[PATCH 08/18] rhashtable: clean up dereference of ->future_tbl.

NeilBrown
2018-06-01T04:44:09Z

Using rht_dereference_bucket() to dereference
->future_tbl looks like a type error, and could be confusing.
Using rht_dereference_rcu() to test a pointer for NULL
adds an unnecessary barrier - rcu_access_pointer() is preferred
for NULL tests when no lock is held.

This uses 3 different ways to access ->future_tbl.
- if we know the mutex is held, use rht_dereference()
- if we don't hold the mutex, and are only testing for NULL,
use rcu_access_pointer()
- otherwise (using RCU protection for true dereference),
use rht_dereference_rcu().

Note that this includes a simplification of the call to
rhashtable_last_table() - we don't do an extra dereference
before the call any more.

Signed-off-by: NeilBrown <neilb@suse.com>
---
include/linux/rhashtable.h | 2 +-
lib/rhashtable.c | 9 ++++-----
2 files changed, 5 insertions(+), 6 deletions(-)

diff --git a/include/linux/rhashtable.h b/include/linux/rhashtable.h
index 49f80506d02e..10435a77b156 100644
--- a/include/linux/rhashtable.h
+++ b/include/linux/rhashtable.h
@@ -605,7 +605,7 @@ static inline void *__rhashtable_insert_fast(
lock = rht_bucket_lock(tbl, hash);
spin_lock_bh(lock);

- if (unlikely(rht_dereference_bucket(tbl->future_tbl, tbl, hash))) {
+ if (unlikely(rcu_access_pointer(tbl->future_tbl))) {
slow_path:
spin_unlock_bh(lock);
rcu_read_unlock();
diff --git a/lib/rhashtable.c b/lib/rhashtable.c
index b5d17bce19ff..1737fbd049da 100644
--- a/lib/rhashtable.c
+++ b/lib/rhashtable.c
@@ -226,8 +226,7 @@ static struct bucket_table *rhashtable_last_table(struct rhashtable *ht,
static int rhashtable_rehash_one(struct rhashtable *ht, unsigned int old_hash)
{
struct bucket_table *old_tbl = rht_dereference(ht->tbl, ht);
- struct bucket_table *new_tbl = rhashtable_last_table(ht,
- rht_dereference_rcu(old_tbl->future_tbl, ht));
+ struct bucket_table *new_tbl = rhashtable_last_table(ht, old_tbl);
struct rhash_head __rcu **pprev = rht_bucket_var(old_tbl, old_hash);
int err = -EAGAIN;
struct rhash_head *head, *next, *entry;
@@ -467,7 +466,7 @@ static int rhashtable_insert_rehash(struct rhashtable *ht,

fail:
/* Do not fail the insert if someone else did a rehash. */
- if (likely(rcu_dereference_raw(tbl->future_tbl)))
+ if (likely(rcu_access_pointer(tbl->future_tbl)))
return 0;

/* Schedule async rehash to retry allocation in process context. */
@@ -540,7 +539,7 @@ static struct bucket_table *rhashtable_insert_one(struct rhashtable *ht,
if (PTR_ERR(data) != -EAGAIN && PTR_ERR(data) != -ENOENT)
return ERR_CAST(data);

- new_tbl = rcu_dereference(tbl->future_tbl);
+ new_tbl = rht_dereference_rcu(tbl->future_tbl, ht);
if (new_tbl)
return new_tbl;

@@ -599,7 +598,7 @@ static void *rhashtable_try_insert(struct rhashtable *ht, const void *key,
break;

spin_unlock_bh(lock);
- tbl = rcu_dereference(tbl->future_tbl);
+ tbl = rht_dereference_rcu(tbl->future_tbl, ht);
}

data = rhashtable_lookup_one(ht, tbl, hash, key, obj);