1 /* $OpenBSD: _atomic_lock.c,v 1.8 2003/01/31 22:10:53 deraadt Exp $ */
2 /*
3 * Atomic lock for sparc
4 */
5
6 #include "spinlock.h"
7
8 int
_atomic_lock(volatile _spinlock_lock_t * lock)9 _atomic_lock(volatile _spinlock_lock_t * lock)
10 {
11 _spinlock_lock_t old;
12
13 /*
14 * " ldstub [address], reg_rd
15 *
16 * The atomic load-store instructions copy a byte from memory
17 * into r[rd]m then rewrite the addressed byte in memory to all
18 * ones [_SPINLOCK_LOCKED]. The operation is performed
19 * atomically, that is, without allowing intervening interrupts
20 * or deferred traps. In a multiprocessor system, two or more
21 * processors executing atomic load-store unsigned byte [...]
22 * addressing the same byte [...] simultaneously are guaranteed
23 * to execute them in an undefined, but serial order."
24 * - p101, The SPARC Architecture Manual (version 8) Prentice-Hall
25 *
26 * "LDSTUB loads a byte value from memory to a register and writes
27 * the value FF_16 into the addressed byte atomically. LDSTUB
28 * is the classic test-and-set instruction. Like SWAP, it has
29 * a consensus number of two and so cannot resolve more than
30 * two contending processes in a wait-free fashion."
31 * - p129, The SPARC Architecture Manual (version 9) Prentice-Hall
32 * (See also section J.6 (spinlocks))
33 *
34 * (No change to the condition codes are documented.)
35 */
36 __asm__("ldstub %0,%1"
37 : "=m" (*lock), "=r" (old)
38 : "0" (*lock));
39
40 return (old == _SPINLOCK_LOCKED);
41 }
42