linux-kernel - [RFC PATCH net] net: ipconfig: Release the rtnl

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Message-Id: <20211027131953.9270-1-maxime.chevallier@bootlin.com>
Date:   Wed, 27 Oct 2021 15:19:53 +0200
From:   Maxime Chevallier <maxime.chevallier@...tlin.com>
To:     davem@...emloft.net, Russell King <linux@...linux.org.uk>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     Maxime Chevallier <maxime.chevallier@...tlin.com>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        thomas.petazzoni@...tlin.com
Subject: [RFC PATCH net] net: ipconfig: Release the rtnl_lock while waiting for carrier

While waiting for a carrier to come on one of the netdevices, some
devices will require to take the rtnl lock at some point to fully
initialize all parts of the link.

That's the case for SFP, where the rtnl is taken when a module gets
detected. This prevents mounting an NFS rootfs over an SFP link.

This means that while ipconfig waits for carriers to be detected, no SFP
modules can be detected in the meantime, it's only detected after
ipconfig times out.

This commit releases the rtnl_lock while waiting for the carrier to come
up, and re-takes it to check the for the init device and carrier status.

At that point, the rtnl_lock seems to be only protecting
ic_is_init_dev().

Fixes: 73970055450e ("sfp: add SFP module support")
Signed-off-by: Maxime Chevallier <maxime.chevallier@...tlin.com>
---
I've sent this patch as an RFC (it doesn't look very clean indeed), since I'm
not fully familiar with the implications of modifying the locking scheme at
that point in the boot process. Please feel free to comment or suggest other
approaches.

 net/ipv4/ipconfig.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/net/ipv4/ipconfig.c b/net/ipv4/ipconfig.c
index 816d8aad5a68..069ae05bd0a5 100644
--- a/net/ipv4/ipconfig.c
+++ b/net/ipv4/ipconfig.c
@@ -278,7 +278,12 @@ static int __init ic_open_devs(void)
 			if (ic_is_init_dev(dev) && netif_carrier_ok(dev))
 				goto have_carrier;

+		/* Give a chance to do complex initialization that
+		 * would require to take the rtnl lock.
+		 */
+		rtnl_unlock();
 		msleep(1);
+		rtnl_lock();

 		if (time_before(jiffies, next_msg))
 			continue;
-- 
2.25.4