1 /* $OpenBSD: _atomic_lock.c,v 1.1 2017/08/15 06:13:24 guenther Exp $ */
2 /*
3 * Copyright (c) 1998 Dale Rahn <drahn@openbsd.org>
4 *
5 * Permission to use, copy, modify, and distribute this software for any
6 * purpose with or without fee is hereby granted, provided that the above
7 * copyright notice and this permission notice appear in all copies.
8 *
9 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
10 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
11 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
12 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
13 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
14 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
15 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
16 */
17
18 /*
19 * Atomic lock for powerpc
20 */
21
22 #include <machine/spinlock.h>
23
24 int
_atomic_lock(volatile _atomic_lock_t * lock)25 _atomic_lock(volatile _atomic_lock_t *lock)
26 {
27 _atomic_lock_t old;
28
29 __asm__("1: lwarx 0,0,%1 \n"
30 " stwcx. %2,0,%1 \n"
31 " bne- 1b \n"
32 " mr %0, 0 \n"
33 : "=r" (old), "=r" (lock)
34 : "r" (_ATOMIC_LOCK_LOCKED), "1" (lock) : "0"
35 );
36
37 return (old != _ATOMIC_LOCK_UNLOCKED);
38
39 /*
40 * Dale <drahn@openbsd.org> says:
41 * Side note. to prevent two processes from accessing
42 * the same address with the lwarx in one instruction
43 * and the stwcx in another process, the current powerpc
44 * kernel uses a stwcx instruction without the corresponding
45 * lwarx which causes any reservation of a process
46 * to be removed. if a context switch occurs
47 * between the two accesses the store will not occur
48 * and the condition code will cause it to loop. If on
49 * a dual processor machine, the reserve will cause
50 * appropriate bus cycle accesses to notify other
51 * processors.
52 */
53 }
54