Subject: Re: [PATCH net-next v2] net: allow to call netif_reset_xps_queues() under cpus_read_lock

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 10 Aug 2018 01:13:14 +0300

On Wed, Aug 08, 2018 at 08:07:35PM -0700, Andrei Vagin wrote:
> From: Andrei Vagin <[email-masked]>
> 
> 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().
> 
> ============================================
> 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()

FYI you change log should go after -- below, not before it.

> Cc: "Nambiar, Amritha" <[email-masked]>
> Cc: "Michael S. Tsirkin" <[email-masked]>
> Cc: Jason Wang <[email-masked]>
> Fixes: 8af2c06ff4b1 ("net-sysfs: Add interface for Rx queue(s) map per Tx queue")
> 
> Signed-off-by: Andrei Vagin <[email-masked]>

Acked-by: Michael S. Tsirkin <[email-masked]>

> ---
>  drivers/net/virtio_net.c |  4 +++-
>  net/core/dev.c           | 20 +++++++++++++++-----
>  net/core/net-sysfs.c     |  4 ++++
>  3 files changed, 22 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 62311dde6e71..39a7f4452587 100644
> --- a/drivers/net/virtio_net.c
> +++ b/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++;
>  	}
>  
> diff --git a/net/core/dev.c b/net/core/dev.c
> index f68122f0ab02..325fc5088370 100644
> --- a/net/core/dev.c
> +++ b/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 @@ int __netif_set_xps_queue(struct net_device *dev, const unsigned long *mask,
>  	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);
>  
> diff --git a/net/core/net-sysfs.c b/net/core/net-sysfs.c
> index 0a95bcf64cdc..bd67c4d0fcfd 100644
> --- a/net/core/net-sysfs.c
> +++ b/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;
>  }
> -- 
> 2.17.1