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Date:	Mon, 18 Aug 2014 11:50:16 +0300
From:	Luca Coelho <>
To:	Linus Torvalds <>
Cc:	Johannes Berg <>,
	Emmanuel Grumbach <>,
	Michal Kazior <>,
	Intel Linux Wireless <>,
	"John W. Linville" <>,
	Linux Wireless List <>,
	Network Development <>
Subject: Re: More wireless problems..

Hi Linus,

On Sun, 2014-08-17 at 23:37 -0500, Linus Torvalds wrote:
> On Sun, Aug 17, 2014 at 11:01 PM, Linus Torvalds
> <> wrote:
> >
> >   BUG: unable to handle kernel NULL pointer dereference at 0000000000000020
> >   IP: ieee80211_vif_use_reserved_switch+0x71c/0xb00 [mac80211]
> Looking at the Code: line and the code generation for that function,
> this *looks* to be this code:
>                         list_del(&sdata->reserved_chanctx_list);
>                         list_move(&sdata->assigned_chanctx_list,
>                                   &new_ctx->assigned_vifs);
>                         sdata->reserved_chanctx = NULL;
> in ieee80211_vif_use_reserved_switch(), where "new_ctx" is NULL, so
> the "list_move()" ends up oopsing. But maybe I screwed up the
> analysis, I don't know the code.
> Looks like that is all-new code introduced by commit 5bcae31d9cb1
> ("mac80211: implement multi-vif in-place reservations")
> And doesn't look at all IWL-specific. Adding Michal Kazior to the list
> of people.

Unfortunately I don't think channel switch on the client side has been
tested very thoroughly yet, from the iwlwifi point of view, we're still
fine-tuning things in the firmware configuration.  So I'd rather disable
channel-switch entirely for iwlwifi.

Probably better to disable it in mac80211, since this oops will probably
happen with other drivers too, since doesn't seem to be iwlwifi related.

I'll sent a patch soon.  Meanwhile, you can apply this directly, to make
the problem go away:

diff --git a/net/mac80211/main.c b/net/mac80211/main.c
index e0ab432..4b8eee8 100644
--- a/net/mac80211/main.c
+++ b/net/mac80211/main.c
@@ -758,6 +758,12 @@ int ieee80211_register_hw(struct ieee80211_hw *hw)
        netdev_features_t feature_whitelist;
        struct cfg80211_chan_def dflt_chandef = {};
+       /* The CSA client code is not very stable yet and seems to be
+        * causing trouble.  Disable it for all drivers until
+        * everything has been fixed and tested properly.
+        */
+       hw->flags &= ~IEEE80211_HW_CHANCTX_STA_CSA;
        if (hw->flags & IEEE80211_HW_QUEUE_CONTROL &&
            (local->hw.offchannel_tx_hw_queue ==
             local->hw.offchannel_tx_hw_queue >= local->hw.queues))


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