Re: [PATCH] mac80211: fix TXQ AC confusion - Toke Høiland-Jørgensen

From: "Toke Høiland-Jørgensen" <toke@redhat.com>
To: Johannes Berg <johannes@sipsolutions.net>,
	linux-wireless@vger.kernel.org
Cc: Johannes Berg <johannes.berg@intel.com>
Subject: Re: [PATCH] mac80211: fix TXQ AC confusion
Date: Tue, 23 Mar 2021 21:09:11 +0100	[thread overview]
Message-ID: <87v99h63a0.fsf@toke.dk> (raw)
In-Reply-To: <20210323210500.bf4d50afea4a.I136ffde910486301f8818f5442e3c9bf8670a9c4@changeid>

Johannes Berg <johannes@sipsolutions.net> writes:

> From: Johannes Berg <johannes.berg@intel.com>
>
> Normally, TXQs have
>
>   txq->tid = tid;
>   txq->ac = ieee80211_ac_from_tid(tid);
>
> However, the special management TXQ actually has
>
>   txq->tid = IEEE80211_NUM_TIDS; // 16
>   txq->ac = IEEE80211_AC_VO;
>
> This makes sense, but ieee80211_ac_from_tid(16) is the same
> as ieee80211_ac_from_tid(0) which is just IEEE80211_AC_BE.
>
> Now, normally this is fine. However, if the netdev queues
> were stopped, then the code in ieee80211_tx_dequeue() will
> propagate the stop from the interface (vif->txqs_stopped[])
> if the AC 2 (ieee80211_ac_from_tid(txq->tid)) is marked as
> stopped. On wake, however, __ieee80211_wake_txqs() will wake
> the TXQ if AC 0 (txq->ac) is woken up.
>
> If a driver stops all queues with ieee80211_stop_tx_queues()
> and then wakes them again with ieee80211_wake_tx_queues(),
> the ieee80211_wake_txqs() tasklet will run to resync queue
> and TXQ state. If all queues were woken, then what'll happen
> is that _ieee80211_wake_txqs() will run in order of HW queues
> 0-3, typically (and certainly for iwlwifi) corresponding to
> ACs 0-3, so it'll call __ieee80211_wake_txqs() for each AC in
> order 0-3.
>
> When __ieee80211_wake_txqs() is called for AC 0 (VO) that'll
> wake up the management TXQ (remember its tid is 16), and the
> driver's wake_tx_queue() will be called. That tries to get a
> frame, which will immediately *stop* the TXQ again, because
> now we check against AC 2, and AC 2 hasn't yet been marked as
> woken up again in sdata->vif.txqs_stopped[] since we're only
> in the __ieee80211_wake_txqs() call for AC 0.
>
> Thus, the management TXQ will never be started again.
>
> Fix this by checking txq->ac directly instead of calculating
> the AC as ieee80211_ac_from_tid(txq->tid).
>
> Fixes: adf8ed01e4fd ("mac80211: add an optional TXQ for other PS-buffered frames")
> Signed-off-by: Johannes Berg <johannes.berg@intel.com>

Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>