[PATCH v4 04/11] futex: Hash only the address for private futexes.

Sebastian Andrzej Siewior
2024-12-03T16:43:40Z

futex_hash() passes the whole futex_key to jhash2. The first two member
are passed as the first argument and the offset as the "initial value".

For private futexes, the mm-part is always the same and it is used only
within the process. By excluding the mm part from the hash, we reduce
the length passed to jhash2 from 4 (16 / 4) to 2 (8 / 2). This avoids
the __jhash_mix() part of jhash.

The resulting code is smaller and based on testing this variant performs
as good as the original or slightly better.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
kernel/futex/core.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index fbfe1f1e94505..14251bbafaffb 100644
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -134,8 +134,8 @@ struct futex_hash_bucket *futex_hash(union futex_key *key)
if (fhb && futex_key_is_private(key)) {
u32 hash_mask = current->mm->futex_hash_mask;

- hash = jhash2((u32 *)key,
- offsetof(typeof(*key), both.offset) / 4,
+ hash = jhash2((void *)&key->private.address,
+ sizeof(key->private.address) / 4,
key->both.offset);
return &fhb[hash & hash_mask];
}
--
2.45.2