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:

quoted

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.

quoted

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@gmail.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.

quoted

 	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