postgreSQL 中的 Lock

最新推荐文章于 2025-11-04 08:54:55 发布

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本文介绍了数据库锁机制中的关键结构，包括LOCK、LOCALLOCK和PROCLOCK等，并详细解释了这些结构的作用及其实现方式。此外还定义了不同类型的锁及其用途。

typedef struct LOCK
{
/* hash key */
LOCKTAG tag; /* unique identifier of lockable object */

/* data */
LOCKMASK grantMask;   /* bitmask for lock types already granted */
LOCKMASK waitMask;   /* bitmask for lock types awaited */
SHM_QUEUE procLocks;   /* list of PROCLOCK objects assoc. with lock */
PROC_QUEUE waitProcs;   /* list of PGPROC objects waiting on lock */
int    requested[MAX_LOCKMODES];   /* counts of requested locks */
int    nRequested;   /* total of requested[] array */
int    granted[MAX_LOCKMODES]; /* counts of granted locks */
int    nGranted;   /* total of granted[] array */
} LOCK;

LOCALLOCK 是在私有进程中的结构，它有指针指向共享内存中实际的锁

typedef struct LOCALLOCK
{
/* tag */
LOCALLOCKTAG tag; /* unique identifier of locallock entry */

/* data */
LOCK    *lock;    /* associated LOCK object in shared mem */ 指向共享内存中的
PROCLOCK   *proclock;   /* associated PROCLOCK object in shmem */
uint32   hashcode;   /* copy of LOCKTAG's hash value */
int64   nLocks;    /* total number of times lock is held */
int    numLockOwners; /* # of relevant ResourceOwners */
int    maxLockOwners; /* allocated size of array */
LOCALLOCKOWNER *lockOwners; /* dynamically resizable array */
} LOCALLOCK;

共享内存中实际的锁

typedef struct LOCK
{
/* hash key */
LOCKTAG tag; /* unique identifier of lockable object */

LOCALLOCK 和 LOCK 为了方便查找，全部放在了 hash table 中

We may have several different backends holding or awaiting locks on the same lockable object. We need to store some per-holder/waiter information for each such holder (or would-be holder). This is kept in a PROCLOCK struct

typedef struct PROCLOCK
{
/* tag */
PROCLOCKTAG tag; /* unique identifier of proclock object */

/* data */
LOCKMASK holdMask;   /* bitmask for lock types currently held */
LOCKMASK releaseMask; /* bitmask for lock types to be released */
SHM_QUEUE lockLink;   /* list link in LOCK's list of proclocks */
SHM_QUEUE procLink;   /* list link in PGPROC's list of proclocks */
} PROCLOCK;

未获得锁的等待是通过每个进程的临界区实现的，当没获得锁时，就进入本进程的临界区，并把本进程放入LOCK的waitProcs队列中，当别的进程解锁后，检查waitProcs，把里面存储的等候的进程出临界区

锁的种类

#define NoLock 0

#define AccessShareLock    1   /* SELECT */
#define RowShareLock    2   /* SELECT FOR UPDATE/FOR SHARE */
#define RowExclusiveLock   3   /* INSERT, UPDATE, DELETE */
#define ShareUpdateExclusiveLock 4   /* VACUUM (non-FULL),ANALYZE, CREATE
           * INDEX CONCURRENTLY */
#define ShareLock     5   /* CREATE INDEX (WITHOUT CONCURRENTLY) */
#define ShareRowExclusiveLock 6   /* like EXCLUSIVE MODE, but allows ROW
           * SHARE */
#define ExclusiveLock    7   /* blocks ROW SHARE/SELECT...FOR
           * UPDATE */
#define AccessExclusiveLock   8   /* ALTER TABLE, DROP TABLE, VACUUM
           * FULL, and unqualified LOCK TABLE */