netdev - Re: [PATCH 8/8] rhashtable: don't hold lock on first table throughout insertion.

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Date:   Mon, 07 May 2018 08:24:41 +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 8/8] rhashtable: don't hold lock on first table throughout insertion.

On Sun, May 06 2018, Herbert Xu wrote:

> On Sun, May 06, 2018 at 08:00:49AM +1000, NeilBrown wrote:
>>
>> The insert function must (and does) take the lock on the bucket before
>> testing if there is a "next" table.
>> If one inserter finds that it has locked the "last" table (because there
>> is no next) and successfully inserts, then the other inserter cannot
>> have locked that table yet, else it would have inserted.  When it does,
>> it will find what the first inserter inserted. 
>
> If you release the lock to the first table then it may be deleted
> by the resize thread.  Hence the other inserter may not have even
> started from the same place.

This is true, but I don't see how it is relevant.
At some point, each thread will find that the table they have just
locked for their search key, has a NULL 'future_tbl' pointer.
At the point, the thread can know that the key is not in any table,
and that no other thread can add the key until the lock is released.

Thanks,
NeilBrown

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