/*-------------------------------------------------------------------------
 *
 * lwlock.h
 *	  Lightweight lock manager
 *
 *
 * Portions Copyright (c) 1996-2015, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/include/storage/lwlock.h
 *
 *-------------------------------------------------------------------------
 */
#ifndef LWLOCK_H
#define LWLOCK_H

#ifdef FRONTEND
#error "lwlock.h may not be included from frontend code"
#endif

#include "lib/ilist.h"
#include "storage/s_lock.h"
#include "port/atomics.h"

struct PGPROC;

/*
 * Prior to PostgreSQL 9.4, every lightweight lock in the system was stored
 * in a single array.  For convenience and for compatibility with past
 * releases, we still have a main array, but it's now also permissible to
 * store LWLocks elsewhere in the main shared memory segment or in a dynamic
 * shared memory segment.  Each array of lwlocks forms a separate "tranche".
 *
 * It's occasionally necessary to identify a particular LWLock "by name"; e.g.
 * because we wish to report the lock to dtrace.  We could store a name or
 * other identifying information in the lock itself, but since it's common
 * to have many nearly-identical locks (e.g. one per buffer) this would end
 * up wasting significant amounts of memory.  Instead, each lwlock stores a
 * tranche ID which tells us which array it's part of.  Based on that, we can
 * figure out where the lwlock lies within the array using the data structure
 * shown below; the lock is then identified based on the tranche name and
 * computed array index.  We need the array stride because the array might not
 * be an array of lwlocks, but rather some larger data structure that includes
 * one or more lwlocks per element.
 */
typedef struct LWLockTranche
{
	const char *name;
	void	   *array_base;
	Size		array_stride;
} LWLockTranche;

/*
 * Code outside of lwlock.c should not manipulate the contents of this
 * structure directly, but we have to declare it here to allow LWLocks to be
 * incorporated into other data structures.
 */
typedef struct LWLock
{
	slock_t		mutex;			/* Protects LWLock and queue of PGPROCs */
	uint16		tranche;		/* tranche ID */

	pg_atomic_uint32 state;		/* state of exclusive/nonexclusive lockers */
#ifdef LOCK_DEBUG
	pg_atomic_uint32 nwaiters;	/* number of waiters */
#endif
	dlist_head	waiters;		/* list of waiting PGPROCs */
#ifdef LOCK_DEBUG
	struct PGPROC *owner;		/* last exclusive owner of the lock */
#endif
} LWLock;

/*
 * In most cases, it's desirable to force each tranche of LWLocks to be aligned
 * on a cache line boundary and make the array stride a power of 2.  This saves
 * a few cycles in indexing, but more importantly ensures that individual
 * LWLocks don't cross cache line boundaries.  This reduces cache contention
 * problems, especially on AMD Opterons.  In some cases, it's useful to add
 * even more padding so that each LWLock takes up an entire cache line; this is
 * useful, for example, in the main LWLock array, where the overall number of
 * locks is small but some are heavily contended.
 *
 * When allocating a tranche that contains data other than LWLocks, it is
 * probably best to include a bare LWLock and then pad the resulting structure
 * as necessary for performance.  For an array that contains only LWLocks,
 * LWLockMinimallyPadded can be used for cases where we just want to ensure
 * that we don't cross cache line boundaries within a single lock, while
 * LWLockPadded can be used for cases where we want each lock to be an entire
 * cache line.
 *
 * On 32-bit platforms, an LWLockMinimallyPadded might actually contain more
 * than the absolute minimum amount of padding required to keep a lock from
 * crossing a cache line boundary, because an unpadded LWLock might fit into
 * 16 bytes.  We ignore that possibility when determining the minimal amount
 * of padding.  Older releases had larger LWLocks, so 32 really was the
 * minimum, and packing them in tighter might hurt performance.
 *
 * LWLOCK_MINIMAL_SIZE should be 32 on basically all common platforms, but
 * because slock_t is more than 2 bytes on some obscure platforms, we allow
 * for the possibility that it might be 64.
 */
#define LWLOCK_PADDED_SIZE	PG_CACHE_LINE_SIZE
#define LWLOCK_MINIMAL_SIZE (sizeof(LWLock) <= 32 ? 32 : 64)

/* LWLock, padded to a full cache line size */
typedef union LWLockPadded
{
	LWLock		lock;
	char		pad[LWLOCK_PADDED_SIZE];
} LWLockPadded;

/* LWLock, minimally padded */
typedef union LWLockMinimallyPadded
{
	LWLock		lock;
	char		pad[LWLOCK_MINIMAL_SIZE];
} LWLockMinimallyPadded;

extern PGDLLIMPORT LWLockPadded *MainLWLockArray;
extern char *MainLWLockNames[];

/* Names for fixed lwlocks */
#include "storage/lwlocknames.h"

/*
 * It's a bit odd to declare NUM_BUFFER_PARTITIONS and NUM_LOCK_PARTITIONS
 * here, but we need them to figure out offsets within MainLWLockArray, and
 * having this file include lock.h or bufmgr.h would be backwards.
 */

/* Number of partitions of the shared buffer mapping hashtable */
#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)

typedef enum LWLockMode
{
	LW_EXCLUSIVE,
	LW_SHARED,
	LW_WAIT_UNTIL_FREE			/* A special mode used in PGPROC->lwlockMode,
								 * when waiting for lock to become free. Not
								 * to be used as LWLockAcquire argument */
} LWLockMode;


#ifdef LOCK_DEBUG
extern bool Trace_lwlocks;
#endif

extern bool LWLockAcquire(LWLock *lock, LWLockMode mode);
extern bool LWLockConditionalAcquire(LWLock *lock, LWLockMode mode);
extern bool LWLockAcquireOrWait(LWLock *lock, LWLockMode mode);
extern void LWLockRelease(LWLock *lock);
extern void LWLockReleaseClearVar(LWLock *lock, uint64 *valptr, uint64 val);
extern void LWLockReleaseAll(void);
extern bool LWLockHeldByMe(LWLock *lock);

extern bool LWLockWaitForVar(LWLock *lock, uint64 *valptr, uint64 oldval, uint64 *newval);
extern void LWLockUpdateVar(LWLock *lock, uint64 *valptr, uint64 value);

extern Size LWLockShmemSize(void);
extern void CreateLWLocks(void);
extern void InitLWLockAccess(void);

/*
 * The traditional method for obtaining an lwlock for use by an extension is
 * to call RequestAddinLWLocks() during postmaster startup; this will reserve
 * space for the indicated number of locks in MainLWLockArray.  Subsequently,
 * a lock can be allocated using LWLockAssign.
 */
extern void RequestAddinLWLocks(int n);
extern LWLock *LWLockAssign(void);

/*
 * There is another, more flexible method of obtaining lwlocks. First, call
 * LWLockNewTrancheId just once to obtain a tranche ID; this allocates from
 * a shared counter.  Next, each individual process using the tranche should
 * call LWLockRegisterTranche() to associate that tranche ID with appropriate
 * metadata.  Finally, LWLockInitialize should be called just once per lwlock,
 * passing the tranche ID as an argument.
 *
 * It may seem strange that each process using the tranche must register it
 * separately, but dynamic shared memory segments aren't guaranteed to be
 * mapped at the same address in all coordinating backends, so storing the
 * registration in the main shared memory segment wouldn't work for that case.
 */
extern int	LWLockNewTrancheId(void);
extern void LWLockRegisterTranche(int tranche_id, LWLockTranche *tranche);
extern void LWLockInitialize(LWLock *lock, int tranche_id);

/*
 * We reserve a few predefined tranche IDs.  A call to LWLockNewTrancheId
 * will never return a value less than LWTRANCHE_FIRST_USER_DEFINED.
 */
typedef enum BuiltinTrancheIds
{
	LWTRANCHE_MAIN,
	LWTRANCHE_WAL_INSERT,
	LWTRANCHE_BUFFER_CONTENT,
	LWTRANCHE_BUFFER_IO_IN_PROGRESS,
	LWTRANCHE_FIRST_USER_DEFINED
}	BuiltinTrancheIds;

/*
 * Prior to PostgreSQL 9.4, we used an enum type called LWLockId to refer
 * to LWLocks.  New code should instead use LWLock *.  However, for the
 * convenience of third-party code, we include the following typedef.
 */
typedef LWLock *LWLockId;

#endif   /* LWLOCK_H */
