linux-kernel - Re: [PATCH v2] rcu: fix a race in hlist_nulls_for_each_entry

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Message-ID: <519CCE49.4050308@yandex-team.ru>
Date:	Wed, 22 May 2013 17:55:21 +0400
From:	Roman Gushchin <klamm@...dex-team.ru>
To:	David Laight <David.Laight@...LAB.COM>
CC:	Eric Dumazet <eric.dumazet@...il.com>, paulmck@...ux.vnet.ibm.com,
	Dipankar Sarma <dipankar@...ibm.com>, zhmurov@...dex-team.ru,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>,
	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 v2] rcu: fix a race in hlist_nulls_for_each_entry_rcu
 macro

On 22.05.2013 17:27, David Laight wrote:
>> So yes, the patch appears to fix the bug, but it sounds not logical to
>> me.
>
> I was confused because the copy of the code I found was different
> (it has some checks for reusaddr - which force a function call in the
> loop).
>
> The code being compiled is:
>
> begin:
> 	result = NULL;
> 	badness = -1;
> 	udp_portaddr_for_each_entry_rcu(sk, node, &hslot2->head) {
> 		score = compute_score2(sk, net, saddr, sport,
> 				      daddr, hnum, dif);
> 		if (score > badness) {
> 			result = sk;
> 			badness = score;
> 			if (score == SCORE2_MAX)
> 				goto exact_match;
> 		}
> 	}
> 	/*
> 	 * if the nulls value we got at the end of this lookup is
> 	 * not the expected one, we must restart lookup.
> 	 * We probably met an item that was moved to another chain.
> 	 */
> 	if (get_nulls_value(node) != slot2)
> 		goto begin;
>
> Which is entirely inlined - so the compiler is allowed to assume
> that no other code modifies any of the data.
> Hence it is allowed to cache the list head value.
> Indeed it could convert the last line to "for (;;);".
>
> A asm volatile ("":::"memory") somewhere would fix it.
>

Yes, it does. It was my first attempt to fix the issue.
But putting such instructions into each such cycle isn't a good idea, IMHO.

Regards,
Roman

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