• 在armv7中：spin_lock調用了wfe指令，讓cpu進入低功耗狀態;在spin_unlock中調用了sev指令，讓cpu退出低功耗模式;
• 在armv8中，spin_lock調用了wfe指令，讓cpu進入低功耗狀態; 在spin_unlock中確刪除了sev指令，為什么這樣呢？

我們在armv8的文檔中找到答案:

stlrh是帶有釋放語義的存儲指令，會改變global monitor的狀態，等效于sevl指令

附上armv7和armv8的spin_lock和spin_unlock的底層代碼:
(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"); }

總結

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