linux-kernel - Re: [PATCH 10/18] rhashtable: remove rhashtable_walk

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Message-ID: <87r2lmnj2c.fsf@notabene.neil.brown.name>
Date:   Tue, 05 Jun 2018 11:00:43 +1000
From:   NeilBrown <neilb@...e.com>
To:     Tom Herbert <tom@...bertland.com>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        Thomas Graf <tgraf@...g.ch>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Tom Herbert <tom@...ntonium.net>
Subject: Re: [PATCH 10/18] rhashtable: remove rhashtable_walk_peek()

On Mon, Jun 04 2018, Tom Herbert wrote:

>>
>> Maybe a useful way forward would be for you to write documentation for
>> the rhashtable_walk_peek() interface which correctly describes what it
>> does and how it is used.  Given that, I can implement that interface
>> with the stability improvements that I'm working on.
>>
>
> Here's how it's documented currently:
>
> "rhashtable_walk_peek - Return the next object but don't advance the iterator"
>
> I don't see what is incorrect about that.

rhashtable_walk_next is documented:

 * rhashtable_walk_next - Return the next object and advance the iterator

So it seems reasonable to assume that you get the same object, no matter
which one you call.  Yet this is not (necessarily) the case.

>                                           Peek returns the next object
> in the walk, however does not move the iterator past that object, so
> sucessive calls to peek return the same object. In other words it's a
> way to inspect the next object but not "consume" it. This is what is
> needed when netlink returns in the middle of a walk. The last object
> retrieved from the table may not have been processed completely, so it
> needs to be the first one processed on the next invocation to netlink.

I completely agree with this last sentence.
We typically need to process the last object retrieved.  This could also
be described as the previously retrieved object.
So rhashtable_walk_last() and rhashtable_walk_prev() might both be
suitable names, though each is open to misinterpretation.
I fail to see how the "last object retrieved" could be the same as
"the next object" which rhashtable_walk_peek claims to return.

>
> This is also easily distinguishable from
>
> "rhashtable_walk_next - Return the next object and advance the iterator"
>
> Where the only difference is that peek and walk is that, walk advances
> the iterator and peek does not. Hence why "peek" is a descriptive name
> for what is happening.

Maybe if we step back and go for a totally different API.

We could change rhashtable_walk_start() to return the object that was
current (most recently returned) when rhashtable_walk_stop() was called,
if it is still in the table, otherwise it returns NULL (as it would the
first time it was called).
This loses the option for rhashtable_walk_start() to return -EAGAIN, but
I would rather than rhashtable_walk_next() were the only thing to return
that.

The only time it really makes sense to call rhashtable_walk_peek() is
immediately after rhashtable_walk_start(), and this change would make
that fact clear in the API.

Thanks,
NeilBrown

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