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

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Date:   Sun, 06 May 2018 08:00:49 +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 Sat, May 05 2018, Herbert Xu wrote:

> On Fri, May 04, 2018 at 01:54:14PM +1000, NeilBrown wrote:
>> rhashtable_try_insert() currently hold a lock on the bucket in
>> the first table, while also locking buckets in subsequent tables.
>> This is unnecessary and looks like a hold-over from some earlier
>> version of the implementation.
>> 
>> As insert and remove always lock a bucket in each table in turn, and
>> as insert only inserts in the final table, there cannot be any races
>> that are not covered by simply locking a bucket in each table in turn.
>> 
>> When an insert call reaches that last table it can be sure that there
>> is no match entry in any other table as it has searched them all, and
>> insertion never happens anywhere but in the last table.  The fact that
>> code tests for the existence of future_tbl while holding a lock on
>> the relevant bucket ensures that two threads inserting the same key
>> will make compatible decisions about which is the "last" table.
>> 
>> This simplifies the code and allows the ->rehash field to be
>> discarded.
>> We still need a way to ensure that a dead bucket_table is never
>> re-linked by rhashtable_walk_stop().  This can be achieved by
>> setting the ->size to 1.  This still allows lookup code to work (and
>> correctly not find anything) but can never happen on an active bucket
>> table (as the minimum size is 4).
>> 
>> Signed-off-by: NeilBrown <neilb@...e.com>
>
> I'm not convinced this is safe.  There can be multiple tables
> in existence.  Unless you hold the lock on the first table, what
> is to stop two parallel inserters each from inserting into their
> "last" tables which may not be the same table?

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. 

Thanks,
NeilBrown

>
> Cheers,
> -- 
> Email: Herbert Xu <herbert@...dor.apana.org.au>
> Home Page: http://gondor.apana.org.au/~herbert/
> PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

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