• 在armv7中：spin_lock调用了wfe指令，让cpu进入低功耗状态;在spin_unlock中调用了sev指令，让cpu退出低功耗模式;
• 在armv8中，spin_lock调用了wfe指令，让cpu进入低功耗状态; 在spin_unlock中确删除了sev指令，为什么这样呢？

我们在armv8的文档中找到答案:

stlrh是带有释放语义的存储指令，会改变global monitor的状态，等效于sevl指令

附上armv7和armv8的spin_lock和spin_unlock的底层代码:
(armv7)

(linux/arch/arm/asm/spinlock.h)
static inline void arch_spin_lock(arch_spinlock_t *lock)
{
	unsigned long tmp;
	u32 newval;
	arch_spinlock_t lockval;

	prefetchw(&lock->slock);
	__asm__ __volatile__(
"1:	ldrex	%0, [%3]\n"
"	add	%1, %0, %4\n"
"	strex	%2, %1, [%3]\n"
"	teq	%2, #0\n"
"	bne	1b"
	: "=&r" (lockval), "=&r" (newval), "=&r" (tmp)
	: "r" (&lock->slock), "I" (1 << TICKET_SHIFT)
	: "cc");

	while (lockval.tickets.next != lockval.tickets.owner) {
		wfe();
		lockval.tickets.owner = ACCESS_ONCE(lock->tickets.owner);
	}

	smp_mb();
}
static inline void arch_spin_unlock(arch_spinlock_t *lock)
{
	smp_mb();
	lock->tickets.owner++;
	dsb_sev();
}

(armv8)

(linux/arch/arm64/asm/spinlock.h)
static inline void arch_spin_lock(arch_spinlock_t *lock)
{
	unsigned int tmp;
	arch_spinlock_t lockval, newval;

	asm volatile(
	/* Atomically increment the next ticket. */
	ARM64_LSE_ATOMIC_INSN(
	/* LL/SC */
"	prfm	pstl1strm, %3\n"
"1:	ldaxr	%w0, %3\n"
"	add	%w1, %w0, %w5\n"
"	stxr	%w2, %w1, %3\n"
"	cbnz	%w2, 1b\n",
	/* LSE atomics */
"	mov	%w2, %w5\n"
"	ldadda	%w2, %w0, %3\n"
"	nop\n"
"	nop\n"
"	nop\n"
	)

	/* Did we get the lock? */
"	eor	%w1, %w0, %w0, ror #16\n"
"	cbz	%w1, 3f\n"
	/*
	 * No: spin on the owner. Send a local event to avoid missing an
	 * unlock before the exclusive load.
	 */
"	sevl\n"
"2:	wfe\n"
"	ldaxrh	%w2, %4\n"
"	eor	%w1, %w2, %w0, lsr #16\n"
"	cbnz	%w1, 2b\n"
	/* We got the lock. Critical section starts here. */
"3:"
	: "=&r" (lockval), "=&r" (newval), "=&r" (tmp), "+Q" (*lock)
	: "Q" (lock->owner), "I" (1 << TICKET_SHIFT)
	: "memory");
}

static inline void arch_spin_unlock(arch_spinlock_t *lock)
{
	unsigned long tmp;

	asm volatile(ARM64_LSE_ATOMIC_INSN(
	/* LL/SC */
	"	ldrh	%w1, %0\n"
	"	add	%w1, %w1, #1\n"
	"	stlrh	%w1, %0",
	/* LSE atomics */
	"	mov	%w1, #1\n"
	"	nop\n"
	"	staddlh	%w1, %0")
	: "=Q" (lock->owner), "=&r" (tmp)
	:
	: "memory");
}