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

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Date:   Mon, 19 Nov 2018 11:54:15 +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 Fri, Nov 16, 2018 at 05:59:19PM +1100, NeilBrown wrote:
>
> NULLS_MARKER assumes a hash value in which the bottom bits are most
> likely to be unique.  To convert this to a pointer which certainly not
> valid, it shifts left by 1 and sets the lsb.
> We aren't passing a hash value, but are passing an address instead.
> In this case the bottom 2 bits are certain to be 0, and the top bit
> could contain valuable information (on a 32bit system).
> The best way to turn a pointer into a certainly-invalid pointer
> is to just set the lsb.  By shifting right by one, we discard an
> uninteresting bit, preserve all the interesting bits, and effectively
> just set the lsb.
> 
> I could add a comment explaining that if you like.

The top-bit is most likely to be fixed and offer no real value.

> >> 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.

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