CFT/CFR, possible fix for ifconfig scan hang

[Attilio Rao]

2010/12/27 Bernhard Schmidt <bschmidt at freebsd.org>:
> Hi,
>
> I recently received some complains about the infamous 'ifconfig scan hang'
> issue again. Finally looking into that I noticed a bunch of inconsistences,
> the most obvious one is that ifconfig(8) is talking about doing a background
> scan by default, which is simply not true according to the implementation.
>
> Anyways.. the generic use-case which triggers the 'hang' is, if 'ifconfig
> scan' is called while a scan is already in progress, net80211 will not start a
> new one which means that no scan flags are updated, though, ifconfig will loop
> until it receives a notification about the scan being done. This does always
> happen after an 'ifconfig up', because net80211 will move the VAP into scan
> state by default, with the scan flags set in such a way that a scan is done
> until there is something to connect to. This also means that no notifications
> about the scan being done are sent to upper layers, because the scan is not
> finished..
>
> If we successfully moved from scan to run state, how so ever, and now want to
> call 'ifconfig scan' we're faced with another issue. Doing a scan while being
> associated means we have to move off our current channel, to do this without
> loosing any imported frames/information, we make use of the power save
> feature. Now if we want to send any traffic while doing the scan, the scan is
> temporary suspended, frames are sent, then the scan is restarted after
> receiving a beacon and there is no frame to send. For this to work though, we
> need to actually request a background scan so appropriate flags are set and
> the scan is actually restarted. Without this, we hang until the bgscan timer
> fires of at that next bgscanintval.
>
> I have a patch available which addresses both of the issues. It requests a
> background scan by default and also honors the return value of
> start_scan_locked():
> - for head
> http://techwires.net/~bschmidt/scan_hang_head.diff
> - for 8-stable/8.2-*:
> http://techwires.net/~bschmidt/scan_hang_stable.diff
>
> Please test and let me know if it works, or not.

Bernard,
thanks a lot for working on this.

There is any receipt for easilly reproduce this type of bug, or I
should just go with 'ifconfig scan' repeteadly?
I recall I could trigger it, but not 100% of times, so if you have
more hints about how this problem could be reproduced, I'd be glad to
hear.

Thanks,
Attilio


-- 
Peace can only be achieved by understanding - A. Einstein