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.

quoted

quoted

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 [off-list ref]
Home Page: http://gondor.apana.org.au/~herbert/
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