[tip: locking/core] futex: Optimise the size check get_futex_key()

["tip-bot2 for Sebastian Andrzej Siewior" <tip-bot2@linutronix.de>]

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     5e601ab3615c86be7c4068ce992f94654693a032
Gitweb:        https://git.kernel.org/tip/5e601ab3615c86be7c4068ce992f94654693a032
Author:        Sebastian Andrzej Siewior <bigeasy@linutronix.de>
AuthorDate:    Wed, 01 Jul 2026 18:17:36 +02:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Wed, 29 Jul 2026 13:17:52 +02:00

futex: Optimise the size check get_futex_key()

The futex address must be naturally aligned and this is checked via
"address % size" where `address' is the supplied address and `size' is
the expected size of futex. It is guaranteed that `size' is power of two
but the compiler does not see it and creates here a `div' operation
(x86, arm, gcc-15).

We can take advantage of the pow2 property and rewrite it as
"address & (size-1)".

As per testing, the command
|perf bench futex hash -f 1 -b 16384 -t 1 -r 30

improved from
| [thread  0] futex: 0x5619f931f740 [ 7001583 ops/sec ]
to
| [thread  0] futex: 0x55da173e5740 [ 7376137 ops/sec ]

or by 5.3%

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260701161736.xYYizA0e@linutronix.de
---
 kernel/futex/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index 3bf6cb7..0864c6e 100644
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -516,7 +516,7 @@ int get_futex_key(u32 __user *uaddr, unsigned int flags, union futex_key *key,
 	 * The futex address must be "naturally" aligned.
 	 */
 	key->both.offset = address % PAGE_SIZE;
-	if (unlikely((address % size) != 0))
+	if (unlikely((address & (size-1)) != 0))
 		return -EINVAL;
 	address -= key->both.offset;