Postgresql源码（67）LWLock锁的内存结构与初始化

/*
 * Compute shmem space needed for LWLocks and named tranches.
 */
Size
LWLockShmemSize(void)
{
	Size		size;
	int			i;
	int			numLocks = NUM_FIXED_LWLOCKS;

	/* Calculate total number of locks needed in the main array. */
	numLocks += NumLWLocksForNamedTranches();

	/* Space for the LWLock array. */
	size = mul_size(numLocks, sizeof(LWLockPadded));

	/* Space for dynamic allocation counter, plus room for alignment. */
	size = add_size(size, sizeof(int) + LWLOCK_PADDED_SIZE);

	/* space for named tranches. */
	size = add_size(size, mul_size(NamedLWLockTrancheRequests, sizeof(NamedLWLockTranche)));

	/* space for name of each tranche. */
	for (i = 0; i < NamedLWLockTrancheRequests; i++)
		size = add_size(size, strlen(NamedLWLockTrancheRequestArray[i].tranche_name) + 1);

	/* Disallow adding any more named tranches. */
	lock_named_request_allowed = false;

	return size;
}

#define NUM_BUFFER_PARTITIONS  128

/* Number of partitions the shared lock tables are divided into */
#define LOG2_NUM_LOCK_PARTITIONS  4
#define NUM_LOCK_PARTITIONS  (1 << LOG2_NUM_LOCK_PARTITIONS)

/* Number of partitions the shared predicate lock tables are divided into */
#define LOG2_NUM_PREDICATELOCK_PARTITIONS  4
#define NUM_PREDICATELOCK_PARTITIONS  (1 << LOG2_NUM_PREDICATELOCK_PARTITIONS)

/* Offsets for various chunks of preallocated lwlocks. */
#define BUFFER_MAPPING_LWLOCK_OFFSET	NUM_INDIVIDUAL_LWLOCKS
#define LOCK_MANAGER_LWLOCK_OFFSET		\
	(BUFFER_MAPPING_LWLOCK_OFFSET + NUM_BUFFER_PARTITIONS)
#define PREDICATELOCK_MANAGER_LWLOCK_OFFSET \
	(LOCK_MANAGER_LWLOCK_OFFSET + NUM_LOCK_PARTITIONS)
#define NUM_FIXED_LWLOCKS \
	(PREDICATELOCK_MANAGER_LWLOCK_OFFSET + NUM_PREDICATELOCK_PARTITIONS)

|--------------------------------------------------|   <----- LWLockCounter（初始=72因为固定锁有72个名字，每次动态锁需要ID，+1使用）
| （一个int对齐LWLOCK_PADDED_SIZE，记录动态申请锁数量） |   （对齐到128字节，int只有4字节，浪费124字节，int放在靠近高内存的位置）
|--------------------------------------------------|    <----- MainLWLockArray
|                                                  |
| （固定锁208 + 动态锁数量） * sizeof(LWLockPadded)   |
|                                                  |
|--------------------------------------------------|    <----- NamedLWLockTrancheArray
|                                                  |  
| （动态申请锁数量） * sizeof(NamedLWLockTranche)     | NamedLWLockTranche{int trancheId, char *trancheName}
|                                                  | 注意：trancheName字符串只有指针，空间下面申请。
|--------------------------------------------------|
|                                                  |
| （遍历数组拿到所有动态申请锁的名字的总字符数）           | 申请空间，给上面使用。
|                                                  |
|--------------------------------------------------|

main
  PostmasterMain
    reset_shared
      CreateSharedMemoryAndSemaphores
        CreateLWLocks
          InitializeLWLocks     // 初始化所有锁，包括fixed lwlock 和 动态注册的named lwlock
          LWLockRegisterTranche // 注册named lwlock到统一命名体系

- 固定锁都可以用MainLWLockArray索引到，找到后按顺序按批次给ID用LWLockInitialize初始化。
- 动态锁不能用MainLWLockArray索引到，用内存偏移寻找并初始化，找到后按LWLockNewTrancheId算ID调用LWLockInitialize初始化。      
    - LWLockNewTrancheId使用共享内存顶部的LWLockCounter，每次+1使用。
    - 固定锁208把，共有72个名字，有一些锁共享名字，也就是共享ID。初始化时会给每个锁结构LWLock添加tranche_id信息，同时将锁状态置为release。

static void
InitializeLWLocks(void)
{
	int			numNamedLocks = NumLWLocksForNamedTranches();
	int			id;
	int			i;
	int			j;
	LWLockPadded *lock;

	/* Initialize all individual LWLocks in main array */
	for (id = 0, lock = MainLWLockArray; id < NUM_INDIVIDUAL_LWLOCKS; id++, lock++)
		LWLockInitialize(&lock->lock, id);

  // 共享名字、ID
	/* Initialize buffer mapping LWLocks in main array */
	lock = MainLWLockArray + BUFFER_MAPPING_LWLOCK_OFFSET;
	for (id = 0; id < NUM_BUFFER_PARTITIONS; id++, lock++)
		LWLockInitialize(&lock->lock, LWTRANCHE_BUFFER_MAPPING);
  
  // 共享名字、ID
	/* Initialize lmgrs' LWLocks in main array */
	lock = MainLWLockArray + LOCK_MANAGER_LWLOCK_OFFSET;
	for (id = 0; id < NUM_LOCK_PARTITIONS; id++, lock++)
		LWLockInitialize(&lock->lock, LWTRANCHE_LOCK_MANAGER);

	/* Initialize predicate lmgrs' LWLocks in main array */
	lock = MainLWLockArray + PREDICATELOCK_MANAGER_LWLOCK_OFFSET;
	for (id = 0; id < NUM_PREDICATELOCK_PARTITIONS; id++, lock++)
		LWLockInitialize(&lock->lock, LWTRANCHE_PREDICATE_LOCK_MANAGER);

	/*
	 * Copy the info about any named tranches into shared memory (so that
	 * other processes can see it), and initialize the requested LWLocks.
	 */
	if (NamedLWLockTrancheRequests > 0)
	{
		char	   *trancheNames;

		NamedLWLockTrancheArray = (NamedLWLockTranche *)
			&MainLWLockArray[NUM_FIXED_LWLOCKS + numNamedLocks];

		trancheNames = (char *) NamedLWLockTrancheArray +
			(NamedLWLockTrancheRequests * sizeof(NamedLWLockTranche));
		lock = &MainLWLockArray[NUM_FIXED_LWLOCKS];

		for (i = 0; i < NamedLWLockTrancheRequests; i++)
		{
			NamedLWLockTrancheRequest *request;
			NamedLWLockTranche *tranche;
			char	   *name;

			request = &NamedLWLockTrancheRequestArray[i];
			tranche = &NamedLWLockTrancheArray[i];

			name = trancheNames;
			trancheNames += strlen(request->tranche_name) + 1;
			strcpy(name, request->tranche_name);
			tranche->trancheId = LWLockNewTrancheId();
			tranche->trancheName = name;

			for (j = 0; j < request->num_lwlocks; j++, lock++)
				LWLockInitialize(&lock->lock, tranche->trancheId);
		}
	}
}

/* 初始化时会给每个锁结构添加tranche_id */
void
LWLockInitialize(LWLock *lock, int tranche_id)
{
	pg_atomic_init_u32(&lock->state, LW_FLAG_RELEASE_OK);
	lock->tranche = tranche_id;
	proclist_init(&lock->waiters);
}

static const char *
GetLWTrancheName(uint16 trancheId)
{
	/* Individual LWLock? */
	if (trancheId < NUM_INDIVIDUAL_LWLOCKS)
		return IndividualLWLockNames[trancheId];

	/* Built-in tranche? */
	if (trancheId < LWTRANCHE_FIRST_USER_DEFINED)
		return BuiltinTrancheNames[trancheId - NUM_INDIVIDUAL_LWLOCKS];

	/*
	 * It's an extension tranche, so look in LWLockTrancheNames[].  However,
	 * it's possible that the tranche has never been registered in the current
	 * process, in which case give up and return "extension".
	 */
	trancheId -= LWTRANCHE_FIRST_USER_DEFINED;

	if (trancheId >= LWLockTrancheNamesAllocated ||
		LWLockTrancheNames[trancheId] == NULL)
		return "extension";

	return LWLockTrancheNames[trancheId];
}