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

[NeilBrown <neilb@suse.com>]

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On Fri, Nov 16 2018, Herbert Xu wrote:

> On Thu, Nov 15, 2018 at 10:32:13AM +1100, NeilBrown wrote:
>>
>> +#define	RHT_NULLS_MARKER(ptr)	\
>> +	((void *)NULLS_MARKER(((unsigned long) (ptr)) >> 1))
>>  #define INIT_RHT_NULLS_HEAD(ptr)	\
>> -	((ptr) = (typeof(ptr)) NULLS_MARKER(0))
>> +	((ptr) = RHT_NULLS_MARKER(&(ptr)))
>
> Why are you shifting this by one?

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.

>
>> 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.

Thanks for the review.

NeilBrown


>
> 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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