linux-kernel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: NeilBrown <neilb@suse.com>
To: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Thomas Graf <tgraf@suug.ch>,
	netdev@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/8] rhashtable: remove nulls_base and related code.
Date: Sun, 06 May 2018 07:37:49 +1000	[thread overview]
Message-ID: <877eoheqc2.fsf@notabene.neil.brown.name> (raw)
In-Reply-To: <20180505091208.tnsxi6hdpjn456yz@gondor.apana.org.au>

[-- Attachment #1: Type: text/plain, Size: 2289 bytes --]

On Sat, May 05 2018, Herbert Xu wrote:

> On Fri, May 04, 2018 at 01:54:14PM +1000, NeilBrown wrote:
>> This "feature" is unused, undocumented, and untested and so
>> doesn't really belong.  If a use for the nulls marker
>> is found, all this code would need to be reviewed to
>> ensure it works as required.  It would be just as easy to
>> just add the code if/when it is needed instead.
>> 
>> This patch actually fixes a bug too.  The table resizing allows a
>> table to grow to 2^31 buckets, but the hash is truncated to 27 bits -
>> any growth beyond 2^27 is wasteful an ineffective.
>> 
>> This patch result in NULLS_MARKER(0) being used for all chains,
>> and leave the use of rht_is_a_null() to test for it.
>> 
>> Signed-off-by: NeilBrown <neilb@suse.com>
>
> I disagree.  This is a fundamental requirement for the use of
> rhashtable in certain networking systems such as TCP/UDP.  So
> we know that there will be a use for this.

I can see no evidence that this is required for anything, as it isn't
use and I'm fairly sure that in it's current form - it cannot be used.

Based on my best guess about how you might intend to use it, I suspect
it would be simpler to store the address of the bucket head in the nuls
rather than the hash and a magic number.  This would make it just as
easy to detect when a search reaches the end of the wrong chain, which I
presume is the purpose.
I would find that useful myself - if the search would repeat when that
happened - as I could then use SLAB_TYPESAFE_BY_RCU.

Were we to take this approach, all the code I've removed here would
still need to be removed.

>
> As to the bug fix, please separate it out of the patch and resubmit.

I don't know how to do that.  I don't know what is safe to change
without "breaking" the nulls_base code because that code is undocumented and
unused, so unmaintainable.
In general the kernel has, I believe, a policy against keeping unused
interfaces.  While that isn't firm and universal, is seems to apply
particularly well to unusable interfaces.

Thanks,
NeilBrown


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

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 832 bytes --]

  reply	other threads:[~2018-05-05 21:38 UTC|newest]

Thread overview: 36+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-05-04  3:54 [PATCH 0/8] Assorted rhashtable fixes and cleanups NeilBrown
2018-05-04  3:54 ` [PATCH 8/8] rhashtable: don't hold lock on first table throughout insertion NeilBrown
2018-05-05  9:41   ` Herbert Xu
2018-05-05 22:00     ` NeilBrown
2018-05-06  5:20       ` Herbert Xu
2018-05-06 22:24         ` NeilBrown
2018-05-07  9:29           ` Herbert Xu
2018-05-08  0:23             ` NeilBrown
2018-05-04  3:54 ` [PATCH 6/8] rhashtable: further improve stability of rhashtable_walk NeilBrown
2018-05-05  9:42   ` Herbert Xu
2018-05-05 21:50     ` NeilBrown
2018-05-07  9:30       ` Herbert Xu
2018-05-08  0:54         ` NeilBrown
2018-05-04  3:54 ` [PATCH 1/8] rhashtable: silence RCU warning in rhashtable_test NeilBrown
2018-05-05  9:10   ` Herbert Xu
2018-05-05 21:49     ` NeilBrown
2018-05-04  3:54 ` [PATCH 2/8] rhashtable: remove nulls_base and related code NeilBrown
2018-05-05  9:12   ` Herbert Xu
2018-05-05 21:37     ` NeilBrown [this message]
2018-05-07  9:27       ` Herbert Xu
2018-05-08  1:14         ` NeilBrown
2018-05-04  3:54 ` [PATCH 5/8] rhashtable: remove rhashtable_walk_peek() NeilBrown
2018-05-05  9:30   ` Herbert Xu
2018-05-04  3:54 ` [PATCH 7/8] rhashtable: add rhashtable_walk_prev() NeilBrown
2018-05-05  9:43   ` Herbert Xu
2018-05-05 15:40     ` Tom Herbert
2018-05-06 22:16       ` NeilBrown
2018-05-04  3:54 ` [PATCH 4/8] rhashtable: fix race in nested_table_alloc() NeilBrown
2018-05-05  9:29   ` Herbert Xu
2018-05-05 21:48     ` NeilBrown
2018-05-06  5:18       ` Herbert Xu
2018-05-06 22:02         ` NeilBrown
2018-05-04  3:54 ` [PATCH 3/8] rhashtable: use cmpxchg() to protect ->future_tbl NeilBrown
2018-05-05  9:27   ` Herbert Xu
2018-05-05 21:45     ` NeilBrown
2018-05-04 17:07 ` [PATCH 0/8] Assorted rhashtable fixes and cleanups David Miller

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=877eoheqc2.fsf@notabene.neil.brown.name \
    --to=neilb@suse.com \
    --cc=herbert@gondor.apana.org.au \
    --cc=linux-kernel@vger.kernel.org \
    --cc=netdev@vger.kernel.org \
    --cc=tgraf@suug.ch \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body. 
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).