full mbuf queue in tap device

[Oleg Dolgov]

In our project we use tap device to make a VPN between clients.
Server make a block read on opened and configured tap device, than send it to client, etc...

If I connect to server with our client and begin to download many
files from FTP throughout tunnel (300 Mb), than, after some time, tap mbuf output queue 
(ifp->if_snd) will full (ifp->if_snd.ifq_len = 49, ifp->if_snd.ifq_maxlen = 50), so,
ip_output will drop packet and return ENOBUFS.

And, even, if I will disconnect client, and just try to ping some network ip address,
that will go throughout tap (except tap ip addrs): no mbufs error returned.

 From ddb, I see, that my server thread locked in waiting queue named "taprd".

I turn on "net.link.tap.debug" and add debug line in ether_output_frame, here is last line from log
(qlen is ifp->if_snd.ifq_len, where ifp is tap device):

...
qlen 0
tap0 starting
tap0 reading, minor = 0
qlen 0
tap0 starting
qlen 1
tap0 starting
qlen 2
tap0 starting
qlen 3
tap0 starting
qlen 4
tap0 starting
qlen 5
tap0 starting
qlen 6
tap0 starting
qlen 7
tap0 starting
qlen 8
tap0 starting
qlen 9
tap0 starting
qlen 10
tap0 starting
qlen 11
tap0 starting
qlen 12
tap0 starting
qlen 13
tap0 starting
qlen 14
tap0 starting
qlen 15
tap0 starting
qlen 16
tap0 starting
qlen 17
tap0 starting
qlen 18
tap0 starting
tap0 writting, minor = 0
qlen 19
tap0 starting
tap0 writting, minor = 0
tap0 writting, minor = 0
qlen 20
tap0 starting
qlen 21
tap0 starting
tap0 writting, minor = 0
tap0 writting, minor = 0
tap0 writting, minor = 0
tap0 writting, minor = 0
tap0 writting, minor = 0
tap0 writting, minor = 0
tap0 writting, minor = 0
tap0 writting, minor = 0
tap0 writting, minor = 0
tap0 writting, minor = 0
tap0 writting, minor = 0
tap0 writting, minor = 0
qlen 22
tap0 starting
qlen 23
tap0 starting
qlen 24
tap0 starting
qlen 25
tap0 starting
qlen 26
tap0 starting
qlen 27
tap0 starting
qlen 28
tap0 starting
qlen 29
tap0 starting
qlen 30
tap0 starting
qlen 31
tap0 starting
qlen 32
tap0 starting
qlen 33
tap0 starting
qlen 34
tap0 starting
qlen 35
tap0 starting
qlen 36
tap0 starting
qlen 37
tap0 starting
qlen 38
tap0 starting
qlen 39
tap0 starting
qlen 40
KDB: enter: beginning
tap0 starting
qlen 41
tap0 starting
tap0 writting, minor = 0
tap0 writting, minor = 0
tap0 writting, minor = 0
qlen 42
tap0 starting
qlen 43
tap0 starting
qlen 44
tap0 starting
qlen 45
tap0 starting
qlen 46
tap0 starting
qlen 47
tap0 starting
tap0 writting, minor = 0
tap0 writting, minor = 0
tap0 writting, minor = 0
tap0 writting, minor = 0
tap0 writting, minor = 0
qlen 48
tap0 starting

As I sad early, seems, that tapifstart doesn't wakeup tapread.
I can't determine a reason of fail, because don't have experience in
the freebsd kernel development.

How and why this can happen, and how to fix?

Please, help me.