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

[NeilBrown]

On Thu, Jul 26 2018, David Miller wrote:

> From: NeilBrown <neilb@xxxxxxxx>
> Date: Mon, 23 Jul 2018 11:56:14 +1000
>
>> Some users of rhashtables might need to move an object from one table
>> to another -  this appears to be the reason for the incomplete usage
>> of NULLS markers.
>> 
>> To support these, we store a unique NULLS_MARKER at the end of
>> each chain, and when a search fails to find a match, we check
>> if the NULLS marker found was the expected one.  If not,
>> the search is repeated.
>  ...
>> This is a simplified version of a previous patch.
>> It provides NULLS_MARKER support only for the specific use case
>> which is currently thought be valuable to in-tree users
>> of rhashtables.
>
> Neil, this doesn't even compile:
>
> In file included from lib/rhashtable.c:28:
> lib/rhashtable.c: In function ârht_bucket_nestedâ:
> ./include/linux/rhashtable.h:79:2: error: initializer element is not computable at load time
>   ((void *)NULLS_MARKER(((unsigned long) (ptr)) >> 1))
>   ^
> lib/rhashtable.c:1178:43: note: in expansion of macro âRHT_NULLS_MARKERâ
>   static struct rhash_head __rcu *rhnull = RHT_NULLS_MARKER(&rhnull);
>                                            ^~~~~~~~~~~~~~~~
> make[1]: *** [scripts/Makefile.build:318: lib/rhashtable.o] Error 1
> make: *** [Makefile:1653: lib/rhashtable.o] Error 2
>
> I imagine you have a mix of other changes or whatever in your tree, so I'll
> give you the benefit of the doubt.
>
> But this is the second time this has happened with your rhashtable changes,
> so...
Your displeasure is understandable.

I had fixed this, but hadn't refreshed the patch - though I had updated
the patch description to explain the change I had to make - second
sentence of:

  The unique NULLS_MARKER is derived from the address of the
  head of the chain.  As this cannot be derived at load-time the
  static rhnull in rht_bucket_nexted() need to be initialised
  at run time.

This is what I meant to send - it does compile.

Thanks,
NeilBrown

From: NeilBrown <neilb@xxxxxxxx>
Date: Mon, 25 Jun 2018 08:09:16 +1000
Subject: [PATCH] rhashtable: detect when object movement between tables might
 have invalidated a lookup

Some users of rhashtables might need to move an object from one table
to another -  this appears to be the reason for the incomplete usage
of NULLS markers.

To support these, we store a unique NULLS_MARKER at the end of
each chain, and when a search fails to find a match, we check
if the NULLS marker found was the expected one.  If not,
the search is repeated.

The unique NULLS_MARKER is derived from the address of the
head of the chain.  As this cannot be derived at load-time the
static rhnull in rht_bucket_nexted() need to be initialised
at run time.

Any caller of a lookup function must be prepared for the possibility
that the object returned is in a different table - it might have been
there for some time.

Note that this does NOT provide support for other uses for
NULLS_MARKERs such as allocating with SLAB_TYPESAFE_BY_RCU or changing
the key of an object and re-inserting it in the table.
These could only be done safely if new objects were inserted
at the *start* of a hash chain, and that is not currently the case.

Signed-off-by: NeilBrown <neilb@xxxxxxxx>
---
 include/linux/rhashtable.h | 25 +++++++++++++++++--------
 lib/rhashtable.c           |  8 +++++---
 2 files changed, 22 insertions(+), 11 deletions(-)

diff --git a/include/linux/rhashtable.h b/include/linux/rhashtable.h
index eb7111039247..8cc240f14834 100644
--- a/include/linux/rhashtable.h
+++ b/include/linux/rhashtable.h
@@ -75,8 +75,10 @@ struct bucket_table {
 	struct rhash_head __rcu *buckets[] ____cacheline_aligned_in_smp;
 };
 
+#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)))
 
 static inline bool rht_is_a_nulls(const struct rhash_head *ptr)
 {
@@ -471,6 +473,7 @@ static inline struct rhash_head *__rhashtable_lookup(
 		.ht = ht,
 		.key = key,
 	};
+	struct rhash_head __rcu * const *head;
 	struct bucket_table *tbl;
 	struct rhash_head *he;
 	unsigned int hash;
@@ -478,13 +481,19 @@ static inline struct rhash_head *__rhashtable_lookup(
 	tbl = rht_dereference_rcu(ht->tbl, ht);
 restart:
 	hash = rht_key_hashfn(ht, tbl, key, params);
-	rht_for_each_rcu(he, tbl, hash) {
-		if (params.obj_cmpfn ?
-		    params.obj_cmpfn(&arg, rht_obj(ht, he)) :
-		    rhashtable_compare(&arg, rht_obj(ht, he)))
-			continue;
-		return he;
-	}
+	head = rht_bucket(tbl, hash);
+	do {
+		rht_for_each_rcu_continue(he, *head, tbl, hash) {
+			if (params.obj_cmpfn ?
+			    params.obj_cmpfn(&arg, rht_obj(ht, he)) :
+			    rhashtable_compare(&arg, rht_obj(ht, he)))
+				continue;
+			return he;
+		}
+		/* An object might have been moved to a different hash chain,
+		 * while we walk along it - better check and retry.
+		 */
+	} while (he != RHT_NULLS_MARKER(head));
 
 	/* Ensure we see any new tables. */
 	smp_rmb();
diff --git a/lib/rhashtable.c b/lib/rhashtable.c
index ae4223e0f5bc..0b1baede369c 100644
--- a/lib/rhashtable.c
+++ b/lib/rhashtable.c
@@ -1175,8 +1175,7 @@ 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;
 	unsigned int index = hash & ((1 << tbl->nest) - 1);
 	unsigned int size = tbl->size >> tbl->nest;
 	unsigned int subhash = hash;
@@ -1194,8 +1193,11 @@ struct rhash_head __rcu **rht_bucket_nested(const struct bucket_table *tbl,
 		subhash >>= shift;
 	}
 
-	if (!ntbl)
+	if (!ntbl) {
+		if (!rhnull)
+			INIT_RHT_NULLS_HEAD(rhnull);
 		return &rhnull;
+	}
 
 	return &ntbl[subhash].bucket;
 
-- 
2.14.0.rc0.dirty

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