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/* $OpenBSD: _atomic_lock.c,v 1.9 2008/10/02 23:27:23 deraadt Exp $ */
/* David Leonard, <d@csee.uq.edu.au>. Public domain. */
/*
* Atomic lock for sparc
*/
#include "spinlock.h"
int
_atomic_lock(volatile _spinlock_lock_t * lock)
{
_spinlock_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 [_SPINLOCK_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 %0,%1"
: "=m" (*lock), "=r" (old)
: "0" (*lock));
return (old == _SPINLOCK_LOCKED);
}
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