linux-kernel - Re: [PATCH 4/8] rhashtable: fix race in nested_table

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Message-ID: <87efiocujf.fsf@notabene.neil.brown.name>
Date:   Mon, 07 May 2018 08:02:12 +1000
From:   NeilBrown <neilb@...e.com>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     Thomas Graf <tgraf@...g.ch>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/8] rhashtable: fix race in nested_table_alloc()

On Sun, May 06 2018, Herbert Xu wrote:

> On Sun, May 06, 2018 at 07:48:20AM +1000, NeilBrown wrote:
>>
>> The spinlock protects 2 or more buckets.  The nested table contains at
>> least 512 buckets, maybe more.
>> It is quite possible for two insertions into 2 different buckets to both
>> get their spinlock and both try to instantiate the same nested table.
>
> I think you missed the fact that when we use nested tables the spin
> lock table is limited to just a single page and hence corresponds
> to the first level in the nested table.  Therefore it's always safe.

Yes I had missed that - thanks for pointing it out.
In fact the lock table is limited to the number of nested_tables
in the second level.
And it is the same low-order bits that choose both the lock
and the set of nested tables.
So there isn't a bug here.  So we don't need this patch. (I still like
it though - it seems more obviously correct).

Thanks,
NeilBrown

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