[PATCH net] net: qualcomm: rmnet: don't over-count statistics

[PATCH net] net: qualcomm: rmnet: don't over-count statistics

[Alex Elder]

The purpose of the loop using u64_stats_fetch_*_irq() is to ensure
statistics on a given CPU are collected atomically. If one of the
statistics values gets updated within the begin/retry window, the
loop will run again.

Currently the statistics totals are updated inside that window.
This means that if the loop ever retries, the statistics for the
CPU will be counted more than once.

Fix this by taking a snapshot of a CPU's statistics inside the
protected window, and then updating the counters with the snapshot
values after exiting the loop.

(Also add a newline at the end of this file...)

Fixes: 192c4b5d48f2a ("net: qualcomm: rmnet: Add support for 64 bit stats")
Signed-off-by: Alex Elder <elder@linaro.org>
---
 .../net/ethernet/qualcomm/rmnet/rmnet_vnd.c    | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/drivers/net/ethernet/qualcomm/rmnet/rmnet_vnd.c b/drivers/net/ethernet/qualcomm/rmnet/rmnet_vnd.c
index fe13017e9a41e..06a74fa0d575d 100644
--- a/drivers/net/ethernet/qualcomm/rmnet/rmnet_vnd.c
+++ b/drivers/net/ethernet/qualcomm/rmnet/rmnet_vnd.c
@@ -126,24 +126,24 @@ static void rmnet_get_stats64(struct net_device *dev,
 			      struct rtnl_link_stats64 *s)
 {
 	struct rmnet_priv *priv = netdev_priv(dev);
-	struct rmnet_vnd_stats total_stats;
+	struct rmnet_vnd_stats total_stats = { };
 	struct rmnet_pcpu_stats *pcpu_ptr;
+	struct rmnet_vnd_stats snapshot;
 	unsigned int cpu, start;
 
-	memset(&total_stats, 0, sizeof(struct rmnet_vnd_stats));
-
 	for_each_possible_cpu(cpu) {
 		pcpu_ptr = per_cpu_ptr(priv->pcpu_stats, cpu);
 
 		do {
 			start = u64_stats_fetch_begin_irq(&pcpu_ptr->syncp);
-			total_stats.rx_pkts += pcpu_ptr->stats.rx_pkts;
-			total_stats.rx_bytes += pcpu_ptr->stats.rx_bytes;
-			total_stats.tx_pkts += pcpu_ptr->stats.tx_pkts;
-			total_stats.tx_bytes += pcpu_ptr->stats.tx_bytes;
+			snapshot = pcpu_ptr->stats;	/* struct assignment */
 		} while (u64_stats_fetch_retry_irq(&pcpu_ptr->syncp, start));
 
-		total_stats.tx_drops += pcpu_ptr->stats.tx_drops;
+		total_stats.rx_pkts += snapshot.rx_pkts;
+		total_stats.rx_bytes += snapshot.rx_bytes;
+		total_stats.tx_pkts += snapshot.tx_pkts;
+		total_stats.tx_bytes += snapshot.tx_bytes;
+		total_stats.tx_drops += snapshot.tx_drops;
 	}
 
 	s->rx_packets = total_stats.rx_pkts;
@@ -355,4 +355,4 @@ int rmnet_vnd_update_dev_mtu(struct rmnet_port *port,
 	}
 
 	return 0;
-}
\ No newline at end of file
+}
-- 
2.27.0

Re: [PATCH net] net: qualcomm: rmnet: don't over-count statistics

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net.git (refs/heads/master):

On Fri, 11 Jun 2021 13:26:00 -0500 you wrote:
> The purpose of the loop using u64_stats_fetch_*_irq() is to ensure
> statistics on a given CPU are collected atomically. If one of the
> statistics values gets updated within the begin/retry window, the
> loop will run again.
> 
> Currently the statistics totals are updated inside that window.
> This means that if the loop ever retries, the statistics for the
> CPU will be counted more than once.
> 
> [...]

Here is the summary with links:
  - [net] net: qualcomm: rmnet: don't over-count statistics
    https://git.kernel.org/netdev/net/c/994c393bb688

You are awesome, thank you!
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