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

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Date: Sun, 14 Jan 2024 15:22:47 +0300
From: Sergey Shtylyov <s.shtylyov@....ru>
To: claudiu beznea <claudiu.beznea@...on.dev>, <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 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?

> Thank you,
> Claudiu Beznea

[...]

MBR, Sergey