netdev - Re: [PATCH] [PATCH net] net: Do not hold the reference for the same sk_rx

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Date:   Thu, 16 Mar 2017 11:16:47 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     kaiwen.xu@...u.com
Cc:     netdev@...r.kernel.org
Subject: Re: [PATCH] [PATCH net] net: Do not hold the reference for the
 same sk_rx_dst.

From: Kevin Xu <kaiwen.xu@...u.com>
Date: Thu, 16 Mar 2017 01:08:30 -0700

> In some rare cases, inet_sk_rx_dst_set() may be called multiple times
> on the same dst, causing reference count leakage. Eventually, it
> prevents net_device to be destroyed. The bug then manifested as
> 
> unregister_netdevice: waiting for lo to become free. Usage count = 1
> 
> in the kernel log, preventing new network namespace creation.
> 
> The patch works around the issue by checking whether the socket already
> has the same dst set.
> 
> Signed-off-by: Kevin Xu <kaiwen.xu@...u.com>

You need to prevent this parallel execution of this function or use
atomic compare-and-exchange to set the rx_dst in order to prevent
the double refcounting.

This patch by itself is even worse than a workaround, because depending
upon how the compiler reloads values from memory, the problem can still
occur.