFIG_DEBUG_NET)
+#define DEBUG_NET_WARN(condition, ...)	WARN(condition, ##__VA_ARGS__)
+#else
+#define DEBUG_NET_WARN(condition, ...)
+#endif
+
 /*
  *	The list of packet types we will receive (as opposed to discard)
  *	and the routines to invoke.
diff --git a/net/Kconfig b/net/Kconfig
index d656716..82e69b0 100644
--- a/net/Kconfig
+++ b/net/Kconfig
@@ -459,6 +459,13 @@ config ETHTOOL_NETLINK
 	  netlink. It provides better extensibility and some new features,
 	  e.g. notification messages.
 
+config DEBUG_NET
+	bool "Net debugging and diagnostics"
+	depends on DEBUG_KERNEL
+	default n
+	help
+	  Say Y here to add some extra checks and diagnostics to networking.
+
 endif   # if NET
 
 # Used by archs to tell that they support BPF JIT compiler plus which flavour.
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 1ba8f01..1834007 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -897,6 +897,10 @@ void napi_consume_skb(struct sk_buff *skb, int budget)
 		return;
 	}
 
+	DEBUG_NET_WARN(!in_softirq(),
+		       "%s is called with non-zero budget outside softirq context.\n",
+		       __func__);
+
 	if (!skb_unref(skb))
 		return;
 
-- 
2.8.1