1 /* $OpenBSD: _atomic_lock.c,v 1.1 2017/08/15 06:13:24 guenther Exp $ */
2 /* David Leonard, <d@csee.uq.edu.au>. Public domain. */
3
4 /*
5 * Atomic lock for sparc64
6 */
7
8 #include <machine/spinlock.h>
9
10 int
_atomic_lock(volatile _atomic_lock_t * lock)11 _atomic_lock(volatile _atomic_lock_t * lock)
12 {
13 _atomic_lock_t old;
14
15 /*
16 * " ldstub [address], reg_rd
17 *
18 * The atomic load-store instructions copy a byte from memory
19 * into r[rd]m then rewrite the addressed byte in memory to all
20 * ones [_ATOMIC_LOCK_LOCKED]. The operation is performed
21 * atomically, that is, without allowing intervening interrupts
22 * or deferred traps. In a multiprocessor system, two or more
23 * processors executing atomic load-store unsigned byte [...]
24 * addressing the same byte [...] simultaneously are guaranteed
25 * to execute them in an undefined, but serial order."
26 * - p101, The SPARC Architecture Manual (version 8) Prentice-Hall
27 *
28 * "LDSTUB loads a byte value from memory to a register and writes
29 * the value FF_16 into the addressed byte atomically. LDSTUB
30 * is the classic test-and-set instruction. Like SWAP, it has
31 * a consensus number of two and so cannot resolve more than
32 * two contending processes in a wait-free fashion."
33 * - p129, The SPARC Architecture Manual (version 9) Prentice-Hall
34 * (See also section J.6 (spinlocks))
35 *
36 * (No change to the condition codes are documented.)
37 */
38 __asm__("ldstub [%1], %0" : "=&r" (old) : "r" (lock) : "memory");
39
40 return (old == _ATOMIC_LOCK_LOCKED);
41 }
42