Re: [LKP] [PATCH net-next] rhashtable: Lower/upper bucket may map to same lock while shrinking
From: Thomas Graf
Each per bucket lock covers a configurable number of buckets. While
shrinking, two buckets in the old table contain entries for a single
bucket in the new table. We need to lock down both while linking.
Check if they are protected by different locks to avoid a recursive
lock.
Thought, could the shrunk table use the same locks as the lower half
of the old table?
I also wonder whether shrinking hash tables is ever actually worth
the effort. Most likely they'll need to grow again very quickly.
spin_lock_bh(old_bucket_lock1);
- spin_lock_bh_nested(old_bucket_lock2, RHT_LOCK_NESTED);
- spin_lock_bh_nested(new_bucket_lock, RHT_LOCK_NESTED2);
+
+ /* Depending on the lock per buckets mapping, the bucket in
+ * the lower and upper region may map to the same lock.
+ */
+ if (old_bucket_lock1 != old_bucket_lock2) {
+ spin_lock_bh_nested(old_bucket_lock2, RHT_LOCK_NESTED);
+ spin_lock_bh_nested(new_bucket_lock, RHT_LOCK_NESTED2);
+ } else {
+ spin_lock_bh_nested(new_bucket_lock, RHT_LOCK_NESTED);
+ }
Acquiring 3 locks of much the same type looks like a locking hierarchy
violation just waiting to happen.
David