linux-kernel - Re: [PATCH net-next 2/3] netlink: Convert netlink

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Message-Id: <20140804.195841.536725238366145203.davem@davemloft.net>
Date:	Mon, 04 Aug 2014 19:58:41 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	sasha.levin@...cle.com
Cc:	tgraf@...g.ch, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, kaber@...sh.net,
	paulmck@...ux.vnet.ibm.com, josh@...htriplett.org,
	challa@...ronetworks.com, walpole@...pdx.edu, dev@...nvswitch.org,
	netfilter-devel@...r.kernel.org, nikolay@...hat.com
Subject: Re: [PATCH net-next 2/3] netlink: Convert netlink_lookup() to use
 RCU protected hash table

From: Sasha Levin <sasha.levin@...cle.com>
Date: Mon, 04 Aug 2014 22:10:19 -0400

> On 08/02/2014 05:47 AM, Thomas Graf wrote:
>>  static void *netlink_seq_start(struct seq_file *seq, loff_t *pos)
>> -	__acquires(nl_table_lock)
>>  {
>> -	read_lock(&nl_table_lock);
>> +	rcu_read_lock();
>>  	return *pos ? netlink_seq_socket_idx(seq, *pos - 1) : SEQ_START_TOKEN;
>>  }
> 
> I'm not sure how you expect this code to work. You're replacing a local lock
> with a RCU critical section. Imagine you're doing spin_lock() and just going
> back to userspace.
> 
> It's quite easy to trigger this issue:

I think he expected the end of the seq sequence to drop the RCU lock,
via netlink_seq_stop().
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