Subject: Re: [PATCH] net: ethernet: ti: cpsw: fix net watchdog timeout

From: Ivan Khoronzhuk <[email-masked]>
Date: Wed, 7 Feb 2018 15:31:06 +0200

On Wed, Feb 07, 2018 at 05:03:19AM +0200, Ivan Khoronzhuk wrote:
> On Tue, Feb 06, 2018 at 07:17:06PM -0600, Grygorii Strashko wrote:
> > It was discovered that simple program which indefinitely sends 200b UDP
> > packets and runs on TI AM574x SoC (SMP) under RT Kernel triggers network
> > watchdog timeout in TI CPSW driver (<6 hours run). The network watchdog
> > timeout is triggered due to race between cpsw_ndo_start_xmit() and
> > cpsw_tx_handler() [NAPI]
> > 
> > cpsw_ndo_start_xmit()
> > 	if (unlikely(!cpdma_check_free_tx_desc(txch))) {
> > 		txq = netdev_get_tx_queue(ndev, q_idx);
> > 		netif_tx_stop_queue(txq);
> > 
> > ^^ as per [1] barier has to be used after set_bit() otherwise new value
> > might not be visible to other cpus
> > 	}
> > 
> > cpsw_tx_handler()
> > 	if (unlikely(netif_tx_queue_stopped(txq)))
> > 		netif_tx_wake_queue(txq);
> > 
> > and when it happens ndev TX queue became disabled forever while driver's HW
> > TX queue is empty.
> I'm sure it fixes test case somehow but there is some strangeness.
> (I've thought about this some X months ago):
> 1. If no free desc, then there is bunch of descs on the queue ready to be sent
> 2. If one of this desc while this process was missed then next will wake queue,
> because there is bunch of them on the fly. So, if desc on top of the sent queue
> missed to enable the queue, then next one more likely will enable it anyway..
> then how it could happen? The described race is possible only on last
> descriptor, yes, packets are small the speed is hight, possibility is very small
> .....but then next situation is also possible:
> - packets are sent fast
> - all packets were sent, but no any descriptors are freed now by sw interrupt (NAPI)
> - when interrupt had started NAPI, the queue was enabled, all other next 
> interrupts are throttled once NAPI not finished it's work yet.
> - when new packet submitted, no free descs are present yet (NAPI has not freed
> any yet), but all packets are sent, so no one can awake tx queue, as interrupt 
> will not arise when NAPI is started to free first descriptor interrupts are 
> disabled.....because h/w queue to be sent is empty...
> - how it can happen as submitting packet and handling packet operations is under 
> channel lock? Not exactly, a period between handling and freeing the descriptor
> to the pool is not under channel lock, here:
> 
> 	spin_unlock_irqrestore(&chan->lock, flags);
> 	if (unlikely(status & CPDMA_DESC_TD_COMPLETE))
> 		cb_status = -ENOSYS;
> 	else
> 		cb_status = status;
> 
> 	__cpdma_chan_free(chan, desc, outlen, cb_status);
> 	return status;
> 
> unlock_ret:
> 	spin_unlock_irqrestore(&chan->lock, flags);
> 	return status;
> 
> And:
> __cpdma_chan_free(chan, desc, outlen, cb_status);
> 	-> cpdma_desc_free(pool, desc, 1);
> 
> As result, queue deadlock as you've described.
> Just thought, not checked, but theoretically possible.
> What do you think?

Better explanation, for rare race:

start conditions:
- all descs are submitted, except last one, and queue is not stopped
- any desc was returned to the pool yet (but packets can be sent)

    time
     ||
     \/

submit process				NAPI poll process
--------------------------------------------------------------------------------
new packet is scheduled for submission
stated that no free descs (with locks)
lock is freed
					returned all descs to the pool
					and queue is enabled
					interrupt enabled, poll exit
queue is disabled
submit exit

Result:
- all descs are returned to the pool, submission to the queue disabled
- NAPI cannot wake queue, as all desc were handled already

According to packet size in 200B
Data size, bits: 200B * 63desc * 10 = 128000bit roughly
Time all of them are sent: 128000 / 1Gb = 128us

That's enough the CPU to be occupied by other process in RT even.

> 
> > 
> > Fix this, by adding smp_mb__after_atomic() after netif_tx_stop_queue()
> > calls and double check for free TX descriptors after stopping ndev TX queue
> > - if there are free TX descriptors wake up ndev TX queue.
> > 
> > [1] https://www.kernel.org/doc/html/latest/core-api/atomic_ops.html
> > Signed-off-by: Grygorii Strashko <[email-masked]>
> > ---
> >  drivers/net/ethernet/ti/cpsw.c | 16 ++++++++++++++--
> >  1 file changed, 14 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/net/ethernet/ti/cpsw.c b/drivers/net/ethernet/ti/cpsw.c
> > index 10d7cbe..3805b13 100644
> > --- a/drivers/net/ethernet/ti/cpsw.c
> > +++ b/drivers/net/ethernet/ti/cpsw.c
> > @@ -1638,6 +1638,7 @@ static netdev_tx_t cpsw_ndo_start_xmit(struct sk_buff *skb,
> >  		q_idx = q_idx % cpsw->tx_ch_num;
> >  
> >  	txch = cpsw->txv[q_idx].ch;
> > +	txq = netdev_get_tx_queue(ndev, q_idx);
> >  	ret = cpsw_tx_packet_submit(priv, skb, txch);
> >  	if (unlikely(ret != 0)) {
> >  		cpsw_err(priv, tx_err, "desc submit failed\n");
> > @@ -1648,15 +1649,26 @@ static netdev_tx_t cpsw_ndo_start_xmit(struct sk_buff *skb,
> >  	 * tell the kernel to stop sending us tx frames.
> >  	 */
> >  	if (unlikely(!cpdma_check_free_tx_desc(txch))) {
> > -		txq = netdev_get_tx_queue(ndev, q_idx);
> >  		netif_tx_stop_queue(txq);
> > +
> > +		/* Barrier, so that stop_queue visible to other cpus */
> > +		smp_mb__after_atomic();
> > +
> > +		if (cpdma_check_free_tx_desc(txch))
> > +			netif_tx_wake_queue(txq);
> >  	}
> >  
> >  	return NETDEV_TX_OK;
> >  fail:
> >  	ndev->stats.tx_dropped++;
> > -	txq = netdev_get_tx_queue(ndev, skb_get_queue_mapping(skb));
> >  	netif_tx_stop_queue(txq);
> > +
> > +	/* Barrier, so that stop_queue visible to other cpus */
> > +	smp_mb__after_atomic();
> > +
> > +	if (cpdma_check_free_tx_desc(txch))
> > +		netif_tx_wake_queue(txq);
> > +
> >  	return NETDEV_TX_BUSY;
> >  }
> >  
> > -- 
> > 2.10.5
> > 
> > --
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> 
> -- 
> Regards,
> Ivan Khoronzhuk

-- 
Regards,
Ivan Khoronzhuk