linux-kernel - Re: [PATCH] rcu: fix a race in hlist_nulls_for_each_entry

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Date:	Tue, 21 May 2013 15:55:05 +0400
From:	Roman Gushchin <klamm@...dex-team.ru>
To:	David Laight <David.Laight@...LAB.COM>
CC:	Dipankar Sarma <dipankar@...ibm.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	zhmurov@...dex-team.ru, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>
Subject: Re: [PATCH] rcu: fix a race in hlist_nulls_for_each_entry_rcu macro

On 21.05.2013 14:40, David Laight wrote:
>> Some network functions (udp4_lib_lookup2(), for instance) use the
>> hlist_nulls_for_each_entry_rcu macro in a way that assumes restarting
>> of a loop. In this case, it is strictly necessary to reread the head->first
>> value from the memory before each scan.
>> Without additional hints, gcc caches this value in a register. In this case,
>> if a cached node is moved to another chain during the scan, we can loop
>> forever getting wrong nulls values and restarting the loop uninterruptedly.
>
> Hmmm.... if either inet_ehashfn() or next_pseudo_random32() is
> called gcc must reread it anyway.
> I'm surprised gcc is generating separate code for all the conditional
> loop endings. So why is it caching head->first.
> The 'list empty' might be short-circuited - but that would only
> be relevant after a rescan.
> I suspect something else is going on.

What do you mean?

> I'd also have thought that this code needs to scan the entire
> hash list. If things are moved under its feet this won't happen.
> If it can end up on a different list (because a node got moved)
> it is also possible for a later node to move it back.
> In that case it would end up on the correct list

Things are always moved to the head of the list, so, it's not a problem.

> ...
>> -#define hlist_nulls_first_rcu(head) \
>> -	(*((struct hlist_nulls_node __rcu __force **)&(head)->first))
>> +#define hlist_nulls_first_rcu(head)			\
>> +	(*((struct hlist_nulls_node __rcu __force **)	\
>> +	   &((volatile typeof(*head) *)head)->first))
>
> I'd have thought it would be better to change hlist_nulls_first_rcu().

It's exactly what I suggest. May be I miss something? Please, clarify.

Regards,
Roman
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