Subject: Re: Possible BUG where mac80211 fails to stop queues
From: Johannes Berg <johannes@sipsolutions.net>
To: Larry Finger <Larry.Finger@lwfinger.net>
Cc: John Linville <linville@tuxdriver.com>,
	Michael Buesch <mb@bu3sch.de>,
	wireless <linux-wireless@vger.kernel.org>
In-Reply-To: <4A6DCE0E.3010103@lwfinger.net>
References: <4A6CDE26.3000409@lwfinger.net>
	 <1248684480.19945.45.camel@johannes.local>  <4A6DCE0E.3010103@lwfinger.net>
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Date: Mon, 27 Jul 2009 18:04:36 +0200
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On Mon, 2009-07-27 at 10:55 -0500, Larry Finger wrote:

> > I've also seen it under extreme stress on Intel hardware, cf.
> > http://thread.gmane.org/gmane.linux.kernel.wireless.general/36497
>=20
> Fortunately, the b43 coding was robust enough to prevent queue
> overrun, thus we just end up with a warning.

Right. iwlwifi wasn't so lucky.

> > Are you seeing this multiple times? I don't think you have fragmentatio=
n
> > on, do you? At least I didn't and still saw the problem, which seemed a
> > bit strange, but I really couldn't see any other way for it to happen.
>=20
> When it occurs, I get just a single warning. Fragmentation was not on.

> I will prepare a patch that acknowledges that mac80211 might send one
> extra fragment after the queues are stopped and only issue a warning
> if we get more than one.

Sounds reasonable. You could even change the threshold from 2 to 4 and
enqueue the frame anyway, if still >=3D 2 slots free, I guess.

However, I think I've seen this happen more than once, which really
makes me think there's a bug in mac80211 too, but I haven't been able to
find that bug if any.

johannes

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