CFT/CFR, possible fix for ifconfig scan hang

[Brandon Gooch]

On Mon, Dec 27, 2010 at 1:24 PM, Bernhard Schmidt <bschmidt at freebsd.org> wrote:
> 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.
>

Works here, on 8.2-PRERELEASE #1 r216690 and 9.0-CURRENT #1 r216702.

I mucked around for about an hour on both systems with varying
configurations (iwn(4) and wpi(4)), and all seemed normal and
consistently well-behaved.

Thanks Bernhard!

-Brandon