本节介绍了spinlock在不同平台(主要是X86_64和aarch64)下的实现. ``` /*------------------------------------------------------------------------- * * s_lock.h * Hardware-dependent implementation of spinlocks. ... ``` ### 一、实现 **X86_64** TAS意思是Test And Set.在X86_64平台下,spinlock的实现使用了汇编语言. ``` #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函数实现(如可用的情况下) ``` /* * 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、参考资料 [s_lock.h](https://github.com/postgres/postgres/blob/master/src/include/storage/s_lock.h) [PostgreSQL中的锁](https://www.leehao.org/postgresql_locks_in_pg/)