Re: [PATCH 2/5] rhashtable: don't hold lock on first table throughout insertion.
From: Herbert Xu <herbert@gondor.apana.org.au>
Date: 2018-07-20 07:54:30
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On Fri, Jul 06, 2018 at 05:22:30PM +1000, NeilBrown wrote:
rhashtable_try_insert() currently hold a lock on the bucket in the first table, while also locking buckets in subsequent tables. This is unnecessary and looks like a hold-over from some earlier version of the implementation. As insert and remove always lock a bucket in each table in turn, and as insert only inserts in the final table, there cannot be any races that are not covered by simply locking a bucket in each table in turn. When an insert call reaches that last table it can be sure that there is no match entry in any other table as it has searched them all, and insertion never happens anywhere but in the last table. The fact that code tests for the existence of future_tbl while holding a lock on the relevant bucket ensures that two threads inserting the same key will make compatible decisions about which is the "last" table. This simplifies the code and allows the ->rehash field to be discarded. We still need a way to ensure that a dead bucket_table is never re-linked by rhashtable_walk_stop(). This can be achieved by calling call_rcu() inside the locked region, and checking ->rcu.func in rhashtable_walk_stop(). If it is not NULL, then the bucket table is empty and dead. Signed-off-by: NeilBrown <redacted>
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quoted hunk ↗ jump to hunk
@@ -339,13 +338,16 @@ static int rhashtable_rehash_table(struct rhashtable *ht) spin_lock(&ht->lock); list_for_each_entry(walker, &old_tbl->walkers, list) walker->tbl = NULL; - spin_unlock(&ht->lock); /* Wait for readers. All new readers will see the new * table, and thus no references to the old table will * remain. + * We do this inside the locked region so that + * rhashtable_walk_stop() can check ->rcu.func and know + * not to re-link the table. */ call_rcu(&old_tbl->rcu, bucket_table_free_rcu); + spin_unlock(&ht->lock); return rht_dereference(new_tbl->future_tbl, ht) ? -EAGAIN : 0; }
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quoted hunk ↗ jump to hunk
@@ -964,7 +942,7 @@ void rhashtable_walk_stop(struct rhashtable_iter *iter) ht = iter->ht; spin_lock(&ht->lock); - if (tbl->rehash < tbl->size) + if (tbl->rcu.func == NULL) list_add(&iter->walker.list, &tbl->walkers); else iter->walker.tbl = NULL;
This appears to be relying on implementation details within RCU. Paul, are you OK with rhashtable doing this trick? Thanks, -- Email: Herbert Xu [off-list ref] Home Page: http://gondor.apana.org.au/~herbert/ PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt