netdev - Re: [PATCH net-next 2/5] rhashtable: Add rhastable_walk

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Date:   Thu, 30 Nov 2017 17:15:16 -0800
From:   Tom Herbert <tom@...ntonium.net>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Rohit LastName <rohit@...ntonium.net>
Subject: Re: [PATCH net-next 2/5] rhashtable: Add rhastable_walk_peek

On Thu, Nov 30, 2017 at 4:38 PM, Herbert Xu <herbert@...dor.apana.org.au> wrote:
> On Thu, Nov 30, 2017 at 04:03:02PM -0800, Tom Herbert wrote:
>> This function is like rhashtable_walk_next except that it only returns
>> the current element in the inter and does not advance the iter.
>>
>> This patch also creates __rhashtable_walk_find_next. It finds the next
>> element in the table when the entry cached in iter is NULL or at the end
>> of a slot. __rhashtable_walk_find_next is called from
>> rhashtable_walk_next and rhastable_walk_peek.
>>
>> Signed-off-by: Tom Herbert <tom@...ntonium.net>
>
> Hi Tom:
>
> Could you add some motivation for this feature into the patch
> description? As it is it's difficult to deduce why we would want
> to add something like this given that hashtable walks are always
> unstable and there is no guarantee that two calls to peek or a
> peek followed by a normal walk will see the same entry.
>
Hi Herbert,

We don't need a guarantee of stability, but what I am seeing is that
we're consisitently dropping entries on when doing a multi-part
netlink walk. We start iterating over the table filling in the netlink
info. But eventually the netlink info fills up and returns an error.
netlink dump gets called again but now the iter of the table returns
the object following the one that would have overflowed the netlink
buffer. So the result I was seeing is that we dropped one object in in
each pass.

This fixes the nldump for ila which will be in a follow on patch set.
In pseudo code it looks something like this:

rhashtable_walk_start(rhiter);

/* Get first entty */
ila = rhashtable_walk_peek(rhiter);

while (ila) {
     if (ila_dump_info(ila) < 0)
         break;
     ila = rhashtable_walk_next(rhiter);
}

rhashtable_walk_stop(rhiter);

return;

So peek is only called one and we only advance iter once the current
entry is successfully processed.

Tom



> Thanks,
> --
> 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