futex: Unbreak futex hashing

commit 8d67743 upstream. The recent futex inode life time fix changed the ordering of the futex key union struct members, but forgot to adjust the hash function accordingly, As a result the hashing omits the leading 64bit and even hashes beyond the futex key causing a bad hash distribution which led to a ~100% performance regression. Hand in the futex key pointer instead of a random struct member and make the size calculation based of the struct offset. Fixes: 8019ad1 ("futex: Fix inode life-time issue") Reported-by: Rong Chen <[email protected]> Decoded-by: Linus Torvalds <[email protected]> Signed-off-by: Thomas Gleixner <[email protected]> Tested-by: Rong Chen <[email protected]> Link: https://lkml.kernel.org/r/[email protected] Signed-off-by: Greg Kroah-Hartman <[email protected]>
LeBlue · Apr 2, 2020 · 95c5383 · 95c5383
1 parent fb099f3
commit 95c5383
Showing 1 changed file with 2 additions and 2 deletions.
diff --git a/kernel/futex.c b/kernel/futex.c
@@ -390,9 +390,9 @@ static inline int hb_waiters_pending(struct futex_hash_bucket *hb)
  */
 static struct futex_hash_bucket *hash_futex(union futex_key *key)
 {
-	u32 hash = jhash2((u32*)&key->both.word,
-			  (sizeof(key->both.word)+sizeof(key->both.ptr))/4,
+	u32 hash = jhash2((u32 *)key, offsetof(typeof(*key), both.offset) / 4,
 			  key->both.offset);
+
 	return &futex_queues[hash & (futex_hashsize - 1)];
 }