本節介紹了spinlock在不同平臺(主要是X86_64和aarch64)下的實現.
```
/*-------------------------------------------------------------------------
 *
 * s_lock.h
 *     Hardware-dependent implementation of spinlocks.
 ...
```

### 一、實現
**X86_64**
TAS意思是Test And Set.在X86_64平臺下,spinlock的實現使用了組合語言.
```
#ifdef __x86_64__       /* AMD Opteron, Intel EM64T */
#define HAS_TEST_AND_SET

typedef unsigned char slock_t;

#define TAS(lock) tas(lock)

/*
 * On Intel EM64T, it's a win to use a non-locking test before the xchg proper,
 * but only when spinning.
 *
 * See also Implementing Scalable Atomic Locks for Multi-Core Intel(tm) EM64T
 * and IA32, by Michael Chynoweth and Mary R. Lee. As of this writing, it is
 * available at:
 * http://software.intel.com/en-us/articles/implementing-scalable-atomic-locks-for-multi-core-intel-em64t-and-ia32-architectures
 */
#define TAS_SPIN(lock)    (*(lock) ? 1 : TAS(lock))

static __inline__ int
tas(volatile slock_t *lock)
{
    register slock_t _res = 1;

__asm__ __volatile__(
        "   lock            \n"
        "   xchgb   %0,%1   \n"
:       "+q"(_res), "+m"(*lock)
:       /* no inputs */
:       "memory", "cc");
    return (int) _res;
}

#define SPIN_DELAY() spin_delay()

static __inline__ void
spin_delay(void)
{
    /*
     * Adding a PAUSE in the spin delay loop is demonstrably a no-op on
     * Opteron, but it may be of some use on EM64T, so we keep it.
     */
    __asm__ __volatile__(
        " rep; nop          \n");
}

#endif   /* __x86_64__ */
```

**aarch64**
在aarch64(ARM64)下,使用了__sync_lock_test_and_set函式實現(如可用的情況下)
```
/*
 * On ARM and ARM64, we use __sync_lock_test_and_set(int *, int) if available.
 *
 * We use the int-width variant of the builtin because it works on more chips
 * than other widths.
 */
#if defined(__arm__) || defined(__arm) || defined(__aarch64__) || defined(__aarch64)
#ifdef HAVE_GCC__SYNC_INT32_TAS
#define HAS_TEST_AND_SET

#define TAS(lock) tas(lock)

typedef int slock_t;

static __inline__ int
tas(volatile slock_t *lock)
{
    return __sync_lock_test_and_set(lock, 1);
}

#define S_UNLOCK(lock) __sync_lock_release(lock)

#endif   /* HAVE_GCC__SYNC_INT32_TAS */
#endif   /* __arm__ || __arm || __aarch64__ || __aarch64 */

```

### 二、參考資料
[s_lock.h](https://github.com/postgres/postgres/blob/master/src/include/storage/s_lock.h)
[PostgreSQL中的鎖](https://www.leehao.org/postgresql_locks_in_pg/)

PostgreSQL 原始碼解讀（218）- spinlock的實現

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