`ll/sc` 指令在`linux`中的软件实现

load-link与store-conditional (LL/SC)是一对用于并发同步访问内存的CPU指令。Load-link返回内存位置处的当前值，随后的store-conditional在该内存位置处保存新值（如果从load-link后没有被修改）。这被用于实现无锁算法与read-modify-write原子操作。

linux在<asm/atomic.h>、<asm/system.h>、<asm/cmpxchg.h>、<asm/bitops.h>、<asm/local.h>中实现了多种基本的原子操作，以最简单的atomic_add来举例：

/*
 * atomic_add - add integer to atomic variable
 * @i: integer value to add
 * @v: pointer of type atomic_t
 *
 * Atomically adds @i to @v.
 */
static __inline__ void atomic_add(int i, atomic_t * v)
{
	if (kernel_uses_llsc && R10000_LLSC_WAR) {
		int temp;

		__asm__ __volatile__(
		"	.set	mips3					\n"
		"1:	ll	%0, %1		# atomic_add		\n"
		"	addu	%0, %2					\n"
		"	sc	%0, %1					\n"
		"	beqzl	%0, 1b					\n"
		"	.set	mips0					\n"
		: "=&r" (temp), "=m" (v->counter)
		: "Ir" (i), "m" (v->counter));
	} else if (kernel_uses_llsc) {
		int temp;

		__asm__ __volatile__(
		"	.set	mips3					\n"
		"1:	ll	%0, %1		# atomic_add		\n"
		"	addu	%0, %2					\n"
		"	sc	%0, %1					\n"
		"	beqz	%0, 2f					\n"
		"	.subsection 2					\n"
		"2:	b	1b					\n"
		"	.previous					\n"
		"	.set	mips0					\n"
		: "=&r" (temp), "=m" (v->counter)
		: "Ir" (i), "m" (v->counter));
	} else {
		unsigned long flags;

		raw_local_irq_save(flags);
		v->counter += i;
		raw_local_irq_restore(flags);
	}
}

其中，kernel_uses_llsc是一个宏定义，当定义为0时就需要软件实现。raw_local_irq_save与raw_local_irq_restore函数定义在<linux/irqflags.h>，对于不同的mips cpu有不同的实现，最简单的实现如下：

	.macro irq_enable_hazard; _ssnop; _ssnop; _ssnop;; .endm
	.macro irq_disable_hazard; nop; nop; nop; .endm

		.macro	raw_local_irq_save result
	.set	push
	.set	reorder
	.set	noat
	mfc0	\result, $12
	ori	$1, \result, 0x1f
	xori	$1, 0x1f
	.set	noreorder
	mtc0	$1, $12
	irq_disable_hazard
	.set	pop
	.endm							

		.macro	raw_local_irq_restore flags
	.set	push
	.set	noreorder
	.set	noat
	mfc0	$1, $12
	andi	\flags, 1
	ori	$1, 0x1f
	xori	$1, 0x1f
	or	\flags, $1
	mtc0	\flags, $12
	irq_disable_hazard
	.set	pop
	.endm

raw_local_irq_save与raw_local_irq_restore被实现为两个mips的宏定义，raw_local_irq_save对cp0的status寄存器进行修改，让cpu进入kernek mode，ERL与EXL置0，同时禁止中断，从而保证了原子性。raw_local_irq_restore将中断使能打开。