postgreSQL 的 lightweight lock

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