netdev - Re: [PATCH net v3] ravb: Fix races between ravb_tx_timeout

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Date: Thu, 16 Nov 2023 21:21:59 +0300
From: Sergey Shtylyov <s.shtylyov@....ru>
To: Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>,
	"davem@...emloft.net" <davem@...emloft.net>, "edumazet@...gle.com"
	<edumazet@...gle.com>, "kuba@...nel.org" <kuba@...nel.org>,
	"pabeni@...hat.com" <pabeni@...hat.com>
CC: "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-renesas-soc@...r.kernel.org" <linux-renesas-soc@...r.kernel.org>,
	Simon Horman <horms@...nel.org>
Subject: Re: [PATCH net v3] ravb: Fix races between ravb_tx_timeout_work() and
 net related ops

On 11/16/23 5:43 AM, Yoshihiro Shimoda wrote:
[...]

>>> Fix races between ravb_tx_timeout_work() and functions of net_device_ops
>>> and ethtool_ops by using rtnl_trylock() and rtnl_unlock(). Note that
>>> since ravb_close() is under the rtnl lock and calls cancel_work_sync(),
>>> ravb_tx_timeout_work() should calls rtnl_trylock(). Otherwise, a deadlock
>>> may happen in ravb_tx_timeout_work() like below:
>>>
>>> CPU0			CPU1
>>> 			ravb_tx_timeout()
>>> 			schedule_work()
>>> ...
>>> __dev_close_many()
>>> // Under rtnl lock
>>> ravb_close()
>>> cancel_work_sync()
>>> // Waiting
>>> 			ravb_tx_timeout_work()
>>> 			rtnl_lock()
>>> 			// This is possible to cause a deadlock
>>>
>>> And, if rtnl_trylock() fails and the netif is still running,
>>> rescheduling the work with 1 msec delayed. So, using

   Ah, you say 1 ms here but 10 ms in the code! Not good... :-)

>>> schedule_delayed_work() instead of schedule_work().
>>>
>>> Fixes: c156633f1353 ("Renesas Ethernet AVB driver proper")
>>> Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>
>>> Reviewed-by: Sergey Shtylyov <s.shtylyov@....ru>
>>
>>    Hm, I haven't reviewed this version... :-)
> 
> Oops, I should have dropped the tag...
> 
>>> Reviewed-by: Simon Horman <horms@...nel.org>
>> [...]
>>> diff --git a/drivers/net/ethernet/renesas/ravb.h b/drivers/net/ethernet/renesas/ravb.h
>>> index e0f8276cffed..e9bb8ee3ba2d 100644
>>> --- a/drivers/net/ethernet/renesas/ravb.h
>>> +++ b/drivers/net/ethernet/renesas/ravb.h
>>> @@ -1081,7 +1081,7 @@ struct ravb_private {
>>>  	u32 cur_tx[NUM_TX_QUEUE];
>>>  	u32 dirty_tx[NUM_TX_QUEUE];
>>>  	struct napi_struct napi[NUM_RX_QUEUE];
>>> -	struct work_struct work;
>>> +	struct delayed_work work;
>>
>>    Not sure this is justified...
>>    Then again, what do I know about workqueues? Not much... :-)
> 
> I thought that the schedule_work() called the work function immediately.
> So, I thought call*ing the schedule_work() from the work function caused
> endless loop. However, it is not true. The schedule_work() just inserts
> a work queue, and then the kernel calls the work function later.
> 
> So, changing from work_struct to delayed_work is not needed for fixing
> this issue, I think now. However, I have another concern about rescheduling
> this work by schedule_work() here because it's possible to cause high CPU load
> while the rtnl_lock() is held. So, I think we should call a sleep function
> like usleep_range(1000, 2000) for instance before schedule_work().
> But, what do you think?

   I think that a sleep before requeuing is pretty much the same as using
a delayed work...

>> [...]
>>> diff --git a/drivers/net/ethernet/renesas/ravb_main.c b/drivers/net/ethernet/renesas/ravb_main.c
>>> index c70cff80cc99..ca7db8a5b412 100644
>>> --- a/drivers/net/ethernet/renesas/ravb_main.c
>>> +++ b/drivers/net/ethernet/renesas/ravb_main.c
>>> @@ -1863,17 +1863,24 @@ static void ravb_tx_timeout(struct net_device *ndev, unsigned int txqueue)
>>>  	/* tx_errors count up */
>>>  	ndev->stats.tx_errors++;
>>>
>>> -	schedule_work(&priv->work);
>>> +	schedule_delayed_work(&priv->work, 0);
>>>  }
>>>
>>>  static void ravb_tx_timeout_work(struct work_struct *work)
>>>  {
>>> -	struct ravb_private *priv = container_of(work, struct ravb_private,
>>> +	struct delayed_work *dwork = to_delayed_work(work);
>>> +	struct ravb_private *priv = container_of(dwork, struct ravb_private,
>>>  						 work);
>>>  	const struct ravb_hw_info *info = priv->info;
>>>  	struct net_device *ndev = priv->ndev;
>>>  	int error;
>>>
>>> +	if (!rtnl_trylock()) {
>>> +		if (netif_running(ndev))
>>> +			schedule_delayed_work(&priv->work, msecs_to_jiffies(10));

  You could reuse dwork instead of &priv->work here...

>>    The delay is rather arbitrary. Why not e.g. 1 ms?
> 
> I think that 1 ms is enough.

   Seeing now that 1 ms was intended...

> Best regards,
> Yoshihiro Shimoda

[...]

MBR, Sergey