linux-kernel - Re: [PATCH v2 0/2] gfs2: Stop using rhashtable_walk

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Message-ID: <871sfx0xeh.fsf@notabene.neil.brown.name>
Date:   Tue, 03 Apr 2018 13:41:26 +1000
From:   NeilBrown <neilb@...e.com>
To:     Herbert Xu <herbert@...dor.apana.org.au>,
        Andreas Gruenbacher <agruenba@...hat.com>
Cc:     cluster-devel <cluster-devel@...hat.com>, netdev@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>, Thomas Graf <tgraf@...g.ch>,
        Tom Herbert <tom@...ntonium.net>
Subject: Re: [PATCH v2 0/2] gfs2: Stop using rhashtable_walk_peek

On Fri, Mar 30 2018, Herbert Xu wrote:

> On Thu, Mar 29, 2018 at 06:52:34PM +0200, Andreas Gruenbacher wrote:
>>
>> Should rhashtable_walk_peek be kept around even if there are no more
>> users? I have my doubts.
>
> Absolutely.  All netlink dumps using rhashtable_walk_next are buggy
> and need to switch over to rhashtable_walk_peek.  As otherwise
> the object that triggers the out-of-space condition will be skipped
> upon resumption.

Do we really need a rhashtable_walk_peek() interface?
I imagine that a seqfile ->start function can do:

  if (*ppos == 0 && last_pos != 0) {
  	rhashtable_walk_exit(&iter);
        rhashtable_walk_enter(&table, &iter);
        last_pos = 0;
  }
  rhashtable_walk_start(&iter);
  if (*ppos == last_pos && iter.p)
  	return iter.p;
  last_pos = *ppos;
  return rhashtable_walk_next(&iter)


and the ->next function just does

  last_pos = *ppos;
  *ppos += 1;
  do p = rhashtable_walk_next(&iter); while (IS_ERR(p));
  return p;

It might be OK to have a function call instead of expecting people to
use iter.p directly.

static inline void *rhashtable_walk_prev(struct rhashtable_iter *iter)
{
	return iter->p;
}

Thoughts?

Thanks,
NeilBrown

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