netdev - Re: [PATCH net-next v3 17/19] net: ravb: Return cached statistics if the interface is down

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Date: Mon, 15 Jan 2024 08:08:22 +0200
From: claudiu beznea <claudiu.beznea@...on.dev>
To: Sergey Shtylyov <s.shtylyov@....ru>, davem@...emloft.net,
 edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com,
 richardcochran@...il.com, p.zabel@...gutronix.de,
 yoshihiro.shimoda.uh@...esas.com, wsa+renesas@...g-engineering.com
Cc: netdev@...r.kernel.org, linux-renesas-soc@...r.kernel.org,
 linux-kernel@...r.kernel.org, geert+renesas@...der.be,
 Claudiu Beznea <claudiu.beznea.uj@...renesas.com>
Subject: Re: [PATCH net-next v3 17/19] net: ravb: Return cached statistics if
 the interface is down



On 14.01.2024 14:22, Sergey Shtylyov wrote:
> On 1/10/24 4:17 PM, claudiu beznea wrote:
> 
> [...]
> 
>>>> From: Claudiu Beznea <claudiu.beznea.uj@...renesas.com>
>>>>
>>>> Return the cached statistics in case the interface is down. There should be
>>>> no drawback to this, as cached statistics are updated in ravb_close().
>>>>
>>>> The commit prepares the code for the addition of runtime PM support.
>>>>
>>>> Suggested-by: Sergey Shtylyov <s.shtylyov@....ru>
>>>> Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@...renesas.com>
>>> [...]
>>>
>>>> diff --git a/drivers/net/ethernet/renesas/ravb_main.c b/drivers/net/ethernet/renesas/ravb_main.c
>>>> index 76035afd4054..168b6208db37 100644
>>>> --- a/drivers/net/ethernet/renesas/ravb_main.c
>>>> +++ b/drivers/net/ethernet/renesas/ravb_main.c
>>>> @@ -2117,6 +2117,9 @@ static struct net_device_stats *ravb_get_stats(struct net_device *ndev)
>>>>  	const struct ravb_hw_info *info = priv->info;
>>>>  	struct net_device_stats *nstats, *stats0, *stats1;
>>>>  
>>>> +	if (!(ndev->flags & IFF_UP))
>>>
>>>    Well, I guess it's OK to read the counters in the reset mode... BUT
>>> won't this race with pm_runtime_put_autosuspend() when its call gets added
>>> to ravb_close()?
>>
>> I re-checked it and, yes, this is true. A sync runtime suspend would be
>> better here. But, as of my current investigation, even with this
> 
>    No, the sync form of the RPM call won't fix the race...
> 
>> ravb_get_stats() can still race with ravb_open()/ravb_close() as they are
>> called though different locking scheme (ravb_open()/ravb_close() is called
>> with rtnl locked while ravb_get_stats() can be called only with
>> dev_base_lock rwlock locked for reading).
>>
>> A mutex in the driver should to help with this.
> 
>    Why don't you want to mimic what the sh_eth driver does?

I thought it can be replaced by the already existing IFF_UP flag from
ndev->flags.

Investigating it further while trying to address this concurrency issue
made me realize that it fits to address the issue you mentioned here.

Thank you,
Claudiu Beznea

> 
>> Thank you,
>> Claudiu Beznea
> 
> [...]
> 
> MBR, Sergey