netns: remove useless synchronize_net()

In dev_change_net_namespace(), synchronize_net() is called at the end of the function, but there is no reason (no deletion occurs).

Signed-off-by Nicolas Dichtel <[email protected]>

From: Nicolas Dichtel <[email protected]>
Date: Thu, 05 Feb 2009 11:21:22 +0100

Quoted 4 lines

In dev_change_net_namespace(), synchronize_net() is called at the end of the function, but there is no reason (no deletion occurs).

Signed-off-by Nicolas Dichtel <[email protected]>

It is necessary to make sure all cpus stop looking at the previous namespace the device was attached to, and only see the new mapping.

That's why this function has two synchronize_net() calls.

If namespace is destroyed after this function, then cleanup_net() will ensure that nobody is looking at it

From: Nicolas Dichtel <[email protected]>
Date: Fri, 06 Feb 2009 14:50:53 +0100

Quoted 2 lines

If namespace is destroyed after this function, then cleanup_net() will ensure that nobody is looking at it

Maybe, but you better get some opinions from the people who wrote and maintain the network namespace code before I can consider your change seriously.

None of them responded to your patch posting, probably because you failed to CC: any of them.

Quoted 10 lines · David Miller

From: Nicolas Dichtel <[email protected]>
Date: Fri, 06 Feb 2009 14:50:53 +0100

Quoted 2 lines

If namespace is destroyed after this function, then cleanup_net() will ensure that nobody is looking at it

Maybe, but you better get some opinions from the people who wrote and maintain the network namespace code before I can consider your change seriously.

Quoted 2 lines

None of them responded to your patch posting, probably because you failed to CC: any of them.

Sorry, I forget to cc them, now it's done.
The thread can be found here: http://marc.info/?l=linux-netdev&m=123382930115535&w=2

So, I'm waiting for maintainers's opinions.

Quoted 13 lines · Nicolas Dichtel

Quoted 12 lines · David Miller

From: Nicolas Dichtel <[email protected]>
Date: Fri, 06 Feb 2009 14:50:53 +0100

Quoted 2 lines

If namespace is destroyed after this function, then cleanup_net() will ensure that nobody is looking at it

Maybe, but you better get some opinions from the people who wrote and maintain the network namespace code before I can consider your change seriously.

None of them responded to your patch posting, probably because you failed to CC: any of them.

Quoted 4 lines

Sorry, I forget to cc them, now it's done.
The thread can be found here: http://marc.info/?l=linux-netdev&m=123382930115535&w=2

So, I'm waiting for maintainers's opinions.

We can move one network device from one namespace to another namespace, and that do not necessarily implies the network namespace will die and call cleanup_net. Without synchronize_net, it would be possible to have netif_receive_skb and dev_change_net_namespace to be executed concurrently, no ?
Wouldn't the execution of one of this function be problematic if we are in the delivery of a packet to the upper protocol in the big rcu_read_lock section of netif_receive_skb ?

   dev_shutdown(dev);

   /* Notify protocols, that we are about to destroy
      this device. They should clean all the things.
   */
   call_netdevice_notifiers(NETDEV_UNREGISTER, dev);

   /*
    *    Flush the unicast and multicast chains
    */
   dev_addr_discard(dev);

   netdev_unregister_kobject(dev);

   /* Actually switch the network namespace */
   dev_net_set(dev, net);

Thanks
-- Daniel

Quoted 18 lines · Daniel Lezcano

Quoted 17 lines · Nicolas Dichtel

Quoted 6 lines · David Miller

From: Nicolas Dichtel <[email protected]>
Date: Fri, 06 Feb 2009 14:50:53 +0100

Quoted 2 lines

If namespace is destroyed after this function, then cleanup_net() will ensure that nobody is looking at it

Quoted 6 lines

Maybe, but you better get some opinions from the people who wrote and maintain the network namespace code before I can consider your change seriously.

None of them responded to your patch posting, probably because you failed to CC: any of them.

Sorry, I forget to cc them, now it's done.
The thread can be found here: http://marc.info/?l=linux-netdev&m=123382930115535&w=2

So, I'm waiting for maintainers's opinions.

Quoted 3 lines

We can move one network device from one namespace to another namespace, and that do not necessarily implies the network namespace will die and call cleanup_net. Without synchronize_net, it would be possible to have netif_receive_skb and dev_change_net_namespace to be executed concurrently, no ?
Wouldn't the execution of one of this function be problematic if we are in the delivery of a packet to the upper protocol in the big rcu_read_lock section of netif_receive_skb ?

Just to be sure: there is two synchronize_net() in dev_change_net_namespace(), and I was talking about the second one. The second one is called just before exiting the function.

Regards,
Nicolas

Hmm, at the first glance I would say it is useless but perhaps there is a trick here I do not understand.
Eric, is there any particular reason to call synchronize_net before exiting the dev_change_net_namespace function ?

Quoted 5 lines · Daniel Lezcano

Hmm, at the first glance I would say it is useless but perhaps there is a trick here I do not understand.
Eric, is there any particular reason to call synchronize_net before exiting the dev_change_net_namespace function ?

I haven't thought about that part of the code path in detail in a long time. dev_change_net_namespace() is a condensed version of register_netdevice() unregister_netdevice(). With the calls down into the driver removed.

On a side note. It looks like we now cope with:
call_netdevice_notifiers(NETDEV_REGISTER, dev); failing in register_netdev, but no one updated dev_change_net_namespace to handle the change, looks like a real pain to cope with.

As for the synchronize_net, and in response to the original comment as best as I can tell we do have things being being deleted that are at least candidates for synchronize_net.

dev_addr_discard(dev);
dev_net_set(dev, net);
netdev_unregister_kobject(dev);

We very much do access dev->net with only rcu protection.

Hmm.

It looks like I originally took the second synchronize_net from what became rollback_registered, which happens just before we start freeing the netdevice.

The nastiest case that I can envision is if we happen to receive a packet (on another cpu) for the network device that we are moving, just after it has registered in the new network namespace. If we read the old network namespace and forward it up the network stack in that context I can imagine it being a recipe for all kinds of strange non-deterministic behavior.

So unless there is a reason for this change beyond general cleanup I would prefer not to think about it potential weirdness, and keep the code the way it is.

I seem to remember a conversation about this synchronize_net when the code was merged as well so if we are going to change it, let's look up those arguments if we can and see if there was something useful said.

Eric

Quoted 3 lines · Eric W. Biederman

Quoted 1 line · Daniel Lezcano

Hmm, at the first glance I would say it is useless but perhaps there is a

trick

Quoted 2 lines

Quoted 2 lines

here I do not understand.
Eric, is there any particular reason to call synchronize_net before exiting

the

Quoted 29 lines

Quoted 2 lines

dev_change_net_namespace function ?

I haven't thought about that part of the code path in detail in a long time. dev_change_net_namespace() is a condensed version of register_netdevice() unregister_netdevice(). With the calls down into the driver removed.

On a side note. It looks like we now cope with:
call_netdevice_notifiers(NETDEV_REGISTER, dev); failing in register_netdev, but no one updated dev_change_net_namespace to handle the change, looks like a real pain to cope with.

As for the synchronize_net, and in response to the original comment as best as I can tell we do have things being being deleted that are at least candidates for synchronize_net.

dev_addr_discard(dev);
dev_net_set(dev, net);
netdev_unregister_kobject(dev);

We very much do access dev->net with only rcu protection.

Hmm.

It looks like I originally took the second synchronize_net from what became rollback_registered, which happens just before we start freeing the netdevice.

Quoted 7 lines

The nastiest case that I can envision is if we happen to receive a packet (on another cpu) for the network device that we are moving, just after it has registered in the new network namespace. If we read the old network namespace and forward it up the network stack in that context I can imagine it being a recipe for all kinds of strange non-deterministic behavior.

The code does:

   dev_close
      dev_deactive
         synchronize_rcu
   synchronize_net
   ...
   dev_shutdown
   ...
   synchronize_net

The network device can no longer receive packets after dev_deactive, no ? The first synchronize_net will wait for the outstanding packets to be delivered to the upper layer and we change the nd_net field after.
Your scenario makes sense for the first synchronize_net but I am not sure that can happen if we remove the second synchronize_net.

Quoted 62 lines · Daniel Lezcano

Quoted 5 lines · Eric W. Biederman

Quoted 2 lines · Daniel Lezcano

Hmm, at the first glance I would say it is useless but perhaps there is a

trick

Quoted 3 lines

Quoted 3 lines

here I do not understand.
Eric, is there any particular reason to call synchronize_net before exiting

the

Quoted 29 lines

Quoted 3 lines

dev_change_net_namespace function ?

I haven't thought about that part of the code path in detail in a long time. dev_change_net_namespace() is a condensed version of register_netdevice() unregister_netdevice(). With the calls down into the driver removed.

On a side note. It looks like we now cope with:
call_netdevice_notifiers(NETDEV_REGISTER, dev); failing in register_netdev, but no one updated dev_change_net_namespace to handle the change, looks like a real pain to cope with.

As for the synchronize_net, and in response to the original comment as best as I can tell we do have things being being deleted that are at least candidates for synchronize_net.

dev_addr_discard(dev);
dev_net_set(dev, net);
netdev_unregister_kobject(dev);

We very much do access dev->net with only rcu protection.

Hmm.

It looks like I originally took the second synchronize_net from what became rollback_registered, which happens just before we start freeing the netdevice.

Quoted 8 lines

The nastiest case that I can envision is if we happen to receive a packet (on another cpu) for the network device that we are moving, just after it has registered in the new network namespace. If we read the old network namespace and forward it up the network stack in that context I can imagine it being a recipe for all kinds of strange non-deterministic behavior.

The code does:

   dev_close
      dev_deactive
         synchronize_rcu
   synchronize_net
   ...
   dev_shutdown
   ...
   synchronize_net

Quoted 6 lines

The network device can no longer receive packets after dev_deactive, no ? The first synchronize_net will wait for the outstanding packets to be delivered to the upper layer and we change the nd_net field after.
Your scenario makes sense for the first synchronize_net but I am not sure that can happen if we remove the second synchronize_net.

Good point. Visibility is key. What can find us after we call list_netdevice() ? Aren't there some pieces of code that do for_each_netdevice under the rcu lock?

Quoted 81 lines · Eric W. Biederman

Quoted 77 lines · Daniel Lezcano

Quoted 7 lines · Eric W. Biederman

Quoted 3 lines · Daniel Lezcano

Hmm, at the first glance I would say it is useless but perhaps there is a

trick

Quoted 4 lines

Quoted 4 lines

here I do not understand.
Eric, is there any particular reason to call synchronize_net before exiting

the

Quoted 30 lines

Quoted 4 lines

dev_change_net_namespace function ?

I haven't thought about that part of the code path in detail in a long time. dev_change_net_namespace() is a condensed version of register_netdevice() unregister_netdevice(). With the calls down into the driver removed.

On a side note. It looks like we now cope with:
call_netdevice_notifiers(NETDEV_REGISTER, dev); failing in register_netdev, but no one updated dev_change_net_namespace to handle the change, looks like a real pain to cope with.

As for the synchronize_net, and in response to the original comment as best as I can tell we do have things being being deleted that are at least candidates for synchronize_net.

dev_addr_discard(dev);
dev_net_set(dev, net);
netdev_unregister_kobject(dev);

We very much do access dev->net with only rcu protection.

Hmm.

It looks like I originally took the second synchronize_net from what became rollback_registered, which happens just before we start freeing the netdevice.

Quoted 9 lines

The nastiest case that I can envision is if we happen to receive a packet (on another cpu) for the network device that we are moving, just after it has registered in the new network namespace. If we read the old network namespace and forward it up the network stack in that context I can imagine it being a recipe for all kinds of strange non-deterministic behavior.

The code does:

   dev_close
      dev_deactive
         synchronize_rcu
   synchronize_net
   ...
   dev_shutdown
   ...
   synchronize_net

The network device can no longer receive packets after dev_deactive, no ? The first synchronize_net will wait for the outstanding packets to be delivered to the upper layer and we change the nd_net field after.
Your scenario makes sense for the first synchronize_net but I am not sure that can happen if we remove the second synchronize_net.

Quoted 5 lines

Good point. Visibility is key. What can find us after we call list_netdevice() ? Aren't there some pieces of code that do for_each_netdevice under the rcu lock?

AFAIR, no. for_each_netdev is protected by rtnl_lock.

Quoted 75 lines · Daniel Lezcano

Quoted 73 lines · Eric W. Biederman

Quoted 70 lines · Daniel Lezcano

Quoted 5 lines · Eric W. Biederman

Quoted 3 lines · Daniel Lezcano

Hmm, at the first glance I would say it is useless but perhaps there is a

trick

Quoted 4 lines

Quoted 4 lines

here I do not understand.
Eric, is there any particular reason to call synchronize_net before exiting

the

Quoted 28 lines

Quoted 2 lines

dev_change_net_namespace function ?

I haven't thought about that part of the code path in detail in a long time. dev_change_net_namespace() is a condensed version of register_netdevice() unregister_netdevice(). With the calls down into the driver removed.

On a side note. It looks like we now cope with:
call_netdevice_notifiers(NETDEV_REGISTER, dev); failing in register_netdev, but no one updated dev_change_net_namespace to handle the change, looks like a real pain to cope with.

As for the synchronize_net, and in response to the original comment as best as I can tell we do have things being being deleted that are at least candidates for synchronize_net.

dev_addr_discard(dev);
dev_net_set(dev, net);
netdev_unregister_kobject(dev);

We very much do access dev->net with only rcu protection.

Hmm.

It looks like I originally took the second synchronize_net from what became rollback_registered, which happens just before we start freeing the netdevice.

Quoted 7 lines

The nastiest case that I can envision is if we happen to receive a packet (on another cpu) for the network device that we are moving, just after it has registered in the new network namespace. If we read the old network namespace and forward it up the network stack in that context I can imagine it being a recipe for all kinds of strange non-deterministic behavior.

The code does:

   dev_close
      dev_deactive
         synchronize_rcu
   synchronize_net
   ...
   dev_shutdown
   ...
   synchronize_net

The network device can no longer receive packets after dev_deactive, no ?
The first synchronize_net will wait for the outstanding packets to be delivered
to the upper layer and we change the nd_net field after. Your scenario makes sense for the first synchronize_net but I am not sure that
can happen if we remove the second synchronize_net.

Quoted 6 lines

Quoted 4 lines

Good point. Visibility is key. What can find us after we call list_netdevice() ? Aren't there some pieces of code that do for_each_netdevice under the rcu lock?

AFAIR, no. for_each_netdev is protected by rtnl_lock.

Nicolas,

At the first glance it looks like the removing of the second synchronize_net is fine, but before posting the patch do you mind to wait a little ?
I would like to do some tests with your patch to check if we don't missed something.