linux-kernel - dead loop for rtnl

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Message-ID: <CY1PR11MB0491C022AD738CCC818BE74AAAAD0@CY1PR11MB0491.namprd11.prod.outlook.com>
Date:	Tue, 16 Feb 2016 10:31:53 +0000
From:	Xianpeng Zhao <xpzhao@...ohive.com>
To:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: dead loop for rtnl_trylock

Hi Group,

         I have find a problem in my system, I found there have a chance that cause the system enter dead loop when try to get the rtnl lock in the sysctl function in net/ipv6/addrconf.c

         The situation should like this, there are 2 processes may need get the rtnl lock, we call them process A and process B, A have high priority than B.
B need get the rtnl lock to do something, when B schedule out without release the lock; At this time, the A start to run "echo 1 > /proc/sys/net/ipv6/conf/<ifname>/disable_ipv6", the echo process will run to this code:

    if (!rtnl_trylock())

        return restart_syscall();

Because the rtnl lock was hold by process B, so here the try will be failure, and run the restart_syscall to let the sys_write do again, even try many times, because the B have very lower priority, the lock was hard to be released, so the echo process created by A will enter a loop of restart system call.

In my case it is the wireless_nlevent_process in process kworker taken the rtnl lock, and another higher priority process need use echo to disable IPv6 met this problem.

I am not very sure, but I think it is better to let the process A sleep a while instead of try it again and again without any delay.

Expects, what's your opinions?

@@ -5304,8 +5308,10 @@ static int addrconf_disable_ipv6(struct ctl_table *table, int *p, int newf)

        struct net *net;

        int old;

-       if (!rtnl_trylock())

+       if (!rtnl_trylock()){

+        schedule_timeout_uninterruptible(HZ/4);

                return restart_syscall();

+    }

        net = (struct net *)table->extra2;

        old = *p;