[tcpdump-workers] TPACKET_V3 timeout bug?

Andrew Lunn andrew at lunn.ch
Sat Apr 15 22:10:50 EDT 2017


On Sat, Apr 15, 2017 at 06:45:36PM -0400, Sowmini Varadhan wrote:
> On (04/15/17 21:40), Andrew Lunn wrote:
> > 
> > In my case, lan3 is up and idle, there are no packets flying around to
> > be captured. So i would expect pcap_next_ex() to exit once a second,
> > with a return value of 0. But it is not, it blocks and stays blocked.
>    :
> > Looking at the libpcap source, the 1000ms timeout is being used as
> > part of the setsockopt(3, SOL_PACKET, PACKET_RX_RING, 0xbe9445c0, 28)
> > call, req.tp_retire_blk_tov is set to the timeoutval.
> 
> right, aiui, the retire_blk_tov will only kick in if we have at
> least one frame in a block, but the block is not filled up yet,
> before the req.tp_retire_blk_tov (1s in your case) expires.
> 
> If there are 0 frames pending, we should not be waking up the app,
> so everything seems to be behaving as it should?

Hi Guy

You wrote the TPACKET3 set_poll_timeout() handling. Please could you
have a look at the thread:

https://www.mail-archive.com/netdev@vger.kernel.org/msg163532.html

Do you think this is a kernel problem, libpcap problem, or an
application problem?

Thanks
	Andrew


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