From mboxrd@z Thu Jan 1 00:00:00 1970 Return-path: Received: from mail-wg0-f52.google.com ([74.125.82.52]:60499 "EHLO mail-wg0-f52.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752033AbaEWI6d convert rfc822-to-8bit (ORCPT ); Fri, 23 May 2014 04:58:33 -0400 Received: by mail-wg0-f52.google.com with SMTP id l18so4574719wgh.35 for ; Fri, 23 May 2014 01:58:31 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <1400834656.4358.1.camel@jlt4.sipsolutions.net> References: <1400767676-15994-1-git-send-email-michal.kazior@tieto.com> <1400767676-15994-5-git-send-email-michal.kazior@tieto.com> <1400770484.4174.35.camel@jlt4.sipsolutions.net> <1400834656.4358.1.camel@jlt4.sipsolutions.net> Date: Fri, 23 May 2014 10:58:31 +0200 Message-ID: (sfid-20140523_105837_759655_314BB643) Subject: Re: [PATCH v6 4/6] mac80211: use chanctx reservation for AP CSA From: Michal Kazior To: Johannes Berg Cc: linux-wireless , Luca Coelho Content-Type: text/plain; charset=UTF-8 Sender: linux-wireless-owner@vger.kernel.org List-ID: On 23 May 2014 10:44, Johannes Berg wrote: > On Fri, 2014-05-23 at 08:49 +0200, Michal Kazior wrote: > >> > But you'll WARN_ON() if they're actually not at the same time and you >> > grab a beacon (or for the template case, call csa_update) in the >> > meantime, right? I'd really like to have all those driver requirements >> > (e.g. to stop beaconing) better documented. >> >> Good point. I suppose it should be stated in the docs that once you >> reach ieee80211_csa_is_complete() being true you must not call >> ieee80211_beacon_get() nor ieee80211_csa_update_counter(). ath9k and >> ath10k conform to this. > > Right, it's still a bit strange that you just have to stop beaconing. > Dunno. I guess I don't care all that much since our driver won't support > multiple AP interfaces anyway :-) If you keep on beaconing for too long on a DFS channel after detecting a radar you risk breaking local law (ETSI and FCC specify quiescing periods). >> I wonder what driver should be supposed to look at before starting to >> beacon again? csa_active isn't well protected to be depended upon. If >> we should create a ieee80211_csa_is_active() that just checks if >> beacon->csa_counter_offset[0] != 0 (assuming my other csa counter >> patches are applied) then it's still racy: >> a) rcu_dereference() across ieee80211_csa_is_active(), _is_complete() >> and _beacon_get() can yield different beacon pointers >> b) cs_count <= 1 yields no beacon update (thus no counters/offsets, >> meaning both _csa_is_complete and _csa_is_active() are `false` thus >> suggesting driver can beacon as if nothing happened) >> >> We could fix (b) by simply not treating cs_count <= 1 so special and >> update the beacon anyway. For (a) to work we'd need either make a >> single-call do-all function or introduce an additional call and a >> generic pointer/structure to be passed to other functions so that a >> beacon pointer is consistent across calls. > > You could technically just return NULL in this period, but I don't know > how well drivers would cope with that (though technically they have to > cope with it due to memory allocation failures anyway) Sounds good to me. I guess you'd use ieee80211_csa_is_complete (or ieee80211_csa_update_counter) for template-based drivers to stop beaconing. Michał