linux-kernel - Re: [PATCH] rhashtable: detect when object movement between tables might have invalidated a lookup

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Message-ID: <20181119035635.2tizfk3vrmxfrr6e@gondor.apana.org.au>
Date:   Mon, 19 Nov 2018 11:56:35 +0800
From:   Herbert Xu <herbert@...dor.apana.org.au>
To:     NeilBrown <neilb@...e.com>
Cc:     David Miller <davem@...emloft.net>, tgraf@...g.ch,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        eric.dumazet@...il.com
Subject: Re: [PATCH] rhashtable: detect when object movement between tables
 might have invalidated a lookup

On Mon, Nov 19, 2018 at 11:54:15AM +0800, Herbert Xu wrote:
>
> > >> diff --git a/lib/rhashtable.c b/lib/rhashtable.c
> > >> index 30526afa8343..852ffa5160f1 100644
> > >> --- a/lib/rhashtable.c
> > >> +++ b/lib/rhashtable.c
> > >> @@ -1179,8 +1179,7 @@ struct rhash_head __rcu **rht_bucket_nested(const struct bucket_table *tbl,
> > >>  					    unsigned int hash)
> > >>  {
> > >>  	const unsigned int shift = PAGE_SHIFT - ilog2(sizeof(void *));
> > >> -	static struct rhash_head __rcu *rhnull =
> > >> -		(struct rhash_head __rcu *)NULLS_MARKER(0);
> > >> +	static struct rhash_head __rcu *rhnull;
> > >
> > > I don't understand why you can't continue to do NULLS_MARKER(0) or
> > > RHT_NULLS_MARKER(0).
> > 
> > Because then the test
> > 
> > +	} while (he != RHT_NULLS_MARKER(head));
> > 
> > in __rhashtable_lookup() would always succeed, and it would loop
> > forever.
> 
> This change is only necessary because of your shifting change
> above, which AFAICS adds no real benefit.

I take that back.  Because of your shift which cancels out the
shift in NULLS_MARKER, it would appear that this should work just
fine with RHT_NULLS_MARRKER(0), no? IOW, it would appear that

	RHT_NULLS_MARKER(0) = RHT_NULLS_MARKER(RHT_NULLS_MARKER(0))

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