netdev - Re: rhashtable issue

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Message-ID: <Z0L8LQeZwtvhJ2Ft@gondor.apana.org.au>
Date: Sun, 24 Nov 2024 18:13:01 +0800
From: Herbert Xu <herbert@...dor.apana.org.au>
To: NeilBrown <neilb@...e.de>
Cc: Kent Overstreet <kent.overstreet@...ux.dev>,
	Thomas Graf <tgraf@...g.ch>, netdev@...r.kernel.org
Subject: Re: rhashtable issue - -EBUSY

On Sun, Nov 24, 2024 at 09:01:26PM +1100, NeilBrown wrote:
>
> But I don't see any justification for refusing an insertion because we
> haven't achieved the short chains yet.  Certainly a WARN_ON_ONCE or a
> rate-limited WARN_ON might be appropriate.  Developers should be told
> when their hash function isn't good enough.
> But requiring developers to test for errors and to come up with some way
> to manage them (sleep and try again is all I can think of) doesn't help anyone.

If someone can show me this occurring in a situation other than
that where multiple entries with identical keys are being added
to the hash table, then I'm certainly happy to change this.

But so far every occurrence of EBUSY has turned out to be caused
by the insertion of duplicate keys into the hash table, which
is very much expected, and one where a solution has already been
provided (rhltable).

If this is genuine then it can be easily proved.  Just make the
EBUSY code-path dump the keys in the chain exceeding 16 entries,
plus the hash secret and the total number of entries in the hash
table plus capacity.  It should then be easy to verify.

Cheers,
-- 
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