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SubjectRe: hdlc_ppp: why no detach()?
Hello Michael,

Michael Barkowski <> writes:

> I am looking at your hdlc_ppp code and I don't understand: why is there
> not the equivalent of fr_detach() in there?

I assume you mean .detach = fr_destroy(). It's used only to kill
subdevices, i.e. it has nothing to do with the interface being up/down.

> pc8300_drv:cpc_remove_one() frees netdevs quite confidently but I wonder
> how it can be so sure that there are not skbs in hdlc_ppp's tx_queue
> associated with those devices before freeing them....q

Theoretically all paths adding skbs to the tx_queue should send them out
before returning (possibly also on behalf of other devices). However I
wonder if it's the case. Let's see: Only ppp_tx_cp() adds to the queue
- ppp_rx() (calls ppp_tx_flush())
- ppp_timer (calls ppp_tx_flush())
- ppp_cp_event():
- ppp_cp_parse_cr() (calls ppp_tx_flush())
- ppp_stop() calls ppp_cp_event(), but it won't queue any skb, it only
marks the connection as closed and does the same to IPCP and IPV6CP.

This means the problematic part is ppp_start() which calls
ppp_cp_event(LCP, START) = IRC | SCR | 3 meaning
Initialize-Restart-Count, Send-Configure-Request and change state to
REQ_SENT. This causes two problems:

1. The SCR packet will be delayed by 2 seconds (both first and second
SCR will be sent the same time). Perhaps we delay only a little
(instead of full 2 seconds) and only then send the initial packet.

2. (as you noted) the skb will be added to tx_queue and left there. If
we happen to "ifconfig up" and "rmmod driver" before receiving any
packet and before ppp->req_timeout (2 seconds) and before any other
PPP interface does the same, we will eventually get skb with invalid
->dev. This is simple to drain in .close (detach is a wrong place
since it may be called long after the interface is deactivated, there
is no need to delay it past .close). The fix for #1 will already fix
#2, but the redundant safety doesn't cost us anything.

Thanks for noting the problem, I'll post a patch shortly.

Also it seems the timeouts etc. should be configurable. ATM we're only
fixing bugs, good.
Krzysztof Halasa

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