netdev - [PATCH net-next] net: add CONFIG_PCPU_DEV

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Date:   Fri, 19 Mar 2021 08:02:32 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: [PATCH net-next] net: add CONFIG_PCPU_DEV_REFCNT

From: Eric Dumazet <edumazet@...gle.com>

I was working on a syzbot issue, claiming one device could not be
dismantled because its refcount was -1

unregister_netdevice: waiting for sit0 to become free. Usage count = -1

It would be nice if syzbot could trigger a warning at the time
this reference count became negative.

This patch adds CONFIG_PCPU_DEV_REFCNT options which defaults
to per cpu variables (as before this patch) on SMP builds.

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 include/linux/netdevice.h | 13 +++++++++++++
 net/Kconfig               |  8 ++++++++
 net/core/dev.c            | 10 ++++++++++
 3 files changed, 31 insertions(+)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 4940509999beeb93ca4ad214e65d68e622a484cb..8f003955c485b81210ed56f7e1c24080b4bb46eb 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -2092,7 +2092,12 @@ struct net_device {
 	u32                     proto_down_reason;
 
 	struct list_head	todo_list;
+
+#ifdef CONFIG_PCPU_DEV_REFCNT
 	int __percpu		*pcpu_refcnt;
+#else
+	refcount_t		dev_refcnt;
+#endif
 
 	struct list_head	link_watch_list;
 
@@ -4044,7 +4049,11 @@ void netdev_run_todo(void);
  */
 static inline void dev_put(struct net_device *dev)
 {
+#ifdef CONFIG_PCPU_DEV_REFCNT
 	this_cpu_dec(*dev->pcpu_refcnt);
+#else
+	refcount_dec(&dev->dev_refcnt);
+#endif
 }
 
 /**
@@ -4055,7 +4064,11 @@ static inline void dev_put(struct net_device *dev)
  */
 static inline void dev_hold(struct net_device *dev)
 {
+#ifdef CONFIG_PCPU_DEV_REFCNT
 	this_cpu_inc(*dev->pcpu_refcnt);
+#else
+	refcount_inc(&dev->dev_refcnt);
+#endif
 }
 
 /* Carrier loss detection, dial on demand. The functions netif_carrier_on
diff --git a/net/Kconfig b/net/Kconfig
index 0ead7ec0d2bd9ccbbfeab32f1892a9afd2a3eb77..9c456acc379e78caa9e45d4f1335802a05663a0f 100644
--- a/net/Kconfig
+++ b/net/Kconfig
@@ -245,6 +245,14 @@ source "net/l3mdev/Kconfig"
 source "net/qrtr/Kconfig"
 source "net/ncsi/Kconfig"
 
+config PCPU_DEV_REFCNT
+	bool "Use percpu variables to maintain network device refcount"
+	depends on SMP
+	default y
+	help
+	  network device refcount are using per cpu variables if this option is set.
+	  This can be forced to N to detect underflows (with a performance drop).
+
 config RPS
 	bool
 	depends on SMP && SYSFS
diff --git a/net/core/dev.c b/net/core/dev.c
index 4961fc2e9b1967c0d63b53cd809d52b43be0ed4b..edde830df1a483535372014034d5b1ee1ff6210a 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -10312,11 +10312,15 @@ EXPORT_SYMBOL(register_netdev);
 
 int netdev_refcnt_read(const struct net_device *dev)
 {
+#ifdef CONFIG_PCPU_DEV_REFCNT
 	int i, refcnt = 0;
 
 	for_each_possible_cpu(i)
 		refcnt += *per_cpu_ptr(dev->pcpu_refcnt, i);
 	return refcnt;
+#else
+	return refcount_read(&dev->dev_refcnt);
+#endif
 }
 EXPORT_SYMBOL(netdev_refcnt_read);
 
@@ -10674,9 +10678,11 @@ struct net_device *alloc_netdev_mqs(int sizeof_priv, const char *name,
 	dev = PTR_ALIGN(p, NETDEV_ALIGN);
 	dev->padded = (char *)dev - (char *)p;
 
+#ifdef CONFIG_PCPU_DEV_REFCNT
 	dev->pcpu_refcnt = alloc_percpu(int);
 	if (!dev->pcpu_refcnt)
 		goto free_dev;
+#endif
 
 	if (dev_addr_init(dev))
 		goto free_pcpu;
@@ -10740,7 +10746,9 @@ struct net_device *alloc_netdev_mqs(int sizeof_priv, const char *name,
 	return NULL;
 
 free_pcpu:
+#ifdef CONFIG_PCPU_DEV_REFCNT
 	free_percpu(dev->pcpu_refcnt);
+#endif
 free_dev:
 	netdev_freemem(dev);
 	return NULL;
@@ -10783,8 +10791,10 @@ void free_netdev(struct net_device *dev)
 	list_for_each_entry_safe(p, n, &dev->napi_list, dev_list)
 		netif_napi_del(p);
 
+#ifdef CONFIG_PCPU_DEV_REFCNT
 	free_percpu(dev->pcpu_refcnt);
 	dev->pcpu_refcnt = NULL;
+#endif
 	free_percpu(dev->xdp_bulkq);
 	dev->xdp_bulkq = NULL;
 
-- 
2.31.0.291.g576ba9dcdaf-goog