From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Miller <davem-fT/PcQaiUtIeIZ0/mPfg9Q@public.gmane.org>
Subject: Re: [PATCH net-next 2/3] netlink: Convert
 netlink_lookup() to use RCU protected hash table
Date: Mon, 04 Aug 2014 19:58:41 -0700 (PDT)
Message-ID: <20140804.195841.536725238366145203.davem@davemloft.net>
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From: Sasha Levin <sasha.levin-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>
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().