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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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      Last update: 2010-04-15 00:51    [W:0.035 / U:2.936 seconds]
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