netdev - Re: [PATCH net] net: systemport: Fix 64-bit stats deadlock

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Date:   Tue, 12 Sep 2017 14:48:25 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, edumazet@...gle.com,
        jqiaoulk@...il.com
Subject: Re: [PATCH net] net: systemport: Fix 64-bit stats deadlock

On 09/12/2017 02:38 PM, Eric Dumazet wrote:
> On Tue, 2017-09-12 at 13:14 -0700, Florian Fainelli wrote:
>> We can enter a deadlock situation because there is no sufficient protection
>> when ndo_get_stats64() runs in process context to guard against RX or TX NAPI
>> contexts running in softirq, this can lead to the following lockdep splat and
>> actual deadlock was experienced as well with an iperf session in the background
>> and a while loop doing ifconfig + ethtool.
> 
>> So just remove the u64_stats_update_begin()/end() pair in ndo_get_stats64()
>> since it does not appear to be useful for anything. No inconsistency was
>> observed with either ifconfig or ethtool, global TX counts equal the sum of
>> per-queue TX counts on a 32-bit architecture.
>>
>> Fixes: 10377ba7673d ("net: systemport: Support 64bit statistics")
>> Signed-off-by: Florian Fainelli <f.fainelli@...il.com>
>> ---
>>  drivers/net/ethernet/broadcom/bcmsysport.c | 3 ---
>>  1 file changed, 3 deletions(-)
>>
>> diff --git a/drivers/net/ethernet/broadcom/bcmsysport.c b/drivers/net/ethernet/broadcom/bcmsysport.c
>> index a6572b51435a..c3c53f6cd9e6 100644
>> --- a/drivers/net/ethernet/broadcom/bcmsysport.c
>> +++ b/drivers/net/ethernet/broadcom/bcmsysport.c
>> @@ -1735,11 +1735,8 @@ static void bcm_sysport_get_stats64(struct net_device *dev,
>>  		stats->tx_packets += tx_packets;
>>  	}
>>  
>> -	/* lockless update tx_bytes and tx_packets */
>> -	u64_stats_update_begin(&priv->syncp);
> 
> Yes, this u64_stats_update_begin()/u64_stats_update_end() is bogus
> 
> But why do we even write on tx_bytes/tx_packets here ??? 

That's for the ethtool -S netdev stats copy (that's on me, I added that
in the driver initial version), so yes, not very robust...

> 
> Seems very wrong anyway.
> 
> (ethtool -S does not call bcm_sysport_get_stats64() to refresh them )

Yes that might actually be the simplest way to get this fixed.

> 
>>  	stats64->tx_bytes = stats->tx_bytes;
>>  	stats64->tx_packets = stats->tx_packets;
>> -	u64_stats_update_end(&priv->syncp);
>>  
>>  	do {
>>  		start = u64_stats_fetch_begin_irq(&priv->syncp);
> 
> 


-- 
Florian