mac80211/scan.c: scanning ceases after a double rescheduling

mac80211/scan.c: scanning ceases after a double rescheduling

[Jean-Pierre TOSONI]

Hello all,

I am using compat-wireless-2012-04-17 in a OpenWrt box, client mode,
kernel 2.6.33.
I arrange to run an active background scan every second under some load
(iperf UDP 40 Mbps).

I observed that the probes can be delayed due to two events flagged as
IEEE80211_STA_BEACON_POLL and IEEE80211_STA_CONNECTION_POLL. They make
__ieee80211_start_scan() return early in mac80211/scan.c when it is
called by ieee80211_request_scan().

When one of these events occurs, the function returns and
__ieee80211_start_scan() is reentered later, called this time by
ieee80211_scan_work(). Now if it finds the other event flagged, it
returns early but will never be called again. As a result later scan
requests result in a "EBUSY" error.

I made a workaround by always returning an error, but I am pretty sure
this is a wrong solution. I wonder if scan_work should not be
rescheduled in this case.

Here is my try:
==================
--- a/mac80211/scan.c   2012-05-31 18:37:41.000000000 +0200
+++ b/mac80211/scan.c   2012-05-31 18:37:48.000000000 +0200
@@ -413,10 +413,7 @@
                return -EBUSY;

        if (!ieee80211_can_scan(local, sdata)) {
-               /* wait for the work to finish/time out */
-               local->scan_req = req;
-               local->scan_sdata = sdata;
-               return 0;
+               return -EAGAIN;
        }

        if (local->ops->hw_scan) {
==================

Any thoughts / suggestions ?

Thanks,

Jean-Pierre Tosoni

Re: mac80211/scan.c: scanning ceases after a double rescheduling

[Stanislaw Gruszka]

On Thu, May 31, 2012 at 06:39:51PM +0200, Jean-Pierre TOSONI wrote:
> Hello all,
> 
> I am using compat-wireless-2012-04-17 in a OpenWrt box, client mode,
> kernel 2.6.33.
> I arrange to run an active background scan every second under some load
> (iperf UDP 40 Mbps).
> 
> I observed that the probes can be delayed due to two events flagged as
> IEEE80211_STA_BEACON_POLL and IEEE80211_STA_CONNECTION_POLL. They make
> __ieee80211_start_scan() return early in mac80211/scan.c when it is
> called by ieee80211_request_scan().
> 
> When one of these events occurs, the function returns and
> __ieee80211_start_scan() is reentered later, called this time by
> ieee80211_scan_work(). Now if it finds the other event flagged, it
> returns early but will never be called again. As a result later scan
> requests result in a "EBUSY" error.
> 
> I made a workaround by always returning an error, but I am pretty sure
> this is a wrong solution. I wonder if scan_work should not be
> rescheduled in this case.

I posted fix here:

http://marc.info/?l=linux-wireless&m=133717501822158&w=2

It is needed for 3.5 and it is still not appied in wireless-testing ...

Stanislaw