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

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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

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