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SubjectRe: [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@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

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