Cc: Felix Fietkau <nbd@openwrt.org>,
	"John W. Linville" <linville@tuxdriver.com>,
	Vasanthakumar Thiagarajan <vthiagar@qti.qualcomm.com>,
	<ath9k-devel@venema.h4ckr.net>,
	linux-wireless <linux-wireless@vger.kernel.org>,
	Jouni Malinen <jouni@qca.qualcomm.com>,
	"Luis R. Rodriguez" <mcgrof@qca.qualcomm.com>,
	Senthil Balasubramanian <senthilb@qti.qualcomm.com>
Date: Tue, 13 May 2014 14:30:58 +0530
From: Rajkumar Manoharan <rmanohar@qti.qualcomm.com>
To: David Herrmann <dh.herrmann@gmail.com>
Subject: Re: [ath9k-devel] [PATCH] ath9k: fix NULL-deref in
 hw_per_calibration() for ar9002
Message-ID: <20140513090056.GA31546@qca.qualcomm.com> (sfid-20140513_110040_191343_6896BB45)
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 <20140512174910.GM17022@tuxdriver.com>
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On Tue, May 13, 2014 at 08:50:00AM +0200, David Herrmann wrote:
> Hi
> 
> On Mon, May 12, 2014 at 8:43 PM, Felix Fietkau <nbd@openwrt.org> wrote:
> > I looked into it again, the scenario where I assumed that this problem
> > could occur didn't turn out to be true. I have no idea how this crash
> > can occur.
> 
> The only path that can set ah->caldata to NULL is through:
> 
> ieee80211_hw_config()
>   ath9k_htc_config()
>     ath9k_htc_set_channel()
>       ath9k_hw_reset()
> 
> This happens whenever IEEE80211_CONF_OFFCHANNEL is set.
> 
> Now mac80211 is way to big for me to review right now and
> ieee80211_hw_config() is used quite often. Given that the described
> call-path does no synchronization against ath9k_htc_ani_work(), all
> the callers of mac80211_hw_config(OFFCHANNEL) must guarantee that no
> ani-work is running. Is that intentional? I cannot see any of those
> functions calling into ath9k_htc_stop_ani(). This might of course be
> implicit.
> 
> One call-path I see is:
> ieee80211_scan_cancel()
>   cancel_delayed_work()
> 
> We cannot use cancel_delayed_work_sync() here due to locking issues.
> However, this obviously races against any following
> set_channel(OFFCHANNEL) request.
> 
> If there's anything I can do to debug this, let me know. I tried
> adding some printk()'s into the hot-path and it turns out to no longer
> fail then. So this really seems to be a quite small race (given that a
> bunch of simple printk()s is slow enough to work around it).

David,

Are you using USB devices? What is the PID/VID? I have not looked at
HTC driver perspective.

-Rajkumar