netdev - Re: [PATCH] iproute2 flush: handle larger tables and deleted entries

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Date:	Wed, 15 Jul 2009 10:50:57 -0700
From:	Gautam Kachroo <gk@...stanetworks.com>
To:	Patrick McHardy <kaber@...sh.net>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH] iproute2 flush: handle larger tables and deleted entries

On Wed, Jul 15, 2009 at 8:19 AM, Patrick McHardy<kaber@...sh.net> wrote:
> Gautam Kachroo wrote:
>> On Tue, Jul 14, 2009 at 2:38 AM, Patrick McHardy<kaber@...sh.net> wrote:
>>> Gautam Kachroo wrote:
>>>> use a new netlink socket when sending flush messages to avoid reading
>>>> any pending data on the existing netlink socket.
>>>>
>>>> read all of the response from the netlink request -- this response can
>>>> be split over multiple recv calls, pretty much one per netlink request
>>>> message. ENOENT errors, which correspond to attempts to delete an
>>>> already deleted entry, are ignored. Other errors are not ignored.
>>>
>>> In which case would there be any pending data? From what I can see,
>>> this can only happen when using batching, but in that case the
>>> previous command should continue reading until it has received all
>>> responses (which the netlink functions appear to be doing properly).
>>
>> What is the "previous command"?
>
> The last command before the one executing when using batching.

This is independent of batching (I assume you're referring to the
-batch option to the ip command).
It happens when running a command like "ip neigh flush to 0.0.0.0/0"
if there are many neighbor entries.

The implementation of flush commands, e.g. ip neigh flush, sends a
dump request, e.g. RTM_GETNEIGH, and then sends requests, e.g.
RTM_DELNEIGH, *while* there can be unread data from the dump request.
There would be unread data if the response to the dump request was
split over multiple calls to recvmsg.

>> Are you referring to rtnl_dump_filter? If rtnl_send_check comes across
>> a failure, rtnl_dump_filter will not continue reading.
>>
>> Here's the situation that I'm referring to:
>>
>> If rtnl_send_check detects an error, it returns -1. rtnl_send_check is
>> called from flush_update. The multiple implementations of flush_update
>> (e.g. in ipneigh.c, ipaddress.c) propagate this return value to their
>> caller, e.g. print_neigh or print_addrinfo.
>>
>> print_neigh, print_addrinfo, etc. are called from rtnl_dump_filter.
>> rtnl_dump_filter sits in a loop calling recvmsg on the netlink socket.
>> However, it returns the error value if the filter function (e.g.
>> print_neigh) returns an error. In this case, rtnl_dump_filter can
>> return before it's read all the responses.
>> The error return from rtnl_dump_filter causes the program to exit.
>
> Yes, and I agree with your patch so far. My question is why you
> need another socket.
>
>> use a new netlink socket when sending flush messages to avoid reading
>> any pending data on the existing netlink socket.
>
> Under what circumstances would there be pending data when
> performing a new iproute operation?

As above, it's not that there is pending data when performing a new
iproute operation, it's that there can be pending data while
performing a single iproute operation, namely ip <object> flush.
The benefit of a new socket is that it won't have any data from the
dump request waiting for it.

thanks,
-gk
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