net: allow to call netif_reset_xps_queues() under cpus_read_lock

The definition of static_key_slow_inc() has cpus_read_lock in place. In the
virtio_net driver, XPS queues are initialized after setting the queue:cpu
affinity in virtnet_set_affinity() which is already protected within
cpus_read_lock. Lockdep prints a warning when we are trying to acquire
cpus_read_lock when it is already held.

This patch adds an ability to call __netif_set_xps_queue under
cpus_read_lock().
Acked-by: Jason Wang <jasowang@redhat.com>

============================================
WARNING: possible recursive locking detected
4.18.0-rc3-next-20180703+ #1 Not tainted
--------------------------------------------
swapper/0/1 is trying to acquire lock:
00000000cf973d46 (cpu_hotplug_lock.rw_sem){++++}, at: static_key_slow_inc+0xe/0x20

but task is already holding lock:
00000000cf973d46 (cpu_hotplug_lock.rw_sem){++++}, at: init_vqs+0x513/0x5a0

other info that might help us debug this:
 Possible unsafe locking scenario:

       CPU0
       ----
  lock(cpu_hotplug_lock.rw_sem);
  lock(cpu_hotplug_lock.rw_sem);

 *** DEADLOCK ***

 May be due to missing lock nesting notation

3 locks held by swapper/0/1:
 #0: 00000000244bc7da (&dev->mutex){....}, at: __driver_attach+0x5a/0x110
 #1: 00000000cf973d46 (cpu_hotplug_lock.rw_sem){++++}, at: init_vqs+0x513/0x5a0
 #2: 000000005cd8463f (xps_map_mutex){+.+.}, at: __netif_set_xps_queue+0x8d/0xc60

v2: move cpus_read_lock() out of __netif_set_xps_queue()

Cc: "Nambiar, Amritha" <amritha.nambiar@intel.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Fixes: 8af2c06ff4b1 ("net-sysfs: Add interface for Rx queue(s) map per Tx queue")

Signed-off-by: Andrei Vagin <avagin@gmail.com>

Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/virtio_net.c

@@ -1903,9 +1903,11 @@ static void virtnet_set_affinity(struct virtnet_info *vi)
 
 	i = 0;
 	for_each_online_cpu(cpu) {
+		const unsigned long *mask = cpumask_bits(cpumask_of(cpu));
+
 		virtqueue_set_affinity(vi->rq[i].vq, cpu);
 		virtqueue_set_affinity(vi->sq[i].vq, cpu);
-		netif_set_xps_queue(vi->dev, cpumask_of(cpu), i);
+		__netif_set_xps_queue(vi->dev, mask, i, false);
 		i++;
 	}
 

net/core/dev.c

@@ -2176,6 +2176,7 @@ static void netif_reset_xps_queues(struct net_device *dev, u16 offset,
 	if (!static_key_false(&xps_needed))
 		return;
 
+	cpus_read_lock();
 	mutex_lock(&xps_map_mutex);
 
 	if (static_key_false(&xps_rxqs_needed)) {
@@ -2199,10 +2200,11 @@ static void netif_reset_xps_queues(struct net_device *dev, u16 offset,
 
 out_no_maps:
 	if (static_key_enabled(&xps_rxqs_needed))
-		static_key_slow_dec(&xps_rxqs_needed);
+		static_key_slow_dec_cpuslocked(&xps_rxqs_needed);
 
-	static_key_slow_dec(&xps_needed);
+	static_key_slow_dec_cpuslocked(&xps_needed);
 	mutex_unlock(&xps_map_mutex);
+	cpus_read_unlock();
 }
 
 static void netif_reset_xps_queues_gt(struct net_device *dev, u16 index)
@@ -2250,6 +2252,7 @@ static struct xps_map *expand_xps_map(struct xps_map *map, int attr_index,
 	return new_map;
 }
 
+/* Must be called under cpus_read_lock */
 int __netif_set_xps_queue(struct net_device *dev, const unsigned long *mask,
 			  u16 index, bool is_rxqs_map)
 {
@@ -2317,9 +2320,9 @@ int __netif_set_xps_queue(struct net_device *dev, const unsigned long *mask,
 	if (!new_dev_maps)
 		goto out_no_new_maps;
 
-	static_key_slow_inc(&xps_needed);
+	static_key_slow_inc_cpuslocked(&xps_needed);
 	if (is_rxqs_map)
-		static_key_slow_inc(&xps_rxqs_needed);
+		static_key_slow_inc_cpuslocked(&xps_rxqs_needed);
 
 	for (j = -1; j = netif_attrmask_next(j, possible_mask, nr_ids),
 	     j < nr_ids;) {
@@ -2448,11 +2451,18 @@ error:
 	kfree(new_dev_maps);
 	return -ENOMEM;
 }
+EXPORT_SYMBOL_GPL(__netif_set_xps_queue);
 
 int netif_set_xps_queue(struct net_device *dev, const struct cpumask *mask,
 			u16 index)
 {
-	return __netif_set_xps_queue(dev, cpumask_bits(mask), index, false);
+	int ret;
+
+	cpus_read_lock();
+	ret =  __netif_set_xps_queue(dev, cpumask_bits(mask), index, false);
+	cpus_read_unlock();
+
+	return ret;
 }
 EXPORT_SYMBOL(netif_set_xps_queue);
 

net/core/net-sysfs.c

@@ -26,6 +26,7 @@
 #include <linux/pm_runtime.h>
 #include <linux/of.h>
 #include <linux/of_net.h>
+#include <linux/cpu.h>
 
 #include "net-sysfs.h"
 
@@ -1400,7 +1401,10 @@ static ssize_t xps_rxqs_store(struct netdev_queue *queue, const char *buf,
 		return err;
 	}
 
+	cpus_read_lock();
 	err = __netif_set_xps_queue(dev, mask, index, true);
+	cpus_read_unlock();
+
 	kfree(mask);
 	return err ? : len;
 }