net: add CONFIG_PCPU_DEV_REFCNT

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.

v2: free_dev label in alloc_netdev_mqs() is moved to avoid
    a compiler warning (-Wunused-label), as reported
    by kernel test robot <lkp@intel.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

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

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

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;
@@ -10740,8 +10746,10 @@ 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);
 free_dev:
+#endif
 	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;