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Commit c0690016 authored by NeilBrown's avatar NeilBrown Committed by David S. Miller
Browse files

rhashtable: clean up dereference of ->future_tbl. 



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.

Acked-by: default avatarHerbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: default avatarNeilBrown <neilb@suse.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 0ad66449

include/linux/rhashtable.h

+1 −1
Original line number Diff line number Diff line
@@ -595,7 +595,7 @@ 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();

lib/rhashtable.c

+4 −5
Original line number Diff line number Diff line
@@ -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);