netdev - RE: [PATCH net-next] rhashtable: Lower/upper bucket may map to same lock while shrinking

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <063D6719AE5E284EB5DD2968C1650D6D1CAC6593@AcuExch.aculab.com>
Date:	Tue, 13 Jan 2015 09:49:19 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Thomas Graf' <tgraf@...g.ch>,
	"davem@...emloft.net" <davem@...emloft.net>,
	Fengguang Wu <fengguang.wu@...el.com>
CC:	LKP <lkp@...org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"netfilter-devel@...r.kernel.org" <netfilter-devel@...r.kernel.org>,
	"coreteam@...filter.org" <coreteam@...filter.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [PATCH net-next] rhashtable: Lower/upper bucket may map to same
 lock while shrinking

From: Thomas Graf
> Each per bucket lock covers a configurable number of buckets. While
> shrinking, two buckets in the old table contain entries for a single
> bucket in the new table. We need to lock down both while linking.
> Check if they are protected by different locks to avoid a recursive
> lock.

Thought, could the shrunk table use the same locks as the lower half
of the old table?

I also wonder whether shrinking hash tables is ever actually worth
the effort. Most likely they'll need to grow again very quickly.

>  		spin_lock_bh(old_bucket_lock1);
> -		spin_lock_bh_nested(old_bucket_lock2, RHT_LOCK_NESTED);
> -		spin_lock_bh_nested(new_bucket_lock, RHT_LOCK_NESTED2);
> +
> +		/* Depending on the lock per buckets mapping, the bucket in
> +		 * the lower and upper region may map to the same lock.
> +		 */
> +		if (old_bucket_lock1 != old_bucket_lock2) {
> +			spin_lock_bh_nested(old_bucket_lock2, RHT_LOCK_NESTED);
> +			spin_lock_bh_nested(new_bucket_lock, RHT_LOCK_NESTED2);
> +		} else {
> +			spin_lock_bh_nested(new_bucket_lock, RHT_LOCK_NESTED);
> +		}

Acquiring 3 locks of much the same type looks like a locking hierarchy
violation just waiting to happen.

	David

--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html