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

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Message-ID: <063D6719AE5E284EB5DD2968C1650D6D1CAC6969@AcuExch.aculab.com>
Date:	Tue, 13 Jan 2015 15:06:14 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Thomas Graf' <tgraf@...g.ch>
CC:	"davem@...emloft.net" <davem@...emloft.net>,
	Fengguang Wu <fengguang.wu@...el.com>, 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
...
> > >  		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.
> 
> I'm not claiming it's extremely pretty, lockless lookup with deferred
> resizing doesn't come for free ;-) If you have a suggestion on how to
> implement this differently I'm all ears.

runs away....

> That said, it's well isolated
> and the user of rhashtable does not have to deal with it. All code paths
> which take multiple locks are mutually exclusive to each other (ht->mutex).

OK, ht->mutes saves the day.
Might be worth a comment to save people looking at the code in isolation
from worrying and doing a bit search.
OTOH it might be obvious from a slightly larger fragment than the diff.

	David

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