netdev - RE: [PATCH 3/4] rhashtable: use bit_spin

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Date:   Wed, 03 Apr 2019 08:10:32 +1100
From:   NeilBrown <neilb@...e.com>
To:     David Laight <David.Laight@...LAB.COM>,
        Thomas Graf <tgraf@...g.ch>,
        Herbert Xu <herbert@...dor.apana.org.au>
Cc:     "netdev\@vger.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 3/4] rhashtable: use bit_spin_locks to protect hash bucket.

On Tue, Apr 02 2019, David Laight wrote:

> From: NeilBrown
>> Sent: 02 April 2019 00:08
>> 
>> This patch changes rhashtables to use a bit_spin_lock on BIT(1) of the
>> bucket pointer to lock the hash chain for that bucket.
> ...
>> To enhance type checking, a new struct is introduced to represent the
>>   pointer plus lock-bit
>> that is stored in the bucket-table.  This is "struct rhash_lock_head"
>> and is empty.  A pointer to this needs to be cast to either an
>> unsigned lock, or a "struct rhash_head *" to be useful.
>> Variables of this type are most often called "bkt".
>
> Did you try using a union of the pointer and an 'unsigned long' ?
> Should remove a lot of the casts.

It might, but I'm not sure it is what we want.
The value is not an unsigned long OR a pointer, it is both blended
together.  So it really isn't a union.
We *want* it to require casts to access, so that it is clear that
something unusual is happening, and care is needed.

Thanks,
NeilBrown

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