futex: Unbreak futex hashing

commit 8d67743653dce5a0e7aa500fcccb237cde7ad88e 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: 8019ad13ef7f ("futex: Fix inode life-time issue")
Reported-by: Rong Chen <rong.a.chen@intel.com>
Decoded-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Rong Chen <rong.a.chen@intel.com>
Link: https://lkml.kernel.org/r/87h7yy90ve.fsf@nanos.tec.linutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/kernel/futex.c b/kernel/futex.c
index 3f1b7c003b70447cc2e4f17d640b2a7c256128d2..2921ebaa14676f3021e662897631146403600508 100644 (file)
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -401,9 +401,9 @@ 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)];
 }