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

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Message-ID: <aed6534b-ad5e-4f5c-9861-9a784968adcc@tuxon.dev>
Date: Wed, 10 Jan 2024 15:17:36 +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 08.01.2024 22:22, Sergey Shtylyov wrote:
> On 1/5/24 11:23 AM, Claudiu 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
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.

Thank you,
Claudiu Beznea

> 
>> +		return &ndev->stats;
>> +
>>  	nstats = &ndev->stats;
>>  	stats0 = &priv->stats[RAVB_BE];
>>  
> [...]
> 
> MBR, Sergey
>