netdev - [RFC PATCH net-next 11/13] net: sysfs: don't hold dev_base

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Date:   Mon,  7 Dec 2020 01:59:17 +0200
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     netdev@...r.kernel.org, Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Jiri Benc <jbenc@...hat.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Eric Dumazet <edumazet@...gle.com>,
        George McCollister <george.mccollister@...il.com>,
        Oleksij Rempel <o.rempel@...gutronix.de>,
        Christian Brauner <christian.brauner@...ntu.com>
Subject: [RFC PATCH net-next 11/13] net: sysfs: don't hold dev_base_lock while retrieving device statistics

In the effort of making .ndo_get_stats64 be able to sleep, we need to
ensure the callers of dev_get_stats do not use atomic context.

I need to preface this by saying that I have no idea why netstat_show
takes the dev_base_lock rwlock. Two things can be observed:
(a) it does not appear to be due to dev_isalive requiring it for some
    reason, because broadcast_show() also calls dev_isalive() and has
    had no problem existing since the beginning of git.
(b) the dev_get_stats function definitely does not need dev_base_lock
    protection either. In fact, holding the dev_base_lock is the entire
    problem here, because we want to make dev_get_stats sleepable, and
    holding a rwlock gives us atomic context.

So since no protection seems to be necessary, just run unlocked while
retrieving the /sys/class/net/eth0/statistics/* values.

Cc: Christian Brauner <christian.brauner@...ntu.com>
Cc: Eric Dumazet <edumazet@...gle.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@....com>
---
 net/core/net-sysfs.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/net/core/net-sysfs.c b/net/core/net-sysfs.c
index 999b70c59761..0782a476b424 100644
--- a/net/core/net-sysfs.c
+++ b/net/core/net-sysfs.c
@@ -585,14 +585,13 @@ static ssize_t netstat_show(const struct device *d,
 	WARN_ON(offset > sizeof(struct rtnl_link_stats64) ||
 		offset % sizeof(u64) != 0);
 
-	read_lock(&dev_base_lock);
 	if (dev_isalive(dev)) {
 		struct rtnl_link_stats64 temp;
 		const struct rtnl_link_stats64 *stats = dev_get_stats(dev, &temp);
 
 		ret = sprintf(buf, fmt_u64, *(u64 *)(((u8 *)stats) + offset));
 	}
-	read_unlock(&dev_base_lock);
+
 	return ret;
 }
 
-- 
2.25.1