LWLocks are normally used to protect not-very-long sections of computation
typedef struct LWLock
{
slock_t mutex; /* Protects LWLock and queue of PGPROCs */
bool releaSEOK; /* T if ok to release waiters */
char exclusive; /* # of exclusive holders (0 or 1) */
int shared; /* # of shared holders (0..MaxBackends) */
PGPROC *head; /* head of list of waiting PGPROCs */
PGPROC *tail; /* tail of list of waiting PGPROCs */
/* tail is undefined when head is NULL */
} LWLock;
LWLock既可以封锁exclusive，又可以封锁shared,而且一个锁可以存在一个等待队列，队列中两种锁都可以有
lw lock 有一部分是固定的，在 lwlock.h 中
typedef enum LWLockId
{
BufFreelistLock,
ShmemIndexLock,
OidGenLock,
XidGenLock,
ProcArrayLock,
SInvalReadLock,
SInvalWriteLock,
WALInsertLock,
WALWriteLock,
ControlFileLock,
CheckpointLock,
CLogControlLock,
SubtransControlLock,
MultixactGenLock,
MultixactOffsetControlLock,
MultixactMemberControlLock,
RelCacheInitLock,
BgWriterCommlock,
TwoPhaseStateLock,
TablespaceCreateLock,
BtreeVacuumlock,
AddinShmemInitLock,
Autovacuumlock,
AutovacuumScheduleLock,
SyncScanLock,
/* Individual lock IDs end here */
FirstBufMappingLock,
FirstLockMgrLock = FirstBufMappingLock + NUM_BUFFER_PARTITIONS,
/* must be last except for MaxDynamicLWLock: */
NumFixedLWLocks = FirstLockMgrLock + NUM_LOCK_PARTITIONS,
MaxDynamicLWLock = 1000000000
} LWLockId;
余下的是可被分配的，总共有8317个，这是多个计算出来的
