RE: [PATCH] ath10k: remove TX lock from ath10k_htt_tx_inc_pending

[Wen Gong <wgong@qti.qualcomm.com>]

> -----Original Message-----
> From: ath10k <ath10k-bounces@lists.infradead.org> On Behalf Of Erik
> Stromdahl
> Sent: Saturday, August 24, 2019 9:49 PM
> To: Kalle Valo <kvalo@qca.qualcomm.com>; linux-wireless@vger.kernel.org;
> ath10k@lists.infradead.org
> Cc: Erik Stromdahl <erik.stromdahl@gmail.com>
> Subject: [EXT] [PATCH] ath10k: remove TX lock from
> ath10k_htt_tx_inc_pending
> 
> This commit removes the call to ath10k_mac_tx_lock() from
> ath10k_htt_tx_inc_pending() in case the high water mark is reached.
> 
> ath10k_mac_tx_lock() calls ieee80211_stop_queues() in order to stop
> mac80211 from pushing more TX data to the driver (this is the TX lock).
> 
> If a driver is trying to fetch an skb from a queue while the queue is
> stopped, ieee80211_tx_dequeue() will return NULL.
> 
> So, in ath10k_mac_tx_push_txq(), there is a risk that the call to
> ath10k_htt_tx_inc_pending() results in a stop of the mac80211 TX queues
> just before the skb is fetched.
> 
> This will cause ieee80211_tx_dequeue() to return NULL and
> ath10k_mac_tx_push_txq() to exit prematurely and return -ENOENT.
> Before the function returns ath10k_htt_tx_dec_pending() will be called.
> This call will re-enable the TX queues through ath10k_mac_tx_unlock().
> When ath10k_mac_tx_push_txq() has returned, the TX queue will be
> returned back to mac80211 with ieee80211_return_txq() without the skb
> being properly consumed.
> 
> Since the TX queues were re-enabled in the error exit path of
> ath10k_mac_tx_push_txq(), mac80211 can continue pushing data to the
> driver. If the hardware does not consume the data, the above mentioned
> case will be repeated over and over.
> 
> A case when the hardware is not able to transmit the data from the host
> is when a STA has been dis-associated from an AP and has not yet been
> able to re-associate. In this case there will be no TX_COMPL_INDs from
> the hardware, resulting in the TX counter not be decremented.
> 
> This phenomenon has been observed in both a real and a test setup.
> 
> In order to fix this, the actual TX locking (the call to
> ath10k_mac_tx_lock()) was removed from ath10k_htt_tx_inc_pending().
> Instead, ath10k_mac_tx_lock() is called separately after the skb has
> been fetched (after the call to ieee80211_tx_dequeue()). At this point
> it is OK to stop the queues.
Is this patch will impact throughput?
> 
> 
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