net: ethernet: ti: cpsw: fix net watchdog timeout

Message ID 20180207011706.13393-1-grygorii.strashko@ti.com
State Accepted
Delegated to: David Miller
Headers show
Series
  • net: ethernet: ti: cpsw: fix net watchdog timeout
Related show

Commit Message

Grygorii Strashko Feb. 7, 2018, 1:17 a.m.
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.

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 <grygorii.strashko@ti.com>
---
 drivers/net/ethernet/ti/cpsw.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

Comments

Ivan Khoronzhuk Feb. 7, 2018, 3:03 a.m. | #1
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?

> 
> 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 <grygorii.strashko@ti.com>
> ---
>  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
> 
> --
> To unsubscribe from this list: send the line "unsubscribe linux-omap" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
Ivan Khoronzhuk Feb. 7, 2018, 1:31 p.m. | #2
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 <grygorii.strashko@ti.com>
> > ---
> >  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
> > 
> > --
> > To unsubscribe from this list: send the line "unsubscribe linux-omap" in
> > the body of a message to majordomo@vger.kernel.org
> > More majordomo info at  http://vger.kernel.org/majordomo-info.html
> 
> -- 
> Regards,
> Ivan Khoronzhuk
Grygorii Strashko Feb. 7, 2018, 6:19 p.m. | #3
On 02/07/2018 07:31 AM, Ivan Khoronzhuk wrote:
> 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] barrier 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?

Yep. And if this happens desc_alloc_fail should be >0 while i usually see it 0 when 
watchdog triggers.
I was able to reproduce this situation once, but with bunch of debug code added
which changes timings:

Prerequisite: only one free desc available.
 cpsw_ndo_start_xmit1				cpsw_tx_poll
  prepare and send packet 
  ->Free desc queue is empty at this moment
 if (unlikely(!cpdma_check_free_tx_desc(txch)))
 	netif_tx_stop_queue(txq);
 --- tx queue stopped
						cpdma_chan_process()
							spin_lock_irqsave(&chan->lock, flags);
 							chan->count--
							spin_unlock_irqrestore(&chan->lock, flags)

							cpsw_tx_handler()
							   if (unlikely(netif_tx_queue_stopped(txq)))
								netif_tx_wake_queue(txq);
 --- tx queue is woken up
 cpsw_ndo_start_xmit2
  prepare packet
 ret = cpsw_tx_packet_submit(priv, skb, txch);
 //fail as desc not returned to the pool yet
	if (unlikely(ret != 0)) {
		cpsw_err(priv, tx_err, "desc submit failed\n");
		goto fail;
	}
							cpdma_desc_free()	

fail:
	ndev->stats.tx_dropped++;
	netif_tx_stop_queue(txq);
// oops.

That's why I added double check and barrier in fail path also
 

> 
> 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.
> 

First, in case of RT, it's not a "rare race" as all processing is done in threads
and both xmit and cpsw_tx_poll can be kicked out any time -
in case of xmit right before netif_tx_stop_queue().

It also better reproducible with low speed as xmit drains Free desc pool faster.

Next, on SMP it not only execution race as netif_tx_stop_queue() is just set_bit(), so
new value might not be visible to other cpus and due to netif_tx_queue_stopped(txq) check 
cpsw_tx_handler() might fail to wake up the queue.

By the way, I'm not inventing smth new here - I've checked bunch of net drivers and found
that a lot of them have exactly the same  netif_tx_stop_queue/netif_tx_wake_queue handling.

Do you want me to update description or there are other comments?
Ivan Khoronzhuk Feb. 7, 2018, 11:07 p.m. | #4
On Wed, Feb 07, 2018 at 12:19:11PM -0600, Grygorii Strashko wrote:
>On 02/07/2018 07:31 AM, Ivan Khoronzhuk wrote:
>> 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] barrier 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?
>
>Yep. And if this happens desc_alloc_fail should be >0 while i usually see it 0 when
>watchdog triggers.
It has to be 0 for situation I trying to describe.
See below re example for keeping all in one place

>I was able to reproduce this situation once, but with bunch of debug code added
>which changes timings:
I also unintentionally observed it several times but usually was thinking
that it was connected with my other experiments. It was with am572x.
But now seems it actually can happen with plane sources. And maybe here not
only 1 fix is needed, that's my concern.

>
>Prerequisite: only one free desc available.
> cpsw_ndo_start_xmit1				cpsw_tx_poll
>  prepare and send packet
>  ->Free desc queue is empty at this moment
> if (unlikely(!cpdma_check_free_tx_desc(txch)))
> 	netif_tx_stop_queue(txq);
> --- tx queue stopped
>						cpdma_chan_process()
>							spin_lock_irqsave(&chan->lock, flags);
> 							chan->count--
>							spin_unlock_irqrestore(&chan->lock, flags)
>
>							cpsw_tx_handler()
>							   if (unlikely(netif_tx_queue_stopped(txq)))
>								netif_tx_wake_queue(txq);
> --- tx queue is woken up
> cpsw_ndo_start_xmit2
>  prepare packet
> ret = cpsw_tx_packet_submit(priv, skb, txch);
> //fail as desc not returned to the pool yet
>	if (unlikely(ret != 0)) {
>		cpsw_err(priv, tx_err, "desc submit failed\n");
>		goto fail;
>	}
>							cpdma_desc_free()	
>
>fail:
>	ndev->stats.tx_dropped++;
>	netif_tx_stop_queue(txq);
>// oops.
>
>That's why I added double check and barrier in fail path also
Seems to be reasonable. It happens along with printing "desc submit failed\n",
so can be coughs easily. Did you observe it? I have no objection, but
implementation prompts that smth wrong in order and locking and rather covers
one usecase than solves issue itself, parts of what could be came out later.
Did you considered case with checking number of descs before submitting,
leaving say 1 desc for later submit, and submit then, and all this under
channel lock, or doing all this stuff in cpsw_submit under channel lock and 
using some analog of cpdma_submit with return codes and w/o lock. Then likely
stop/wake/check queue also can be under channel lock and probably no need in
smp_mb__after_atomic() then, no sure.

>
>
>>
>> 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.
>>
>
>First, in case of RT, it's not a "rare race" as all processing is done in threads
>and both xmit and cpsw_tx_poll can be kicked out any time -
>in case of xmit right before netif_tx_stop_queue().
sure, processing is done always in threads, but it happens frequently
with your good test case :).

>
>It also better reproducible with low speed as xmit drains Free desc pool faster.
Low speed of interface? Strange. Seems you have good test case. Could be
useful to check version with correct channel locking.

>
>Next, on SMP it not only execution race as netif_tx_stop_queue() is just set_bit(), so
>new value might not be visible to other cpus and due to netif_tx_queue_stopped(txq) check
>cpsw_tx_handler() might fail to wake up the queue.
I have nothing against smp_mb__after_atomic(), just want to show that issue can
have different sources not only caused by not "atomicity" of bitwise operation,
critical section is a little wider here.

>
>By the way, I'm not inventing smth new here - I've checked bunch of net drivers and found
>that a lot of them have exactly the same  netif_tx_stop_queue/netif_tx_wake_queue handling.
Let it be, if it covers all potential races, not sure if can cover all cases for
cpsw.

>
>Do you want me to update description or there are other comments?
Seems here some misunderstanding, above description are not for your fix, above
description is for alternative reason why queue deadlock can happen, actually
here can be two cases, your and one more. Let me rewrite it a little:

Prerequisite: only one free desc available.

cpsw_ndo_start_xmit				cpsw_tx_poll
--------------------------------------------------------------------------------
prepare and send packet
->Free desc queue is empty at this moment
if (unlikely(!cpdma_check_free_tx_desc(txch)))
--- tx queue is not stopped yet, but decision to stop is made

						->All packets were sent to the network at this moment
						for() {
						   cpdma_chan_process()
							spin_lock_irqsave(&chan->lock, flags);
 							chan->count--
							spin_unlock_irqrestore(&chan->lock, flags)

							cpsw_tx_handler()
							   if (unlikely(netif_tx_queue_stopped(txq)))
								netif_tx_wake_queue(txq);
						}
						->Returned all descs to the pool
						->handled all channels
						->cpsw_tx_poll exit

netif_tx_stop_queue(txq);
--- tx queue is disabled

Result:
- desc_alloc_fail = 0 (your main criteria)
- queue is disabled
- no one is going to wake queue
- watchdog is scheduled

>Do you want me to update description or there are other comments?
You can leave this patch as is, description is enough, if upper is not your
case...basically you've extended critical area and double checked pool,
masking other possible issue and removed only race for "tx_queue_stopped()"
flag check between CPUs. By a big account, it can never happen, but criteria of
check can cover not one source of issue and can mask others. If no lock or no
need in it, then memory barrier is only choice, as obviously, it has to be
added for existing code.

>
>-- 
>regards,
>-grygorii
Ivan Khoronzhuk Feb. 7, 2018, 11:30 p.m. | #5
Rechecked once again, seems it covers every case, at this moment, so

Reviewed-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
David Miller Feb. 8, 2018, 2:57 a.m. | #6
From: Grygorii Strashko <grygorii.strashko@ti.com>
Date: Tue, 6 Feb 2018 19:17:06 -0600

> 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.
> 
> 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 <grygorii.strashko@ti.com>

Applied, thanks.
Grygorii Strashko Feb. 8, 2018, 4:04 p.m. | #7
On 02/07/2018 08:57 PM, David Miller wrote:
> From: Grygorii Strashko <grygorii.strashko@ti.com>
> Date: Tue, 6 Feb 2018 19:17:06 -0600
> 
>> 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.
>>
>> 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 <grygorii.strashko@ti.com>
> 
> Applied, thanks.
> 

Thank you David.

Could this be marked as stable material 4.9+?
David Miller Feb. 8, 2018, 6:50 p.m. | #8
From: Grygorii Strashko <grygorii.strashko@ti.com>
Date: Thu, 8 Feb 2018 10:04:31 -0600

> Could this be marked as stable material 4.9+?

Sure, queued up.

Patch

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;
 }