/*      $OpenBSD: _atomic_lock.c,v 1.1 2017/08/15 06:13:24 guenther Exp $       */
/* David Leonard, <d@csee.uq.edu.au>. Public domain. */

/*
 * Atomic lock for sparc64
 */
 
#include <machine/spinlock.h>

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

        /*
         *  "  ldstub  [address], reg_rd
         * 
         *  The atomic load-store instructions copy a byte from memory
         *  into r[rd]m then rewrite the addressed byte in memory to all
         *  ones [_ATOMIC_LOCK_LOCKED]. The operation is performed
         *  atomically, that is, without allowing intervening interrupts
         *  or deferred traps. In a multiprocessor system, two or more
         *  processors executing atomic load-store unsigned byte [...]
         *  addressing the same byte [...] simultaneously are guaranteed
         *  to execute them in an undefined, but serial order."
         *    - p101, The SPARC Architecture Manual (version 8) Prentice-Hall
         *
         * "LDSTUB loads a byte value from memory to a register and writes
         *  the value FF_16 into the addressed byte atomically. LDSTUB
         *  is the classic test-and-set instruction. Like SWAP, it has
         *  a consensus number of two and so cannot resolve more than
         *  two contending processes in a wait-free fashion."
         *    - p129, The SPARC Architecture Manual (version 9) Prentice-Hall
         *  (See also section J.6 (spinlocks))
         *
         * (No change to the condition codes are documented.)
         */
        __asm__("ldstub [%1], %0" : "=&r" (old) : "r" (lock) : "memory");

        return (old == _ATOMIC_LOCK_LOCKED);
}