[PATCHv1 net 0/2] xen-netfront: multi-queue related locking fixes

[PATCHv1 net 0/2] xen-netfront: multi-queue related locking fixes

[David Vrabel]

Two fixes to the per-queue locking bugs in xen-netfront that were
introduced in 3.16-rc1 with the multi-queue support.

David

[PATCH 1/2] xen-netfront: don't nest queue locks in xennet_connect()

[David Vrabel]

The nesting of the per-queue rx_lock and tx_lock in xennet_connect()
is confusing to both humans and lockdep.  The locking is safe because
this is the only place where the locks are nested in this way but
lockdep still warns.

Instead of adding the missing lockdep annotations, refactor the
locking to avoid the confusing nesting.  This is still safe, because
the xenbus connection state changes are all serialized by the xenwatch
thread.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reported-by: Sander Eikelenboom <linux@eikelenboom.it>
---
 drivers/net/xen-netfront.c |   16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
index 2ccb4a0..6a37d62 100644
--- a/drivers/net/xen-netfront.c
+++ b/drivers/net/xen-netfront.c
@@ -2046,13 +2046,15 @@ static int xennet_connect(struct net_device *dev)
 	/* By now, the queue structures have been set up */
 	for (j = 0; j < num_queues; ++j) {
 		queue = &np->queues[j];
-		spin_lock_bh(&queue->rx_lock);
-		spin_lock_irq(&queue->tx_lock);
 
 		/* Step 1: Discard all pending TX packet fragments. */
+		spin_lock_irq(&queue->tx_lock);
 		xennet_release_tx_bufs(queue);
+		spin_unlock_irq(&queue->tx_lock);
 
 		/* Step 2: Rebuild the RX buffer freelist and the RX ring itself. */
+		spin_lock_bh(&queue->rx_lock);
+
 		for (requeue_idx = 0, i = 0; i < NET_RX_RING_SIZE; i++) {
 			skb_frag_t *frag;
 			const struct page *page;
@@ -2076,6 +2078,8 @@ static int xennet_connect(struct net_device *dev)
 		}
 
 		queue->rx.req_prod_pvt = requeue_idx;
+
+		spin_unlock_bh(&queue->rx_lock);
 	}
 
 	/*
@@ -2087,13 +2091,17 @@ static int xennet_connect(struct net_device *dev)
 	netif_carrier_on(np->netdev);
 	for (j = 0; j < num_queues; ++j) {
 		queue = &np->queues[j];
+
 		notify_remote_via_irq(queue->tx_irq);
 		if (queue->tx_irq != queue->rx_irq)
 			notify_remote_via_irq(queue->rx_irq);
-		xennet_tx_buf_gc(queue);
-		xennet_alloc_rx_buffers(queue);
 
+		spin_lock_irq(&queue->tx_lock);
+		xennet_tx_buf_gc(queue);
 		spin_unlock_irq(&queue->tx_lock);
+
+		spin_lock_bh(&queue->rx_lock);
+		xennet_alloc_rx_buffers(queue);
 		spin_unlock_bh(&queue->rx_lock);
 	}
 
-- 
1.7.10.4

Re: [PATCH 1/2] xen-netfront: don't nest queue locks in xennet_connect()

[Boris Ostrovsky]

On 07/02/2014 11:09 AM, David Vrabel wrote:
> The nesting of the per-queue rx_lock and tx_lock in xennet_connect()
> is confusing to both humans and lockdep.  The locking is safe because
> this is the only place where the locks are nested in this way but
> lockdep still warns.
>
> Instead of adding the missing lockdep annotations, refactor the
> locking to avoid the confusing nesting.  This is still safe, because
> the xenbus connection state changes are all serialized by the xenwatch
> thread.

Tulin (GSoC student) is working on adding some concurrency to the 
xenwatch thread (possibly eliminating it and replacing with a 
workqueue). Which means that we will need to add some sort of 
synchronization in watch callbacks, whenever they are required.

-boris

>
> Signed-off-by: David Vrabel <david.vrabel@citrix.com>
> Reported-by: Sander Eikelenboom <linux@eikelenboom.it>
> ---
>   drivers/net/xen-netfront.c |   16 ++++++++++++----
>   1 file changed, 12 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
> index 2ccb4a0..6a37d62 100644
> --- a/drivers/net/xen-netfront.c
> +++ b/drivers/net/xen-netfront.c
> @@ -2046,13 +2046,15 @@ static int xennet_connect(struct net_device *dev)
>   	/* By now, the queue structures have been set up */
>   	for (j = 0; j < num_queues; ++j) {
>   		queue = &np->queues[j];
> -		spin_lock_bh(&queue->rx_lock);
> -		spin_lock_irq(&queue->tx_lock);
>   
>   		/* Step 1: Discard all pending TX packet fragments. */
> +		spin_lock_irq(&queue->tx_lock);
>   		xennet_release_tx_bufs(queue);
> +		spin_unlock_irq(&queue->tx_lock);
>   
>   		/* Step 2: Rebuild the RX buffer freelist and the RX ring itself. */
> +		spin_lock_bh(&queue->rx_lock);
> +
>   		for (requeue_idx = 0, i = 0; i < NET_RX_RING_SIZE; i++) {
>   			skb_frag_t *frag;
>   			const struct page *page;
> @@ -2076,6 +2078,8 @@ static int xennet_connect(struct net_device *dev)
>   		}
>   
>   		queue->rx.req_prod_pvt = requeue_idx;
> +
> +		spin_unlock_bh(&queue->rx_lock);
>   	}
>   
>   	/*
> @@ -2087,13 +2091,17 @@ static int xennet_connect(struct net_device *dev)
>   	netif_carrier_on(np->netdev);
>   	for (j = 0; j < num_queues; ++j) {
>   		queue = &np->queues[j];
> +
>   		notify_remote_via_irq(queue->tx_irq);
>   		if (queue->tx_irq != queue->rx_irq)
>   			notify_remote_via_irq(queue->rx_irq);
> -		xennet_tx_buf_gc(queue);
> -		xennet_alloc_rx_buffers(queue);
>   
> +		spin_lock_irq(&queue->tx_lock);
> +		xennet_tx_buf_gc(queue);
>   		spin_unlock_irq(&queue->tx_lock);
> +
> +		spin_lock_bh(&queue->rx_lock);
> +		xennet_alloc_rx_buffers(queue);
>   		spin_unlock_bh(&queue->rx_lock);
>   	}
>   

Re: [PATCH 1/2] xen-netfront: don't nest queue locks in xennet_connect()

[David Vrabel]

On 02/07/14 18:36, Boris Ostrovsky wrote:
> On 07/02/2014 11:09 AM, David Vrabel wrote:
>> The nesting of the per-queue rx_lock and tx_lock in xennet_connect()
>> is confusing to both humans and lockdep.  The locking is safe because
>> this is the only place where the locks are nested in this way but
>> lockdep still warns.
>>
>> Instead of adding the missing lockdep annotations, refactor the
>> locking to avoid the confusing nesting.  This is still safe, because
>> the xenbus connection state changes are all serialized by the xenwatch
>> thread.
> 
> Tulin (GSoC student) is working on adding some concurrency to the
> xenwatch thread (possibly eliminating it and replacing with a
> workqueue). Which means that we will need to add some sort of
> synchronization in watch callbacks, whenever they are required.

The back/frontend changed callbacks should be serialized with the device
mutex in the same way that the device core does for probe and remove etc.

David

[PATCH 2/2] xen-netfront: call netif_carrier_off() only once when disconnecting

[David Vrabel]

In xennet_disconnect_backend(), netif_carrier_off() was called once
per queue when it needs to only be called once.

The queue locking around the netif_carrier_off() call looked very
odd. I think they were supposed to synchronize any NAPI instances with
the expectation that no further NAPI instances would be scheduled
because of the carrier being off (see the check in
xennet_rx_interrupt()).  But I can't easily tell if this works
correctly.

Instead, add a napi_synchronize() call after disabling the interrupts.
This is obviously correct as with no Rx interrupts, no further NAPI
instances will be scheduled.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
---
 drivers/net/xen-netfront.c |   11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)

diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
index 6a37d62..055222b 100644
--- a/drivers/net/xen-netfront.c
+++ b/drivers/net/xen-netfront.c
@@ -1439,16 +1439,11 @@ static void xennet_disconnect_backend(struct netfront_info *info)
 	unsigned int i = 0;
 	unsigned int num_queues = info->netdev->real_num_tx_queues;
 
+	netif_carrier_off(info->netdev);
+
 	for (i = 0; i < num_queues; ++i) {
 		struct netfront_queue *queue = &info->queues[i];
 
-		/* Stop old i/f to prevent errors whilst we rebuild the state. */
-		spin_lock_bh(&queue->rx_lock);
-		spin_lock_irq(&queue->tx_lock);
-		netif_carrier_off(queue->info->netdev);
-		spin_unlock_irq(&queue->tx_lock);
-		spin_unlock_bh(&queue->rx_lock);
-
 		if (queue->tx_irq && (queue->tx_irq == queue->rx_irq))
 			unbind_from_irqhandler(queue->tx_irq, queue);
 		if (queue->tx_irq && (queue->tx_irq != queue->rx_irq)) {
@@ -1458,6 +1453,8 @@ static void xennet_disconnect_backend(struct netfront_info *info)
 		queue->tx_evtchn = queue->rx_evtchn = 0;
 		queue->tx_irq = queue->rx_irq = 0;
 
+		napi_synchronize(&queue->napi);
+
 		/* End access and free the pages */
 		xennet_end_access(queue->tx_ring_ref, queue->tx.sring);
 		xennet_end_access(queue->rx_ring_ref, queue->rx.sring);
-- 
1.7.10.4

Re: [PATCHv1 net 0/2] xen-netfront: multi-queue related locking fixes

[David Miller]

From: David Vrabel <david.vrabel@citrix.com>
Date: Wed, 2 Jul 2014 16:09:13 +0100

> Two fixes to the per-queue locking bugs in xen-netfront that were
> introduced in 3.16-rc1 with the multi-queue support.

Series applied, thanks.