/*      $OpenBSD: _atomic_lock.c,v 1.2 2020/06/25 02:22:31 drahn Exp $  */
/*
 * Copyright (c) 1998 Dale Rahn <drahn@openbsd.org>
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */

/*
 * Atomic lock for powerpc
 */

#include <machine/spinlock.h>

int
_atomic_lock(volatile _atomic_lock_t *lock)
{
        _atomic_lock_t old;

        __asm__("1: lwarx 0,0,%1   \n"
                "   stwcx. %2,0,%1 \n"
                "   bne- 1b        \n"
                "   mr %0, 0       \n"
                : "=r" (old), "=r" (lock)
                : "r" (_ATOMIC_LOCK_LOCKED), "1" (lock) : "0"
        );

        return (old != _ATOMIC_LOCK_UNLOCKED);

        /*
         * Dale <drahn@openbsd.org> says:
         *   Side note. to prevent two processes from accessing
         *   the same address with the lwarx in one instruction
         *   and the stwcx in another process, the current powerpc
         *   kernel uses a stwcx instruction without the corresponding
         *   lwarx which causes any reservation of a process
         *   to be removed.  if a context switch occurs
         *   between the two accesses the store will not occur
         *   and the condition code will cause it to loop. If on
         *   a dual processor machine, the reserve will cause
         *   appropriate bus cycle accesses to notify other
         *   processors.
         */
}