src/backend/storage/lmgr/lwlock.c - master - githubmirror/postgres

src/backend/storage/lmgr/lwlock.c
1 939 строк54 KB
Heikki Linnakangas
Convert lwlock.c to use the new shmem allocation functions
06 апр 2026, 02:12
06 апр 2026, 02:12a006bc7
/*-------------------------------------------------------------------------
 *
 * lwlock.c
 *	  Lightweight lock manager
 *
 * Lightweight locks are intended primarily to provide mutual exclusion of
 * access to shared-memory data structures.  Therefore, they offer both
 * exclusive and shared lock modes (to support read/write and read-only
 * access to a shared object).  There are few other frammishes.  User-level
 * locking should be done with the full lock manager --- which depends on
 * LWLocks to protect its shared state.
 *
 * In addition to exclusive and shared modes, lightweight locks can be used to
 * wait until a variable changes value.  The variable is initially not set
 * when the lock is acquired with LWLockAcquire, i.e. it remains set to the
 * value it was set to when the lock was released last, and can be updated
 * without releasing the lock by calling LWLockUpdateVar.  LWLockWaitForVar
 * waits for the variable to be updated, or until the lock is free.  When
 * releasing the lock with LWLockReleaseClearVar() the value can be set to an
 * appropriate value for a free lock.  The meaning of the variable is up to
 * the caller, the lightweight lock code just assigns and compares it.
 *
 * Portions Copyright (c) 1996-2026, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *	  src/backend/storage/lmgr/lwlock.c
 *
 * NOTES:
 *
 * This used to be a pretty straight forward reader-writer lock
 * implementation, in which the internal state was protected by a
 * spinlock. Unfortunately the overhead of taking the spinlock proved to be
 * too high for workloads/locks that were taken in shared mode very
 * frequently. Often we were spinning in the (obviously exclusive) spinlock,
 * while trying to acquire a shared lock that was actually free.
 *
 * Thus a new implementation was devised that provides wait-free shared lock
 * acquisition for locks that aren't exclusively locked.
 *
 * The basic idea is to have a single atomic variable 'lockcount' instead of
 * the formerly separate shared and exclusive counters and to use atomic
 * operations to acquire the lock. That's fairly easy to do for plain
 * rw-spinlocks, but a lot harder for something like LWLocks that want to wait
 * in the OS.
 *
 * For lock acquisition we use an atomic compare-and-exchange on the lockcount
 * variable. For exclusive lock we swap in a sentinel value
 * (LW_VAL_EXCLUSIVE), for shared locks we count the number of holders.
 *
 * To release the lock we use an atomic decrement to release the lock. If the
 * new value is zero (we get that atomically), we know we can/have to release
 * waiters.
 *
 * Obviously it is important that the sentinel value for exclusive locks
 * doesn't conflict with the maximum number of possible share lockers -
 * luckily MAX_BACKENDS makes that easily possible.
 *
 *
 * The attentive reader might have noticed that naively doing the above has a
 * glaring race condition: We try to lock using the atomic operations and
 * notice that we have to wait. Unfortunately by the time we have finished
 * queuing, the former locker very well might have already finished its
 * work. That's problematic because we're now stuck waiting inside the OS.

 * To mitigate those races we use a two phased attempt at locking:
 *	 Phase 1: Try to do it atomically, if we succeed, nice
 *	 Phase 2: Add ourselves to the waitqueue of the lock
 *	 Phase 3: Try to grab the lock again, if we succeed, remove ourselves from
 *			  the queue
 *	 Phase 4: Sleep till wake-up, goto Phase 1
 *
 * This protects us against the problem from above as nobody can release too
 *	  quick, before we're queued, since after Phase 2 we're already queued.
 * -------------------------------------------------------------------------
 */
#include "postgres.h"

#include "miscadmin.h"
#include "pg_trace.h"
#include "pgstat.h"
#include "port/pg_bitutils.h"
#include "storage/proc.h"
#include "storage/proclist.h"
#include "storage/procnumber.h"
#include "storage/spin.h"
#include "storage/subsystems.h"
#include "utils/memutils.h"
#include "utils/wait_event.h"

#ifdef LWLOCK_STATS
#include "utils/hsearch.h"
#endif


#define LW_FLAG_HAS_WAITERS			((uint32) 1 << 31)
#define LW_FLAG_WAKE_IN_PROGRESS	((uint32) 1 << 30)
#define LW_FLAG_LOCKED				((uint32) 1 << 29)
#define LW_FLAG_BITS				3
#define LW_FLAG_MASK				(((1<<LW_FLAG_BITS)-1)<<(32-LW_FLAG_BITS))

/* assumes MAX_BACKENDS is a (power of 2) - 1, checked below */
#define LW_VAL_EXCLUSIVE			(MAX_BACKENDS + 1)
#define LW_VAL_SHARED				1

/* already (power of 2)-1, i.e. suitable for a mask */
#define LW_SHARED_MASK				MAX_BACKENDS
#define LW_LOCK_MASK				(MAX_BACKENDS | LW_VAL_EXCLUSIVE)


StaticAssertDecl(((MAX_BACKENDS + 1) & MAX_BACKENDS) == 0,
				 "MAX_BACKENDS + 1 needs to be a power of 2");

StaticAssertDecl((MAX_BACKENDS & LW_FLAG_MASK) == 0,
				 "MAX_BACKENDS and LW_FLAG_MASK overlap");

StaticAssertDecl((LW_VAL_EXCLUSIVE & LW_FLAG_MASK) == 0,
				 "LW_VAL_EXCLUSIVE and LW_FLAG_MASK overlap");

/*
 * There are three sorts of LWLock "tranches":
 *
 * 1. The individually-named locks defined in lwlocklist.h each have their
 * own tranche.  We absorb the names of these tranches from there into
 * BuiltinTrancheNames here.
 *
 * 2. There are some predefined tranches for built-in groups of locks defined
 * in lwlocklist.h.  We absorb the names of these tranches, too.
 *
 * 3. Extensions can create new tranches, via either RequestNamedLWLockTranche
 * or LWLockNewTrancheId.  These are stored in shared memory and can be
 * accessed via LWLockTranches.
 *
 * All these names are user-visible as wait event names, so choose with care
 * ... and do not forget to update the documentation's list of wait events.
 */
static const char *const BuiltinTrancheNames[] = {
#define PG_LWLOCK(id, lockname) [id] = CppAsString(lockname),
#define PG_LWLOCKTRANCHE(id, lockname) [LWTRANCHE_##id] = CppAsString(lockname),
#include "storage/lwlocklist.h"
#undef PG_LWLOCK
#undef PG_LWLOCKTRANCHE
};

StaticAssertDecl(lengthof(BuiltinTrancheNames) ==
				 LWTRANCHE_FIRST_USER_DEFINED,
				 "missing entries in BuiltinTrancheNames[]");

/* Main array of LWLocks in shared memory */
LWLockPadded *MainLWLockArray = NULL;

/*
 * We use this structure to keep track of locked LWLocks for release
 * during error recovery.  Normally, only a few will be held at once, but
 * occasionally the number can be much higher.
 */
#define MAX_SIMUL_LWLOCKS	200

/* struct representing the LWLocks we're holding */
typedef struct LWLockHandle
{
	LWLock	   *lock;
	LWLockMode	mode;
} LWLockHandle;

static int	num_held_lwlocks = 0;
static LWLockHandle held_lwlocks[MAX_SIMUL_LWLOCKS];

/* Maximum number of LWLock tranches that can be assigned by extensions */
#define MAX_USER_DEFINED_TRANCHES 256

/*
 * Shared memory structure holding user-defined tranches.
 */
typedef struct LWLockTrancheShmemData
{
	/* This is indexed by tranche ID minus LWTRANCHE_FIRST_USER_DEFINED */
	struct
	{
		char		name[NAMEDATALEN];

		/*
		 * Index of the tranche's locks in MainLWLockArray if this tranche was
		 * allocated with RequestNamedLWLockTranche(), or -1 if the tranche
		 * was allocated with LWLockNewTrancheId()
		 */
		int			main_array_idx;
	}			user_defined[MAX_USER_DEFINED_TRANCHES];

	int			num_user_defined;	/* 'user_defined' entries in use */

	slock_t		lock;			/* protects the above */
} LWLockTrancheShmemData;

static LWLockTrancheShmemData *LWLockTranches;

/* backend-local copy of LWLockTranches->num_user_defined */
static int	LocalNumUserDefinedTranches;

/*
 * NamedLWLockTrancheRequests is a list of tranches requested with
 * RequestNamedLWLockTranche().  It is only valid in the postmaster; after
 * startup the tranches are tracked in LWLockTranches in shared memory.
 */
typedef struct NamedLWLockTrancheRequest
{
	char		tranche_name[NAMEDATALEN];
	int			num_lwlocks;
} NamedLWLockTrancheRequest;

static List *NamedLWLockTrancheRequests = NIL;

/* Size of MainLWLockArray.  Only valid in postmaster. */
static int	num_main_array_locks;

static void LWLockShmemRequest(void *arg);
static void LWLockShmemInit(void *arg);

const ShmemCallbacks LWLockCallbacks = {
	.request_fn = LWLockShmemRequest,
	.init_fn = LWLockShmemInit,
};


static inline void LWLockReportWaitStart(LWLock *lock);
static inline void LWLockReportWaitEnd(void);
static const char *GetLWTrancheName(uint16 trancheId);

#define T_NAME(lock) \
	GetLWTrancheName((lock)->tranche)

#ifdef LWLOCK_STATS
typedef struct lwlock_stats_key
{
	int			tranche;
	void	   *instance;
}			lwlock_stats_key;

typedef struct lwlock_stats
{
	lwlock_stats_key key;
	int			sh_acquire_count;
	int			ex_acquire_count;
	int			block_count;
	int			dequeue_self_count;
	int			spin_delay_count;
}			lwlock_stats;

static HTAB *lwlock_stats_htab;
static lwlock_stats lwlock_stats_dummy;
#endif

#ifdef LOCK_DEBUG
bool		Trace_lwlocks = false;

inline static void
PRINT_LWDEBUG(const char *where, LWLock *lock, LWLockMode mode)
{
	/* hide statement & context here, otherwise the log is just too verbose */
	if (Trace_lwlocks)
	{
		uint32		state = pg_atomic_read_u32(&lock->state);

		ereport(LOG,
				(errhidestmt(true),
				 errhidecontext(true),
				 errmsg_internal("%d: %s(%s %p): excl %u shared %u haswaiters %u waiters %u waking %d",
								 MyProcPid,
								 where, T_NAME(lock), lock,
								 (state & LW_VAL_EXCLUSIVE) != 0,
								 state & LW_SHARED_MASK,
								 (state & LW_FLAG_HAS_WAITERS) != 0,
								 pg_atomic_read_u32(&lock->nwaiters),
								 (state & LW_FLAG_WAKE_IN_PROGRESS) != 0)));
	}
}

inline static void
LOG_LWDEBUG(const char *where, LWLock *lock, const char *msg)
{
	/* hide statement & context here, otherwise the log is just too verbose */
	if (Trace_lwlocks)
	{
		ereport(LOG,
				(errhidestmt(true),
				 errhidecontext(true),
				 errmsg_internal("%s(%s %p): %s", where,
								 T_NAME(lock), lock, msg)));
	}
}

#else							/* not LOCK_DEBUG */
#define PRINT_LWDEBUG(a,b,c) ((void)0)
#define LOG_LWDEBUG(a,b,c) ((void)0)
#endif							/* LOCK_DEBUG */

#ifdef LWLOCK_STATS

static void init_lwlock_stats(void);
static void print_lwlock_stats(int code, Datum arg);
static lwlock_stats * get_lwlock_stats_entry(LWLock *lock);

static void
init_lwlock_stats(void)
{
	HASHCTL		ctl;
	static MemoryContext lwlock_stats_cxt = NULL;
	static bool exit_registered = false;

	if (lwlock_stats_cxt != NULL)
		MemoryContextDelete(lwlock_stats_cxt);

	/*
	 * The LWLock stats will be updated within a critical section, which
	 * requires allocating new hash entries. Allocations within a critical
	 * section are normally not allowed because running out of memory would
	 * lead to a PANIC, but LWLOCK_STATS is debugging code that's not normally
	 * turned on in production, so that's an acceptable risk. The hash entries
	 * are small, so the risk of running out of memory is minimal in practice.
	 */
	lwlock_stats_cxt = AllocSetContextCreate(TopMemoryContext,
											 "LWLock stats",
											 ALLOCSET_DEFAULT_SIZES);
	MemoryContextAllowInCriticalSection(lwlock_stats_cxt, true);

	ctl.keysize = sizeof(lwlock_stats_key);
	ctl.entrysize = sizeof(lwlock_stats);
	ctl.hcxt = lwlock_stats_cxt;
	lwlock_stats_htab = hash_create("lwlock stats", 16384, &ctl,
									HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);
	if (!exit_registered)
	{
		on_shmem_exit(print_lwlock_stats, 0);
		exit_registered = true;
	}
}

static void
print_lwlock_stats(int code, Datum arg)
{
	HASH_SEQ_STATUS scan;
	lwlock_stats *lwstats;

	hash_seq_init(&scan, lwlock_stats_htab);

	/* Grab an LWLock to keep different backends from mixing reports */
	LWLockAcquire(&MainLWLockArray[0].lock, LW_EXCLUSIVE);

	while ((lwstats = (lwlock_stats *) hash_seq_search(&scan)) != NULL)
	{
		fprintf(stderr,
				"PID %d lwlock %s %p: shacq %u exacq %u blk %u spindelay %u dequeue self %u\n",
				MyProcPid, GetLWTrancheName(lwstats->key.tranche),
				lwstats->key.instance, lwstats->sh_acquire_count,
				lwstats->ex_acquire_count, lwstats->block_count,
				lwstats->spin_delay_count, lwstats->dequeue_self_count);
	}

	LWLockRelease(&MainLWLockArray[0].lock);
}

static lwlock_stats *
get_lwlock_stats_entry(LWLock *lock)
{
	lwlock_stats_key key;
	lwlock_stats *lwstats;
	bool		found;

	/*
	 * During shared memory initialization, the hash table doesn't exist yet.
	 * Stats of that phase aren't very interesting, so just collect operations
	 * on all locks in a single dummy entry.
	 */
	if (lwlock_stats_htab == NULL)
		return &lwlock_stats_dummy;

	/* Fetch or create the entry. */
	MemSet(&key, 0, sizeof(key));
	key.tranche = lock->tranche;
	key.instance = lock;
	lwstats = hash_search(lwlock_stats_htab, &key, HASH_ENTER, &found);
	if (!found)
	{
		lwstats->sh_acquire_count = 0;
		lwstats->ex_acquire_count = 0;
		lwstats->block_count = 0;
		lwstats->dequeue_self_count = 0;
		lwstats->spin_delay_count = 0;
	}
	return lwstats;
}
#endif							/* LWLOCK_STATS */


/*
 * Compute number of LWLocks required by user-defined tranches requested with
 * RequestNamedLWLockTranche().  These will be allocated in the main array.
 */
static int
NumLWLocksForNamedTranches(void)
{
	int			numLocks = 0;

	foreach_ptr(NamedLWLockTrancheRequest, request, NamedLWLockTrancheRequests)
	{
		numLocks += request->num_lwlocks;
	}

	return numLocks;
}

/*
 * Request shmem space for user-defined tranches and the main LWLock array.
 */
static void
LWLockShmemRequest(void *arg)
{
	size_t		size;

	/* Space for user-defined tranches */
	ShmemRequestStruct(.name = "LWLock tranches",
					   .size = sizeof(LWLockTrancheShmemData),
					   .ptr = (void **) &LWLockTranches,
		);

	/* Space for the LWLock array */
	if (!IsUnderPostmaster)
	{
		num_main_array_locks = NUM_FIXED_LWLOCKS + NumLWLocksForNamedTranches();
		size = num_main_array_locks * sizeof(LWLockPadded);
	}
	else
		size = SHMEM_ATTACH_UNKNOWN_SIZE;

	ShmemRequestStruct(.name = "Main LWLock array",
					   .size = size,
					   .ptr = (void **) &MainLWLockArray,
		);
}

/*
 * Initialize shmem space for user-defined tranches and the main LWLock array.
 */
static void
LWLockShmemInit(void *arg)
{
	int			pos;

	/* Initialize the dynamic-allocation counter for tranches */
	LWLockTranches->num_user_defined = 0;

	SpinLockInit(&LWLockTranches->lock);

	/*
	 * Allocate and initialize all LWLocks in the main array.  It includes all
	 * LWLocks for built-in tranches and those requested with
	 * RequestNamedLWLockTranche().
	 */
	pos = 0;

	/* Initialize all individual LWLocks in main array */
	for (int id = 0; id < NUM_INDIVIDUAL_LWLOCKS; id++)
		LWLockInitialize(&MainLWLockArray[pos++].lock, id);

	/* Initialize buffer mapping LWLocks in main array */
	Assert(pos == BUFFER_MAPPING_LWLOCK_OFFSET);
	for (int i = 0; i < NUM_BUFFER_PARTITIONS; i++)
		LWLockInitialize(&MainLWLockArray[pos++].lock, LWTRANCHE_BUFFER_MAPPING);

	/* Initialize lmgrs' LWLocks in main array */
	Assert(pos == LOCK_MANAGER_LWLOCK_OFFSET);
	for (int i = 0; i < NUM_LOCK_PARTITIONS; i++)
		LWLockInitialize(&MainLWLockArray[pos++].lock, LWTRANCHE_LOCK_MANAGER);

	/* Initialize predicate lmgrs' LWLocks in main array */
	Assert(pos == PREDICATELOCK_MANAGER_LWLOCK_OFFSET);
	for (int i = 0; i < NUM_PREDICATELOCK_PARTITIONS; i++)
		LWLockInitialize(&MainLWLockArray[pos++].lock, LWTRANCHE_PREDICATE_LOCK_MANAGER);

	/*
	 * Copy the info about any user-defined tranches into shared memory (so
	 * that other processes can see it), and initialize the requested LWLocks.
	 */
	Assert(pos == NUM_FIXED_LWLOCKS);
	foreach_ptr(NamedLWLockTrancheRequest, request, NamedLWLockTrancheRequests)
	{
		int			idx = (LWLockTranches->num_user_defined++);

		strlcpy(LWLockTranches->user_defined[idx].name,
				request->tranche_name,
				NAMEDATALEN);
		LWLockTranches->user_defined[idx].main_array_idx = pos;

		for (int i = 0; i < request->num_lwlocks; i++)
			LWLockInitialize(&MainLWLockArray[pos++].lock, LWTRANCHE_FIRST_USER_DEFINED + idx);
	}

	/* Cross-check that we agree on the total size with LWLockShmemRequest() */
	Assert(pos == num_main_array_locks);
}

/*
 * InitLWLockAccess - initialize backend-local state needed to hold LWLocks
 */
void
InitLWLockAccess(void)
{
#ifdef LWLOCK_STATS
	init_lwlock_stats();
#endif
}

/*
 * GetNamedLWLockTranche - returns the base address of LWLock from the
 *		specified tranche.
 *
 * Caller needs to retrieve the requested number of LWLocks starting from
 * the base lock address returned by this API.  This can be used for
 * tranches that are requested by using RequestNamedLWLockTranche() API.
 */
LWLockPadded *
GetNamedLWLockTranche(const char *tranche_name)
{
	SpinLockAcquire(&LWLockTranches->lock);
	LocalNumUserDefinedTranches = LWLockTranches->num_user_defined;
	SpinLockRelease(&LWLockTranches->lock);

	/*
	 * Obtain the position of base address of LWLock belonging to requested
	 * tranche_name in MainLWLockArray.  LWLocks for user-defined tranches
	 * requested with RequestNamedLWLockTranche() are placed in
	 * MainLWLockArray after fixed locks.
	 */
	for (int i = 0; i < LocalNumUserDefinedTranches; i++)
	{
		if (strcmp(LWLockTranches->user_defined[i].name,
				   tranche_name) == 0)
		{
			int			lock_pos = LWLockTranches->user_defined[i].main_array_idx;

			/*
			 * GetNamedLWLockTranche() should only be used for locks requested
			 * with RequestNamedLWLockTranche(), not those allocated with
			 * LWLockNewTrancheId().
			 */
			if (lock_pos == -1)
				elog(ERROR, "requested tranche was not registered with RequestNamedLWLockTranche()");
			return &MainLWLockArray[lock_pos];
		}
	}

	elog(ERROR, "requested tranche is not registered");

	/* just to keep compiler quiet */
	return NULL;
}

/*
 * Allocate a new tranche ID with the provided name.
 */
int
LWLockNewTrancheId(const char *name)
{
	int			idx;

	if (!name)
		ereport(ERROR,
				(errcode(ERRCODE_INVALID_NAME),
				 errmsg("tranche name cannot be NULL")));

	if (strlen(name) >= NAMEDATALEN)
		ereport(ERROR,
				(errcode(ERRCODE_NAME_TOO_LONG),
				 errmsg("tranche name too long"),
				 errdetail("LWLock tranche names must be no longer than %d bytes.",
						   NAMEDATALEN - 1)));

	/* The counter and the tranche names are protected by the spinlock */
	SpinLockAcquire(&LWLockTranches->lock);

	if (LWLockTranches->num_user_defined >= MAX_USER_DEFINED_TRANCHES)
	{
		SpinLockRelease(&LWLockTranches->lock);
		ereport(ERROR,
				(errmsg("maximum number of tranches already registered"),
				 errdetail("No more than %d tranches may be registered.",
						   MAX_USER_DEFINED_TRANCHES)));
	}

	/* Allocate an entry in the user_defined array */
	idx = (LWLockTranches->num_user_defined)++;

	/* update our local copy while we're at it */
	LocalNumUserDefinedTranches = LWLockTranches->num_user_defined;

	/* Initialize it */
	strlcpy(LWLockTranches->user_defined[idx].name, name, NAMEDATALEN);

	/* the locks are not in the main array */
	LWLockTranches->user_defined[idx].main_array_idx = -1;

	SpinLockRelease(&LWLockTranches->lock);

	return LWTRANCHE_FIRST_USER_DEFINED + idx;
}

/*
 * RequestNamedLWLockTranche
 *		Request that extra LWLocks be allocated during postmaster
 *		startup.
 *
 * This may only be called via the shmem_request_hook of a library that is
 * loaded into the postmaster via shared_preload_libraries.  Calls from
 * elsewhere will fail.
 *
 * The tranche name will be user-visible as a wait event name, so try to
 * use a name that fits the style for those.
 */
void
RequestNamedLWLockTranche(const char *tranche_name, int num_lwlocks)
{
	NamedLWLockTrancheRequest *request;
	MemoryContext oldcontext;

	if (!process_shmem_requests_in_progress)
		elog(FATAL, "cannot request additional LWLocks outside shmem_request_hook");

	if (!tranche_name)
		ereport(ERROR,
				(errcode(ERRCODE_INVALID_NAME),
				 errmsg("tranche name cannot be NULL")));

	if (strlen(tranche_name) >= NAMEDATALEN)
		ereport(ERROR,
				(errcode(ERRCODE_NAME_TOO_LONG),
				 errmsg("tranche name too long"),
				 errdetail("LWLock tranche names must be no longer than %d bytes.",
						   NAMEDATALEN - 1)));

	if (list_length(NamedLWLockTrancheRequests) >= MAX_USER_DEFINED_TRANCHES)
		ereport(ERROR,
				(errmsg("maximum number of tranches already registered"),
				 errdetail("No more than %d tranches may be registered.",
						   MAX_USER_DEFINED_TRANCHES)));

	/* Check that the name isn't already in use */
	foreach_ptr(NamedLWLockTrancheRequest, existing, NamedLWLockTrancheRequests)
	{
		if (strcmp(existing->tranche_name, tranche_name) == 0)
			elog(ERROR, "requested tranche \"%s\" is already registered", tranche_name);
	}

	if (IsPostmasterEnvironment)
		oldcontext = MemoryContextSwitchTo(PostmasterContext);
	else
		oldcontext = MemoryContextSwitchTo(TopMemoryContext);

	request = palloc0(sizeof(NamedLWLockTrancheRequest));
	strlcpy(request->tranche_name, tranche_name, NAMEDATALEN);
	request->num_lwlocks = num_lwlocks;
	NamedLWLockTrancheRequests = lappend(NamedLWLockTrancheRequests, request);

	MemoryContextSwitchTo(oldcontext);
}

/*
 * LWLockInitialize - initialize a new lwlock; it's initially unlocked
 */
void
LWLockInitialize(LWLock *lock, int tranche_id)
{
	/* verify the tranche_id is valid */
	(void) GetLWTrancheName(tranche_id);

	pg_atomic_init_u32(&lock->state, 0);
#ifdef LOCK_DEBUG
	pg_atomic_init_u32(&lock->nwaiters, 0);
#endif
	lock->tranche = tranche_id;
	proclist_init(&lock->waiters);
}

/*
 * Report start of wait event for light-weight locks.
 *
 * This function will be used by all the light-weight lock calls which
 * needs to wait to acquire the lock.  This function distinguishes wait
 * event based on tranche and lock id.
 */
static inline void
LWLockReportWaitStart(LWLock *lock)
{
	pgstat_report_wait_start(PG_WAIT_LWLOCK | lock->tranche);
}

/*
 * Report end of wait event for light-weight locks.
 */
static inline void
LWLockReportWaitEnd(void)
{
	pgstat_report_wait_end();
}

/*
 * Return the name of an LWLock tranche.
 */
static const char *
GetLWTrancheName(uint16 trancheId)
{
	int			idx;

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

	/*
	 * It's an extension tranche, so look in LWLockTranches->user_defined.
	 */
	idx = trancheId - LWTRANCHE_FIRST_USER_DEFINED;

	/*
	 * We only ever add new entries to LWLockTranches->user_defined, so most
	 * lookups can avoid taking the spinlock as long as the backend-local
	 * counter (LocalNumUserDefinedTranches) is greater than the requested
	 * tranche ID.  Else, we need to first update the backend-local counter
	 * with the spinlock held before attempting the lookup again.  In
	 * practice, the latter case is probably rare.
	 */
	if (idx >= LocalNumUserDefinedTranches)
	{
		SpinLockAcquire(&LWLockTranches->lock);
		LocalNumUserDefinedTranches = LWLockTranches->num_user_defined;
		SpinLockRelease(&LWLockTranches->lock);

		if (idx >= LocalNumUserDefinedTranches)
			elog(ERROR, "tranche %d is not registered", trancheId);
	}

	return LWLockTranches->user_defined[idx].name;
}

/*
 * Return an identifier for an LWLock based on the wait class and event.
 */
const char *
GetLWLockIdentifier(uint32 classId, uint16 eventId)
{
	Assert(classId == PG_WAIT_LWLOCK);
	/* The event IDs are just tranche numbers. */
	return GetLWTrancheName(eventId);
}

/*
 * Internal function that tries to atomically acquire the lwlock in the passed
 * in mode.
 *
 * This function will not block waiting for a lock to become free - that's the
 * caller's job.
 *
 * Returns true if the lock isn't free and we need to wait.
 */
static bool
LWLockAttemptLock(LWLock *lock, LWLockMode mode)
{
	uint32		old_state;

	Assert(mode == LW_EXCLUSIVE || mode == LW_SHARED);

	/*
	 * Read once outside the loop, later iterations will get the newer value
	 * via compare & exchange.
	 */
	old_state = pg_atomic_read_u32(&lock->state);

	/* loop until we've determined whether we could acquire the lock or not */
	while (true)
	{
		uint32		desired_state;
		bool		lock_free;

		desired_state = old_state;

		if (mode == LW_EXCLUSIVE)
		{
			lock_free = (old_state & LW_LOCK_MASK) == 0;
			if (lock_free)
				desired_state += LW_VAL_EXCLUSIVE;
		}
		else
		{
			lock_free = (old_state & LW_VAL_EXCLUSIVE) == 0;
			if (lock_free)
				desired_state += LW_VAL_SHARED;
		}

		/*
		 * Attempt to swap in the state we are expecting. If we didn't see
		 * lock to be free, that's just the old value. If we saw it as free,
		 * we'll attempt to mark it acquired. The reason that we always swap
		 * in the value is that this doubles as a memory barrier. We could try
		 * to be smarter and only swap in values if we saw the lock as free,
		 * but benchmark haven't shown it as beneficial so far.
		 *
		 * Retry if the value changed since we last looked at it.
		 */
		if (pg_atomic_compare_exchange_u32(&lock->state,
										   &old_state, desired_state))
		{
			if (lock_free)
			{
				/* Great! Got the lock. */
#ifdef LOCK_DEBUG
				if (mode == LW_EXCLUSIVE)
					lock->owner = MyProc;
#endif
				return false;
			}
			else
				return true;	/* somebody else has the lock */
		}
	}
	pg_unreachable();
}

/*
 * Lock the LWLock's wait list against concurrent activity.
 *
 * NB: even though the wait list is locked, non-conflicting lock operations
 * may still happen concurrently.
 *
 * Time spent holding mutex should be short!
 */
static void
LWLockWaitListLock(LWLock *lock)
{
	uint32		old_state;
#ifdef LWLOCK_STATS
	lwlock_stats *lwstats;
	uint32		delays = 0;

	lwstats = get_lwlock_stats_entry(lock);
#endif

	while (true)
	{
		/*
		 * Always try once to acquire the lock directly, without setting up
		 * the spin-delay infrastructure. The work necessary for that shows up
		 * in profiles and is rarely necessary.
		 */
		old_state = pg_atomic_fetch_or_u32(&lock->state, LW_FLAG_LOCKED);
		if (likely(!(old_state & LW_FLAG_LOCKED)))
			break;				/* got lock */

		/* and then spin without atomic operations until lock is released */
		{
			SpinDelayStatus delayStatus;

			init_local_spin_delay(&delayStatus);

			while (old_state & LW_FLAG_LOCKED)
			{
				perform_spin_delay(&delayStatus);
				old_state = pg_atomic_read_u32(&lock->state);
			}
#ifdef LWLOCK_STATS
			delays += delayStatus.delays;
#endif
			finish_spin_delay(&delayStatus);
		}

		/*
		 * Retry. The lock might obviously already be re-acquired by the time
		 * we're attempting to get it again.
		 */
	}

#ifdef LWLOCK_STATS
	lwstats->spin_delay_count += delays;
#endif
}

/*
 * Unlock the LWLock's wait list.
 *
 * Note that it can be more efficient to manipulate flags and release the
 * locks in a single atomic operation.
 */
static void
LWLockWaitListUnlock(LWLock *lock)
{
	uint32		old_state PG_USED_FOR_ASSERTS_ONLY;

	old_state = pg_atomic_fetch_and_u32(&lock->state, ~LW_FLAG_LOCKED);

	Assert(old_state & LW_FLAG_LOCKED);
}

/*
 * Wakeup all the lockers that currently have a chance to acquire the lock.
 */
static void
LWLockWakeup(LWLock *lock)
{
	bool		new_wake_in_progress = false;
	bool		wokeup_somebody = false;
	proclist_head wakeup;
	proclist_mutable_iter iter;

	proclist_init(&wakeup);

	/* lock wait list while collecting backends to wake up */
	LWLockWaitListLock(lock);

	proclist_foreach_modify(iter, &lock->waiters, lwWaitLink)
	{
		PGPROC	   *waiter = GetPGProcByNumber(iter.cur);

		if (wokeup_somebody && waiter->lwWaitMode == LW_EXCLUSIVE)
			continue;

		proclist_delete(&lock->waiters, iter.cur, lwWaitLink);
		proclist_push_tail(&wakeup, iter.cur, lwWaitLink);

		if (waiter->lwWaitMode != LW_WAIT_UNTIL_FREE)
		{
			/*
			 * Prevent additional wakeups until retryer gets to run. Backends
			 * that are just waiting for the lock to become free don't retry
			 * automatically.
			 */
			new_wake_in_progress = true;

			/*
			 * Don't wakeup (further) exclusive locks.
			 */
			wokeup_somebody = true;
		}

		/*
		 * Signal that the process isn't on the wait list anymore. This allows
		 * LWLockDequeueSelf() to remove itself of the waitlist with a
		 * proclist_delete(), rather than having to check if it has been
		 * removed from the list.
		 */
		Assert(waiter->lwWaiting == LW_WS_WAITING);
		waiter->lwWaiting = LW_WS_PENDING_WAKEUP;

		/*
		 * Once we've woken up an exclusive lock, there's no point in waking
		 * up anybody else.
		 */
		if (waiter->lwWaitMode == LW_EXCLUSIVE)
			break;
	}

	Assert(proclist_is_empty(&wakeup) || pg_atomic_read_u32(&lock->state) & LW_FLAG_HAS_WAITERS);

	/* unset required flags, and release lock, in one fell swoop */
	{
		uint32		old_state;
		uint32		desired_state;

		old_state = pg_atomic_read_u32(&lock->state);
		while (true)
		{
			desired_state = old_state;

			/* compute desired flags */

			if (new_wake_in_progress)
				desired_state |= LW_FLAG_WAKE_IN_PROGRESS;
			else
				desired_state &= ~LW_FLAG_WAKE_IN_PROGRESS;

			if (proclist_is_empty(&lock->waiters))
				desired_state &= ~LW_FLAG_HAS_WAITERS;

			desired_state &= ~LW_FLAG_LOCKED;	/* release lock */

			if (pg_atomic_compare_exchange_u32(&lock->state, &old_state,
											   desired_state))
				break;
		}
	}

	/* Awaken any waiters I removed from the queue. */
	proclist_foreach_modify(iter, &wakeup, lwWaitLink)
	{
		PGPROC	   *waiter = GetPGProcByNumber(iter.cur);

		LOG_LWDEBUG("LWLockRelease", lock, "release waiter");
		proclist_delete(&wakeup, iter.cur, lwWaitLink);

		/*
		 * Guarantee that lwWaiting being unset only becomes visible once the
		 * unlink from the link has completed. Otherwise the target backend
		 * could be woken up for other reason and enqueue for a new lock - if
		 * that happens before the list unlink happens, the list would end up
		 * being corrupted.
		 *
		 * The barrier pairs with the LWLockWaitListLock() when enqueuing for
		 * another lock.
		 */
		pg_write_barrier();
		waiter->lwWaiting = LW_WS_NOT_WAITING;
		PGSemaphoreUnlock(waiter->sem);
	}
}

/*
 * Add ourselves to the end of the queue.
 *
 * NB: Mode can be LW_WAIT_UNTIL_FREE here!
 */
static void
LWLockQueueSelf(LWLock *lock, LWLockMode mode)
{
	/*
	 * If we don't have a PGPROC structure, there's no way to wait. This
	 * should never occur, since MyProc should only be null during shared
	 * memory initialization.
	 */
	if (MyProc == NULL)
		elog(PANIC, "cannot wait without a PGPROC structure");

	if (MyProc->lwWaiting != LW_WS_NOT_WAITING)
		elog(PANIC, "queueing for lock while waiting on another one");

	LWLockWaitListLock(lock);

	/* setting the flag is protected by the spinlock */
	pg_atomic_fetch_or_u32(&lock->state, LW_FLAG_HAS_WAITERS);

	MyProc->lwWaiting = LW_WS_WAITING;
	MyProc->lwWaitMode = mode;

	/* LW_WAIT_UNTIL_FREE waiters are always at the front of the queue */
	if (mode == LW_WAIT_UNTIL_FREE)
		proclist_push_head(&lock->waiters, MyProcNumber, lwWaitLink);
	else
		proclist_push_tail(&lock->waiters, MyProcNumber, lwWaitLink);

	/* Can release the mutex now */
	LWLockWaitListUnlock(lock);

#ifdef LOCK_DEBUG
	pg_atomic_fetch_add_u32(&lock->nwaiters, 1);
#endif
}

/*
 * Remove ourselves from the waitlist.
 *
 * This is used if we queued ourselves because we thought we needed to sleep
 * but, after further checking, we discovered that we don't actually need to
 * do so.
 */
static void
LWLockDequeueSelf(LWLock *lock)
{
	bool		on_waitlist;

#ifdef LWLOCK_STATS
	lwlock_stats *lwstats;

	lwstats = get_lwlock_stats_entry(lock);

	lwstats->dequeue_self_count++;
#endif

	LWLockWaitListLock(lock);

	/*
	 * Remove ourselves from the waitlist, unless we've already been removed.
	 * The removal happens with the wait list lock held, so there's no race in
	 * this check.
	 */
	on_waitlist = MyProc->lwWaiting == LW_WS_WAITING;
	if (on_waitlist)
		proclist_delete(&lock->waiters, MyProcNumber, lwWaitLink);

	if (proclist_is_empty(&lock->waiters) &&
		(pg_atomic_read_u32(&lock->state) & LW_FLAG_HAS_WAITERS) != 0)
	{
		pg_atomic_fetch_and_u32(&lock->state, ~LW_FLAG_HAS_WAITERS);
	}

	/* XXX: combine with fetch_and above? */
	LWLockWaitListUnlock(lock);

	/* clear waiting state again, nice for debugging */
	if (on_waitlist)
		MyProc->lwWaiting = LW_WS_NOT_WAITING;
	else
	{
		int			extraWaits = 0;

		/*
		 * Somebody else dequeued us and has or will wake us up. Deal with the
		 * superfluous absorption of a wakeup.
		 */

		/*
		 * Clear LW_FLAG_WAKE_IN_PROGRESS if somebody woke us before we
		 * removed ourselves - they'll have set it.
		 */
		pg_atomic_fetch_and_u32(&lock->state, ~LW_FLAG_WAKE_IN_PROGRESS);

		/*
		 * Now wait for the scheduled wakeup, otherwise our ->lwWaiting would
		 * get reset at some inconvenient point later. Most of the time this
		 * will immediately return.
		 */
		for (;;)
		{
			PGSemaphoreLock(MyProc->sem);
			if (MyProc->lwWaiting == LW_WS_NOT_WAITING)
				break;
			extraWaits++;
		}

		/*
		 * Fix the process wait semaphore's count for any absorbed wakeups.
		 */
		while (extraWaits-- > 0)
			PGSemaphoreUnlock(MyProc->sem);
	}

#ifdef LOCK_DEBUG
	{
		/* not waiting anymore */
		uint32		nwaiters PG_USED_FOR_ASSERTS_ONLY = pg_atomic_fetch_sub_u32(&lock->nwaiters, 1);

		Assert(nwaiters < MAX_BACKENDS);
	}
#endif
}

/*
 * LWLockAcquire - acquire a lightweight lock in the specified mode
 *
 * If the lock is not available, sleep until it is.  Returns true if the lock
 * was available immediately, false if we had to sleep.
 *
 * Side effect: cancel/die interrupts are held off until lock release.
 */
bool
LWLockAcquire(LWLock *lock, LWLockMode mode)
{
	PGPROC	   *proc = MyProc;
	bool		result = true;
	int			extraWaits = 0;
#ifdef LWLOCK_STATS
	lwlock_stats *lwstats;

	lwstats = get_lwlock_stats_entry(lock);
#endif

	Assert(mode == LW_SHARED || mode == LW_EXCLUSIVE);

	PRINT_LWDEBUG("LWLockAcquire", lock, mode);

#ifdef LWLOCK_STATS
	/* Count lock acquisition attempts */
	if (mode == LW_EXCLUSIVE)
		lwstats->ex_acquire_count++;
	else
		lwstats->sh_acquire_count++;
#endif							/* LWLOCK_STATS */

	/*
	 * We can't wait if we haven't got a PGPROC.  This should only occur
	 * during bootstrap or shared memory initialization.  Put an Assert here
	 * to catch unsafe coding practices.
	 */
	Assert(!(proc == NULL && IsUnderPostmaster));

	/* Ensure we will have room to remember the lock */
	if (num_held_lwlocks >= MAX_SIMUL_LWLOCKS)
		elog(ERROR, "too many LWLocks taken");

	/*
	 * Lock out cancel/die interrupts until we exit the code section protected
	 * by the LWLock.  This ensures that interrupts will not interfere with
	 * manipulations of data structures in shared memory.
	 */
	HOLD_INTERRUPTS();

	/*
	 * Loop here to try to acquire lock after each time we are signaled by
	 * LWLockRelease.
	 *
	 * NOTE: it might seem better to have LWLockRelease actually grant us the
	 * lock, rather than retrying and possibly having to go back to sleep. But
	 * in practice that is no good because it means a process swap for every
	 * lock acquisition when two or more processes are contending for the same
	 * lock.  Since LWLocks are normally used to protect not-very-long
	 * sections of computation, a process needs to be able to acquire and
	 * release the same lock many times during a single CPU time slice, even
	 * in the presence of contention.  The efficiency of being able to do that
	 * outweighs the inefficiency of sometimes wasting a process dispatch
	 * cycle because the lock is not free when a released waiter finally gets
	 * to run.  See pgsql-hackers archives for 29-Dec-01.
	 */
	for (;;)
	{
		bool		mustwait;

		/*
		 * Try to grab the lock the first time, we're not in the waitqueue
		 * yet/anymore.
		 */
		mustwait = LWLockAttemptLock(lock, mode);

		if (!mustwait)
		{
			LOG_LWDEBUG("LWLockAcquire", lock, "immediately acquired lock");
			break;				/* got the lock */
		}

		/*
		 * Ok, at this point we couldn't grab the lock on the first try. We
		 * cannot simply queue ourselves to the end of the list and wait to be
		 * woken up because by now the lock could long have been released.
		 * Instead add us to the queue and try to grab the lock again. If we
		 * succeed we need to revert the queuing and be happy, otherwise we
		 * recheck the lock. If we still couldn't grab it, we know that the
		 * other locker will see our queue entries when releasing since they
		 * existed before we checked for the lock.
		 */

		/* add to the queue */
		LWLockQueueSelf(lock, mode);

		/* we're now guaranteed to be woken up if necessary */
		mustwait = LWLockAttemptLock(lock, mode);

		/* ok, grabbed the lock the second time round, need to undo queueing */
		if (!mustwait)
		{
			LOG_LWDEBUG("LWLockAcquire", lock, "acquired, undoing queue");

			LWLockDequeueSelf(lock);
			break;
		}

		/*
		 * Wait until awakened.
		 *
		 * It is possible that we get awakened for a reason other than being
		 * signaled by LWLockRelease.  If so, loop back and wait again.  Once
		 * we've gotten the LWLock, re-increment the sema by the number of
		 * additional signals received.
		 */
		LOG_LWDEBUG("LWLockAcquire", lock, "waiting");

#ifdef LWLOCK_STATS
		lwstats->block_count++;
#endif

		LWLockReportWaitStart(lock);
		if (TRACE_POSTGRESQL_LWLOCK_WAIT_START_ENABLED())
			TRACE_POSTGRESQL_LWLOCK_WAIT_START(T_NAME(lock), mode);

		for (;;)
		{
			PGSemaphoreLock(proc->sem);
			if (proc->lwWaiting == LW_WS_NOT_WAITING)
				break;
			extraWaits++;
		}

		/* Retrying, allow LWLockRelease to release waiters again. */
		pg_atomic_fetch_and_u32(&lock->state, ~LW_FLAG_WAKE_IN_PROGRESS);

#ifdef LOCK_DEBUG
		{
			/* not waiting anymore */
			uint32		nwaiters PG_USED_FOR_ASSERTS_ONLY = pg_atomic_fetch_sub_u32(&lock->nwaiters, 1);

			Assert(nwaiters < MAX_BACKENDS);
		}
#endif

		if (TRACE_POSTGRESQL_LWLOCK_WAIT_DONE_ENABLED())
			TRACE_POSTGRESQL_LWLOCK_WAIT_DONE(T_NAME(lock), mode);
		LWLockReportWaitEnd();

		LOG_LWDEBUG("LWLockAcquire", lock, "awakened");

		/* Now loop back and try to acquire lock again. */
		result = false;
	}

	if (TRACE_POSTGRESQL_LWLOCK_ACQUIRE_ENABLED())
		TRACE_POSTGRESQL_LWLOCK_ACQUIRE(T_NAME(lock), mode);

	/* Add lock to list of locks held by this backend */
	held_lwlocks[num_held_lwlocks].lock = lock;
	held_lwlocks[num_held_lwlocks++].mode = mode;

	/*
	 * Fix the process wait semaphore's count for any absorbed wakeups.
	 */
	while (extraWaits-- > 0)
		PGSemaphoreUnlock(proc->sem);

	return result;
}

/*
 * LWLockConditionalAcquire - acquire a lightweight lock in the specified mode
 *
 * If the lock is not available, return false with no side-effects.
 *
 * If successful, cancel/die interrupts are held off until lock release.
 */
bool
LWLockConditionalAcquire(LWLock *lock, LWLockMode mode)
{
	bool		mustwait;

	Assert(mode == LW_SHARED || mode == LW_EXCLUSIVE);

	PRINT_LWDEBUG("LWLockConditionalAcquire", lock, mode);

	/* Ensure we will have room to remember the lock */
	if (num_held_lwlocks >= MAX_SIMUL_LWLOCKS)
		elog(ERROR, "too many LWLocks taken");

	/*
	 * Lock out cancel/die interrupts until we exit the code section protected
	 * by the LWLock.  This ensures that interrupts will not interfere with
	 * manipulations of data structures in shared memory.
	 */
	HOLD_INTERRUPTS();

	/* Check for the lock */
	mustwait = LWLockAttemptLock(lock, mode);

	if (mustwait)
	{
		/* Failed to get lock, so release interrupt holdoff */
		RESUME_INTERRUPTS();

		LOG_LWDEBUG("LWLockConditionalAcquire", lock, "failed");
		if (TRACE_POSTGRESQL_LWLOCK_CONDACQUIRE_FAIL_ENABLED())
			TRACE_POSTGRESQL_LWLOCK_CONDACQUIRE_FAIL(T_NAME(lock), mode);
	}
	else
	{
		/* Add lock to list of locks held by this backend */
		held_lwlocks[num_held_lwlocks].lock = lock;
		held_lwlocks[num_held_lwlocks++].mode = mode;
		if (TRACE_POSTGRESQL_LWLOCK_CONDACQUIRE_ENABLED())
			TRACE_POSTGRESQL_LWLOCK_CONDACQUIRE(T_NAME(lock), mode);
	}
	return !mustwait;
}

/*
 * LWLockAcquireOrWait - Acquire lock, or wait until it's free
 *
 * The semantics of this function are a bit funky.  If the lock is currently
 * free, it is acquired in the given mode, and the function returns true.  If
 * the lock isn't immediately free, the function waits until it is released
 * and returns false, but does not acquire the lock.
 *
 * This is currently used for WALWriteLock: when a backend flushes the WAL,
 * holding WALWriteLock, it can flush the commit records of many other
 * backends as a side-effect.  Those other backends need to wait until the
 * flush finishes, but don't need to acquire the lock anymore.  They can just
 * wake up, observe that their records have already been flushed, and return.
 */
bool
LWLockAcquireOrWait(LWLock *lock, LWLockMode mode)
{
	PGPROC	   *proc = MyProc;
	bool		mustwait;
	int			extraWaits = 0;
#ifdef LWLOCK_STATS
	lwlock_stats *lwstats;

	lwstats = get_lwlock_stats_entry(lock);
#endif

	Assert(mode == LW_SHARED || mode == LW_EXCLUSIVE);

	PRINT_LWDEBUG("LWLockAcquireOrWait", lock, mode);

	/* Ensure we will have room to remember the lock */
	if (num_held_lwlocks >= MAX_SIMUL_LWLOCKS)
		elog(ERROR, "too many LWLocks taken");

	/*
	 * Lock out cancel/die interrupts until we exit the code section protected
	 * by the LWLock.  This ensures that interrupts will not interfere with
	 * manipulations of data structures in shared memory.
	 */
	HOLD_INTERRUPTS();

	/*
	 * NB: We're using nearly the same twice-in-a-row lock acquisition
	 * protocol as LWLockAcquire(). Check its comments for details.
	 */
	mustwait = LWLockAttemptLock(lock, mode);

	if (mustwait)
	{
		LWLockQueueSelf(lock, LW_WAIT_UNTIL_FREE);

		mustwait = LWLockAttemptLock(lock, mode);

		if (mustwait)
		{
			/*
			 * Wait until awakened.  Like in LWLockAcquire, be prepared for
			 * bogus wakeups.
			 */
			LOG_LWDEBUG("LWLockAcquireOrWait", lock, "waiting");

#ifdef LWLOCK_STATS
			lwstats->block_count++;
#endif

			LWLockReportWaitStart(lock);
			if (TRACE_POSTGRESQL_LWLOCK_WAIT_START_ENABLED())
				TRACE_POSTGRESQL_LWLOCK_WAIT_START(T_NAME(lock), mode);

			for (;;)
			{
				PGSemaphoreLock(proc->sem);
				if (proc->lwWaiting == LW_WS_NOT_WAITING)
					break;
				extraWaits++;
			}

#ifdef LOCK_DEBUG
			{
				/* not waiting anymore */
				uint32		nwaiters PG_USED_FOR_ASSERTS_ONLY = pg_atomic_fetch_sub_u32(&lock->nwaiters, 1);

				Assert(nwaiters < MAX_BACKENDS);
			}
#endif
			if (TRACE_POSTGRESQL_LWLOCK_WAIT_DONE_ENABLED())
				TRACE_POSTGRESQL_LWLOCK_WAIT_DONE(T_NAME(lock), mode);
			LWLockReportWaitEnd();

			LOG_LWDEBUG("LWLockAcquireOrWait", lock, "awakened");
		}
		else
		{
			LOG_LWDEBUG("LWLockAcquireOrWait", lock, "acquired, undoing queue");

			/*
			 * Got lock in the second attempt, undo queueing. We need to treat
			 * this as having successfully acquired the lock, otherwise we'd
			 * not necessarily wake up people we've prevented from acquiring
			 * the lock.
			 */
			LWLockDequeueSelf(lock);
		}
	}

	/*
	 * Fix the process wait semaphore's count for any absorbed wakeups.
	 */
	while (extraWaits-- > 0)
		PGSemaphoreUnlock(proc->sem);

	if (mustwait)
	{
		/* Failed to get lock, so release interrupt holdoff */
		RESUME_INTERRUPTS();
		LOG_LWDEBUG("LWLockAcquireOrWait", lock, "failed");
		if (TRACE_POSTGRESQL_LWLOCK_ACQUIRE_OR_WAIT_FAIL_ENABLED())
			TRACE_POSTGRESQL_LWLOCK_ACQUIRE_OR_WAIT_FAIL(T_NAME(lock), mode);
	}
	else
	{
		LOG_LWDEBUG("LWLockAcquireOrWait", lock, "succeeded");
		/* Add lock to list of locks held by this backend */
		held_lwlocks[num_held_lwlocks].lock = lock;
		held_lwlocks[num_held_lwlocks++].mode = mode;
		if (TRACE_POSTGRESQL_LWLOCK_ACQUIRE_OR_WAIT_ENABLED())
			TRACE_POSTGRESQL_LWLOCK_ACQUIRE_OR_WAIT(T_NAME(lock), mode);
	}

	return !mustwait;
}

/*
 * Does the lwlock in its current state need to wait for the variable value to
 * change?
 *
 * If we don't need to wait, and it's because the value of the variable has
 * changed, store the current value in newval.
 *
 * *result is set to true if the lock was free, and false otherwise.
 */
static bool
LWLockConflictsWithVar(LWLock *lock, pg_atomic_uint64 *valptr, uint64 oldval,
					   uint64 *newval, bool *result)
{
	bool		mustwait;
	uint64		value;

	/*
	 * Test first to see if it the slot is free right now.
	 *
	 * XXX: the unique caller of this routine, WaitXLogInsertionsToFinish()
	 * via LWLockWaitForVar(), uses an implied barrier with a spinlock before
	 * this, so we don't need a memory barrier here as far as the current
	 * usage is concerned.  But that might not be safe in general.
	 */
	mustwait = (pg_atomic_read_u32(&lock->state) & LW_VAL_EXCLUSIVE) != 0;

	if (!mustwait)
	{
		*result = true;
		return false;
	}

	*result = false;

	/*
	 * Reading this value atomically is safe even on platforms where uint64
	 * cannot be read without observing a torn value.
	 */
	value = pg_atomic_read_u64(valptr);

	if (value != oldval)
	{
		mustwait = false;
		*newval = value;
	}
	else
	{
		mustwait = true;
	}

	return mustwait;
}

/*
 * LWLockWaitForVar - Wait until lock is free, or a variable is updated.
 *
 * If the lock is held and *valptr equals oldval, waits until the lock is
 * either freed, or the lock holder updates *valptr by calling
 * LWLockUpdateVar.  If the lock is free on exit (immediately or after
 * waiting), returns true.  If the lock is still held, but *valptr no longer
 * matches oldval, returns false and sets *newval to the current value in
 * *valptr.
 *
 * Note: this function ignores shared lock holders; if the lock is held
 * in shared mode, returns 'true'.
 *
 * Be aware that LWLockConflictsWithVar() does not include a memory barrier,
 * hence the caller of this function may want to rely on an explicit barrier or
 * an implied barrier via spinlock or LWLock to avoid memory ordering issues.
 */
bool
LWLockWaitForVar(LWLock *lock, pg_atomic_uint64 *valptr, uint64 oldval,
				 uint64 *newval)
{
	PGPROC	   *proc = MyProc;
	int			extraWaits = 0;
	bool		result = false;
#ifdef LWLOCK_STATS
	lwlock_stats *lwstats;

	lwstats = get_lwlock_stats_entry(lock);
#endif

	PRINT_LWDEBUG("LWLockWaitForVar", lock, LW_WAIT_UNTIL_FREE);

	/*
	 * Lock out cancel/die interrupts while we sleep on the lock.  There is no
	 * cleanup mechanism to remove us from the wait queue if we got
	 * interrupted.
	 */
	HOLD_INTERRUPTS();

	/*
	 * Loop here to check the lock's status after each time we are signaled.
	 */
	for (;;)
	{
		bool		mustwait;

		mustwait = LWLockConflictsWithVar(lock, valptr, oldval, newval,
										  &result);

		if (!mustwait)
			break;				/* the lock was free or value didn't match */

		/*
		 * Add myself to wait queue. Note that this is racy, somebody else
		 * could wakeup before we're finished queuing. NB: We're using nearly
		 * the same twice-in-a-row lock acquisition protocol as
		 * LWLockAcquire(). Check its comments for details. The only
		 * difference is that we also have to check the variable's values when
		 * checking the state of the lock.
		 */
		LWLockQueueSelf(lock, LW_WAIT_UNTIL_FREE);

		/*
		 * Clear LW_FLAG_WAKE_IN_PROGRESS flag, to make sure we get woken up
		 * as soon as the lock is released.
		 */
		pg_atomic_fetch_and_u32(&lock->state, ~LW_FLAG_WAKE_IN_PROGRESS);

		/*
		 * We're now guaranteed to be woken up if necessary. Recheck the lock
		 * and variables state.
		 */
		mustwait = LWLockConflictsWithVar(lock, valptr, oldval, newval,
										  &result);

		/* Ok, no conflict after we queued ourselves. Undo queueing. */
		if (!mustwait)
		{
			LOG_LWDEBUG("LWLockWaitForVar", lock, "free, undoing queue");

			LWLockDequeueSelf(lock);
			break;
		}

		/*
		 * Wait until awakened.
		 *
		 * It is possible that we get awakened for a reason other than being
		 * signaled by LWLockRelease.  If so, loop back and wait again.  Once
		 * we've gotten the LWLock, re-increment the sema by the number of
		 * additional signals received.
		 */
		LOG_LWDEBUG("LWLockWaitForVar", lock, "waiting");

#ifdef LWLOCK_STATS
		lwstats->block_count++;
#endif

		LWLockReportWaitStart(lock);
		if (TRACE_POSTGRESQL_LWLOCK_WAIT_START_ENABLED())
			TRACE_POSTGRESQL_LWLOCK_WAIT_START(T_NAME(lock), LW_EXCLUSIVE);

		for (;;)
		{
			PGSemaphoreLock(proc->sem);
			if (proc->lwWaiting == LW_WS_NOT_WAITING)
				break;
			extraWaits++;
		}

#ifdef LOCK_DEBUG
		{
			/* not waiting anymore */
			uint32		nwaiters PG_USED_FOR_ASSERTS_ONLY = pg_atomic_fetch_sub_u32(&lock->nwaiters, 1);

			Assert(nwaiters < MAX_BACKENDS);
		}
#endif

		if (TRACE_POSTGRESQL_LWLOCK_WAIT_DONE_ENABLED())
			TRACE_POSTGRESQL_LWLOCK_WAIT_DONE(T_NAME(lock), LW_EXCLUSIVE);
		LWLockReportWaitEnd();

		LOG_LWDEBUG("LWLockWaitForVar", lock, "awakened");

		/* Now loop back and check the status of the lock again. */
	}

	/*
	 * Fix the process wait semaphore's count for any absorbed wakeups.
	 */
	while (extraWaits-- > 0)
		PGSemaphoreUnlock(proc->sem);

	/*
	 * Now okay to allow cancel/die interrupts.
	 */
	RESUME_INTERRUPTS();

	return result;
}


/*
 * LWLockUpdateVar - Update a variable and wake up waiters atomically
 *
 * Sets *valptr to 'val', and wakes up all processes waiting for us with
 * LWLockWaitForVar().  It first sets the value atomically and then wakes up
 * waiting processes so that any process calling LWLockWaitForVar() on the same
 * lock is guaranteed to see the new value, and act accordingly.
 *
 * The caller must be holding the lock in exclusive mode.
 */
void
LWLockUpdateVar(LWLock *lock, pg_atomic_uint64 *valptr, uint64 val)
{
	proclist_head wakeup;
	proclist_mutable_iter iter;

	PRINT_LWDEBUG("LWLockUpdateVar", lock, LW_EXCLUSIVE);

	/*
	 * Note that pg_atomic_exchange_u64 is a full barrier, so we're guaranteed
	 * that the variable is updated before waking up waiters.
	 */
	pg_atomic_exchange_u64(valptr, val);

	proclist_init(&wakeup);

	LWLockWaitListLock(lock);

	Assert(pg_atomic_read_u32(&lock->state) & LW_VAL_EXCLUSIVE);

	/*
	 * See if there are any LW_WAIT_UNTIL_FREE waiters that need to be woken
	 * up. They are always in the front of the queue.
	 */
	proclist_foreach_modify(iter, &lock->waiters, lwWaitLink)
	{
		PGPROC	   *waiter = GetPGProcByNumber(iter.cur);

		if (waiter->lwWaitMode != LW_WAIT_UNTIL_FREE)
			break;

		proclist_delete(&lock->waiters, iter.cur, lwWaitLink);
		proclist_push_tail(&wakeup, iter.cur, lwWaitLink);

		/* see LWLockWakeup() */
		Assert(waiter->lwWaiting == LW_WS_WAITING);
		waiter->lwWaiting = LW_WS_PENDING_WAKEUP;
	}

	/* We are done updating shared state of the lock itself. */
	LWLockWaitListUnlock(lock);

	/*
	 * Awaken any waiters I removed from the queue.
	 */
	proclist_foreach_modify(iter, &wakeup, lwWaitLink)
	{
		PGPROC	   *waiter = GetPGProcByNumber(iter.cur);

		proclist_delete(&wakeup, iter.cur, lwWaitLink);
		/* check comment in LWLockWakeup() about this barrier */
		pg_write_barrier();
		waiter->lwWaiting = LW_WS_NOT_WAITING;
		PGSemaphoreUnlock(waiter->sem);
	}
}


/*
 * LWLockRelease - release a previously acquired lock
 *
 * NB: This will leave lock->owner pointing to the current backend (if
 * LOCK_DEBUG is set). This is somewhat intentional, as it makes it easier to
 * debug cases of missing wakeups during lock release.
 */
void
LWLockRelease(LWLock *lock)
{
	LWLockMode	mode;
	uint32		oldstate;
	bool		check_waiters;
	int			i;

	/*
	 * Remove lock from list of locks held.  Usually, but not always, it will
	 * be the latest-acquired lock; so search array backwards.
	 */
	for (i = num_held_lwlocks; --i >= 0;)
		if (lock == held_lwlocks[i].lock)
			break;

	if (i < 0)
		elog(ERROR, "lock %s is not held", T_NAME(lock));

	mode = held_lwlocks[i].mode;

	num_held_lwlocks--;
	for (; i < num_held_lwlocks; i++)
		held_lwlocks[i] = held_lwlocks[i + 1];

	PRINT_LWDEBUG("LWLockRelease", lock, mode);

	/*
	 * Release my hold on lock, after that it can immediately be acquired by
	 * others, even if we still have to wakeup other waiters.
	 */
	if (mode == LW_EXCLUSIVE)
		oldstate = pg_atomic_sub_fetch_u32(&lock->state, LW_VAL_EXCLUSIVE);
	else
		oldstate = pg_atomic_sub_fetch_u32(&lock->state, LW_VAL_SHARED);

	/* nobody else can have that kind of lock */
	Assert(!(oldstate & LW_VAL_EXCLUSIVE));

	if (TRACE_POSTGRESQL_LWLOCK_RELEASE_ENABLED())
		TRACE_POSTGRESQL_LWLOCK_RELEASE(T_NAME(lock));

	/*
	 * Check if we're still waiting for backends to get scheduled, if so,
	 * don't wake them up again.
	 */
	if ((oldstate & LW_FLAG_HAS_WAITERS) &&
		!(oldstate & LW_FLAG_WAKE_IN_PROGRESS) &&
		(oldstate & LW_LOCK_MASK) == 0)
		check_waiters = true;
	else
		check_waiters = false;

	/*
	 * As waking up waiters requires the spinlock to be acquired, only do so
	 * if necessary.
	 */
	if (check_waiters)
	{
		/* XXX: remove before commit? */
		LOG_LWDEBUG("LWLockRelease", lock, "releasing waiters");
		LWLockWakeup(lock);
	}

	/*
	 * Now okay to allow cancel/die interrupts.
	 */
	RESUME_INTERRUPTS();
}

/*
 * LWLockReleaseClearVar - release a previously acquired lock, reset variable
 */
void
LWLockReleaseClearVar(LWLock *lock, pg_atomic_uint64 *valptr, uint64 val)
{
	/*
	 * Note that pg_atomic_exchange_u64 is a full barrier, so we're guaranteed
	 * that the variable is updated before releasing the lock.
	 */
	pg_atomic_exchange_u64(valptr, val);

	LWLockRelease(lock);
}


/*
 * LWLockReleaseAll - release all currently-held locks
 *
 * Used to clean up after ereport(ERROR). An important difference between this
 * function and retail LWLockRelease calls is that InterruptHoldoffCount is
 * unchanged by this operation.  This is necessary since InterruptHoldoffCount
 * has been set to an appropriate level earlier in error recovery. We could
 * decrement it below zero if we allow it to drop for each released lock!
 *
 * Note that this function must be safe to call even before the LWLock
 * subsystem has been initialized (e.g., during early startup failures).
 * In that case, num_held_lwlocks will be 0 and we do nothing.
 */
void
LWLockReleaseAll(void)
{
	while (num_held_lwlocks > 0)
	{
		HOLD_INTERRUPTS();		/* match the upcoming RESUME_INTERRUPTS */

		LWLockRelease(held_lwlocks[num_held_lwlocks - 1].lock);
	}

	Assert(num_held_lwlocks == 0);
}


/*
 * LWLockHeldByMe - test whether my process holds a lock in any mode
 *
 * This is meant as debug support only.
 */
bool
LWLockHeldByMe(LWLock *lock)
{
	int			i;

	for (i = 0; i < num_held_lwlocks; i++)
	{
		if (held_lwlocks[i].lock == lock)
			return true;
	}
	return false;
}

/*
 * LWLockAnyHeldByMe - test whether my process holds any of an array of locks
 *
 * This is meant as debug support only.
 */
bool
LWLockAnyHeldByMe(LWLock *lock, int nlocks, size_t stride)
{
	char	   *held_lock_addr;
	char	   *begin;
	char	   *end;
	int			i;

	begin = (char *) lock;
	end = begin + nlocks * stride;
	for (i = 0; i < num_held_lwlocks; i++)
	{
		held_lock_addr = (char *) held_lwlocks[i].lock;
		if (held_lock_addr >= begin &&
			held_lock_addr < end &&
			(held_lock_addr - begin) % stride == 0)
			return true;
	}
	return false;
}

/*
 * LWLockHeldByMeInMode - test whether my process holds a lock in given mode
 *
 * This is meant as debug support only.
 */
bool
LWLockHeldByMeInMode(LWLock *lock, LWLockMode mode)
{
	int			i;

	for (i = 0; i < num_held_lwlocks; i++)
	{
		if (held_lwlocks[i].lock == lock && held_lwlocks[i].mode == mode)
			return true;
	}
	return false;
}