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

NeilBrown
2018-07-26T22:04:40Z

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On Thu, Jul 26 2018, David Miller wrote:

> From: NeilBrown <neilb@suse.com>
> 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 ‘rht_bucket_nested’:
> ./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 ‘RHT_NULLS_MARKER’
> 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@suse.com>
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@suse.com>
---
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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