lwlock.c

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/*-------------------------------------------------------------------------
 *
 * 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) 1996-2020, 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 it's
 * 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 "postmaster/postmaster.h"
#include "replication/slot.h"
#include "storage/ipc.h"
#include "storage/predicate.h"
#include "storage/proc.h"
#include "storage/proclist.h"
#include "storage/spin.h"
#include "utils/memutils.h"

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


/* We use the ShmemLock spinlock to protect LWLockCounter */
extern slock_t *ShmemLock;

#define LW_FLAG_HAS_WAITERS         ((uint32) 1 << 30)
#define LW_FLAG_RELEASE_OK          ((uint32) 1 << 29)
#define LW_FLAG_LOCKED              ((uint32) 1 << 28)

#define LW_VAL_EXCLUSIVE            ((uint32) 1 << 24)
#define LW_VAL_SHARED               1

#define LW_LOCK_MASK                ((uint32) ((1 << 25)-1))
/* Must be greater than MAX_BACKENDS - which is 2^23-1, so we're fine. */
#define LW_SHARED_MASK              ((uint32) ((1 << 24)-1))

/*
 * This is indexed by tranche ID and stores the names of all tranches known
 * to the current backend.
 */
static const char **LWLockTrancheArray = NULL;
static int  LWLockTranchesAllocated = 0;

#define T_NAME(lock) \
    (LWLockTrancheArray[(lock)->tranche])

/*
 * This points to the main array of LWLocks in shared memory.  Backends inherit
 * the pointer by fork from the postmaster (except in the EXEC_BACKEND case,
 * where we have special measures to pass it down).
 */
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; for example, the pg_buffercache
 * extension locks all buffer partitions simultaneously.
 */
#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];

/* struct representing the LWLock tranche request for named tranche */
typedef struct NamedLWLockTrancheRequest
{
    char        tranche_name[NAMEDATALEN];
    int         num_lwlocks;
} NamedLWLockTrancheRequest;

NamedLWLockTrancheRequest *NamedLWLockTrancheRequestArray = NULL;
static int  NamedLWLockTrancheRequestsAllocated = 0;
int         NamedLWLockTrancheRequests = 0;

NamedLWLockTranche *NamedLWLockTrancheArray = NULL;

static bool lock_named_request_allowed = true;

static void InitializeLWLocks(void);
static void RegisterLWLockTranches(void);

static inline void LWLockReportWaitStart(LWLock *lock);
static inline void LWLockReportWaitEnd(void);

#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 rOK %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_RELEASE_OK) != 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);

    MemSet(&ctl, 0, sizeof(ctl));
    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, LWLockTrancheArray[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. */
    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 named tranches.  These will be
 * allocated in the main array.
 */
static int
NumLWLocksByNamedTranches(void)
{
    int         numLocks = 0;
    int         i;

    for (i = 0; i < NamedLWLockTrancheRequests; i++)
        numLocks += NamedLWLockTrancheRequestArray[i].num_lwlocks;

    return numLocks;
}

/*
 * Compute shmem space needed for LWLocks and named tranches.
 */
Size
LWLockShmemSize(void)
{
    Size        size;
    int         i;
    int         numLocks = NUM_FIXED_LWLOCKS;

    numLocks += NumLWLocksByNamedTranches();

    /* Space for the LWLock array. */
    size = mul_size(numLocks, sizeof(LWLockPadded));

    /* Space for dynamic allocation counter, plus room for alignment. */
    size = add_size(size, sizeof(int) + LWLOCK_PADDED_SIZE);

    /* space for named tranches. */
    size = add_size(size, mul_size(NamedLWLockTrancheRequests, sizeof(NamedLWLockTranche)));

    /* space for name of each tranche. */
    for (i = 0; i < NamedLWLockTrancheRequests; i++)
        size = add_size(size, strlen(NamedLWLockTrancheRequestArray[i].tranche_name) + 1);

    /* Disallow named LWLocks' requests after startup */
    lock_named_request_allowed = false;

    return size;
}

/*
 * Allocate shmem space for the main LWLock array and all tranches and
 * initialize it.  We also register all the LWLock tranches here.
 */
void
CreateLWLocks(void)
{
    StaticAssertStmt(LW_VAL_EXCLUSIVE > (uint32) MAX_BACKENDS,
                     "MAX_BACKENDS too big for lwlock.c");

    StaticAssertStmt(sizeof(LWLock) <= LWLOCK_MINIMAL_SIZE &&
                     sizeof(LWLock) <= LWLOCK_PADDED_SIZE,
                     "Miscalculated LWLock padding");

    if (!IsUnderPostmaster)
    {
        Size        spaceLocks = LWLockShmemSize();
        int        *LWLockCounter;
        char       *ptr;

        /* Allocate space */
        ptr = (char *) ShmemAlloc(spaceLocks);

        /* Leave room for dynamic allocation of tranches */
        ptr += sizeof(int);

        /* Ensure desired alignment of LWLock array */
        ptr += LWLOCK_PADDED_SIZE - ((uintptr_t) ptr) % LWLOCK_PADDED_SIZE;

        MainLWLockArray = (LWLockPadded *) ptr;

        /*
         * Initialize the dynamic-allocation counter for tranches, which is
         * stored just before the first LWLock.
         */
        LWLockCounter = (int *) ((char *) MainLWLockArray - sizeof(int));
        *LWLockCounter = LWTRANCHE_FIRST_USER_DEFINED;

        /* Initialize all LWLocks */
        InitializeLWLocks();
    }

    /* Register all LWLock tranches */
    RegisterLWLockTranches();
}

/*
 * Initialize LWLocks that are fixed and those belonging to named tranches.
 */
static void
InitializeLWLocks(void)
{
    int         numNamedLocks = NumLWLocksByNamedTranches();
    int         id;
    int         i;
    int         j;
    LWLockPadded *lock;

    /* Initialize all individual LWLocks in main array */
    for (id = 0, lock = MainLWLockArray; id < NUM_INDIVIDUAL_LWLOCKS; id++, lock++)
        LWLockInitialize(&lock->lock, id);

    /* Initialize buffer mapping LWLocks in main array */
    lock = MainLWLockArray + NUM_INDIVIDUAL_LWLOCKS;
    for (id = 0; id < NUM_BUFFER_PARTITIONS; id++, lock++)
        LWLockInitialize(&lock->lock, LWTRANCHE_BUFFER_MAPPING);

    /* Initialize lmgrs' LWLocks in main array */
    lock = MainLWLockArray + NUM_INDIVIDUAL_LWLOCKS + NUM_BUFFER_PARTITIONS;
    for (id = 0; id < NUM_LOCK_PARTITIONS; id++, lock++)
        LWLockInitialize(&lock->lock, LWTRANCHE_LOCK_MANAGER);

    /* Initialize predicate lmgrs' LWLocks in main array */
    lock = MainLWLockArray + NUM_INDIVIDUAL_LWLOCKS +
        NUM_BUFFER_PARTITIONS + NUM_LOCK_PARTITIONS;
    for (id = 0; id < NUM_PREDICATELOCK_PARTITIONS; id++, lock++)
        LWLockInitialize(&lock->lock, LWTRANCHE_PREDICATE_LOCK_MANAGER);

    /* Initialize named tranches. */
    if (NamedLWLockTrancheRequests > 0)
    {
        char       *trancheNames;

        NamedLWLockTrancheArray = (NamedLWLockTranche *)
            &MainLWLockArray[NUM_FIXED_LWLOCKS + numNamedLocks];

        trancheNames = (char *) NamedLWLockTrancheArray +
            (NamedLWLockTrancheRequests * sizeof(NamedLWLockTranche));
        lock = &MainLWLockArray[NUM_FIXED_LWLOCKS];

        for (i = 0; i < NamedLWLockTrancheRequests; i++)
        {
            NamedLWLockTrancheRequest *request;
            NamedLWLockTranche *tranche;
            char       *name;

            request = &NamedLWLockTrancheRequestArray[i];
            tranche = &NamedLWLockTrancheArray[i];

            name = trancheNames;
            trancheNames += strlen(request->tranche_name) + 1;
            strcpy(name, request->tranche_name);
            tranche->trancheId = LWLockNewTrancheId();
            tranche->trancheName = name;

            for (j = 0; j < request->num_lwlocks; j++, lock++)
                LWLockInitialize(&lock->lock, tranche->trancheId);
        }
    }
}

/*
 * Register named tranches and tranches for fixed LWLocks.
 */
static void
RegisterLWLockTranches(void)
{
    int         i;

    if (LWLockTrancheArray == NULL)
    {
        LWLockTranchesAllocated = 128;
        LWLockTrancheArray = (const char **)
            MemoryContextAllocZero(TopMemoryContext,
                                   LWLockTranchesAllocated * sizeof(char *));
        Assert(LWLockTranchesAllocated >= LWTRANCHE_FIRST_USER_DEFINED);
    }

    for (i = 0; i < NUM_INDIVIDUAL_LWLOCKS; ++i)
        LWLockRegisterTranche(i, MainLWLockNames[i]);

    LWLockRegisterTranche(LWTRANCHE_BUFFER_MAPPING, "buffer_mapping");
    LWLockRegisterTranche(LWTRANCHE_LOCK_MANAGER, "lock_manager");
    LWLockRegisterTranche(LWTRANCHE_PREDICATE_LOCK_MANAGER,
                          "predicate_lock_manager");
    LWLockRegisterTranche(LWTRANCHE_PARALLEL_QUERY_DSA,
                          "parallel_query_dsa");
    LWLockRegisterTranche(LWTRANCHE_SESSION_DSA,
                          "session_dsa");
    LWLockRegisterTranche(LWTRANCHE_SESSION_RECORD_TABLE,
                          "session_record_table");
    LWLockRegisterTranche(LWTRANCHE_SESSION_TYPMOD_TABLE,
                          "session_typmod_table");
    LWLockRegisterTranche(LWTRANCHE_SHARED_TUPLESTORE,
                          "shared_tuplestore");
    LWLockRegisterTranche(LWTRANCHE_TBM, "tbm");
    LWLockRegisterTranche(LWTRANCHE_PARALLEL_APPEND, "parallel_append");
    LWLockRegisterTranche(LWTRANCHE_PARALLEL_HASH_JOIN, "parallel_hash_join");
    LWLockRegisterTranche(LWTRANCHE_SXACT, "serializable_xact");

    /* Register named tranches. */
    for (i = 0; i < NamedLWLockTrancheRequests; i++)
        LWLockRegisterTranche(NamedLWLockTrancheArray[i].trancheId,
                              NamedLWLockTrancheArray[i].trancheName);
}

/*
 * 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)
{
    int         lock_pos;
    int         i;

    /*
     * Obtain the position of base address of LWLock belonging to requested
     * tranche_name in MainLWLockArray.  LWLocks for named tranches are placed
     * in MainLWLockArray after fixed locks.
     */
    lock_pos = NUM_FIXED_LWLOCKS;
    for (i = 0; i < NamedLWLockTrancheRequests; i++)
    {
        if (strcmp(NamedLWLockTrancheRequestArray[i].tranche_name,
                   tranche_name) == 0)
            return &MainLWLockArray[lock_pos];

        lock_pos += NamedLWLockTrancheRequestArray[i].num_lwlocks;
    }

    if (i >= NamedLWLockTrancheRequests)
        elog(ERROR, "requested tranche is not registered");

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

/*
 * Allocate a new tranche ID.
 */
int
LWLockNewTrancheId(void)
{
    int         result;
    int        *LWLockCounter;

    LWLockCounter = (int *) ((char *) MainLWLockArray - sizeof(int));
    SpinLockAcquire(ShmemLock);
    result = (*LWLockCounter)++;
    SpinLockRelease(ShmemLock);

    return result;
}

/*
 * Register a tranche ID in the lookup table for the current process.  This
 * routine will save a pointer to the tranche name passed as an argument,
 * so the name should be allocated in a backend-lifetime context
 * (TopMemoryContext, static variable, or similar).
 */
void
LWLockRegisterTranche(int tranche_id, const char *tranche_name)
{
    Assert(LWLockTrancheArray != NULL);

    if (tranche_id >= LWLockTranchesAllocated)
    {
        int         i = LWLockTranchesAllocated;
        int         j = LWLockTranchesAllocated;

        while (i <= tranche_id)
            i *= 2;

        LWLockTrancheArray = (const char **)
            repalloc(LWLockTrancheArray, i * sizeof(char *));
        LWLockTranchesAllocated = i;
        while (j < LWLockTranchesAllocated)
            LWLockTrancheArray[j++] = NULL;
    }

    LWLockTrancheArray[tranche_id] = tranche_name;
}

/*
 * RequestNamedLWLockTranche
 *      Request that extra LWLocks be allocated during postmaster
 *      startup.
 *
 * This is only useful for extensions if called from the _PG_init hook
 * of a library that is loaded into the postmaster via
 * shared_preload_libraries.  Once shared memory has been allocated, calls
 * will be ignored.  (We could raise an error, but it seems better to make
 * it a no-op, so that libraries containing such calls can be reloaded if
 * needed.)
 */
void
RequestNamedLWLockTranche(const char *tranche_name, int num_lwlocks)
{
    NamedLWLockTrancheRequest *request;

    if (IsUnderPostmaster || !lock_named_request_allowed)
        return;                 /* too late */

    if (NamedLWLockTrancheRequestArray == NULL)
    {
        NamedLWLockTrancheRequestsAllocated = 16;
        NamedLWLockTrancheRequestArray = (NamedLWLockTrancheRequest *)
            MemoryContextAlloc(TopMemoryContext,
                               NamedLWLockTrancheRequestsAllocated
                               * sizeof(NamedLWLockTrancheRequest));
    }

    if (NamedLWLockTrancheRequests >= NamedLWLockTrancheRequestsAllocated)
    {
        int         i = NamedLWLockTrancheRequestsAllocated;

        while (i <= NamedLWLockTrancheRequests)
            i *= 2;

        NamedLWLockTrancheRequestArray = (NamedLWLockTrancheRequest *)
            repalloc(NamedLWLockTrancheRequestArray,
                     i * sizeof(NamedLWLockTrancheRequest));
        NamedLWLockTrancheRequestsAllocated = i;
    }

    request = &NamedLWLockTrancheRequestArray[NamedLWLockTrancheRequests];
    Assert(strlen(tranche_name) + 1 < NAMEDATALEN);
    StrNCpy(request->tranche_name, tranche_name, NAMEDATALEN);
    request->num_lwlocks = num_lwlocks;
    NamedLWLockTrancheRequests++;
}

/*
 * LWLockInitialize - initialize a new lwlock; it's initially unlocked
 */
void
LWLockInitialize(LWLock *lock, int tranche_id)
{
    pg_atomic_init_u32(&lock->state, LW_FLAG_RELEASE_OK);
#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 an identifier for an LWLock based on the wait class and event.
 */
const char *
GetLWLockIdentifier(uint32 classId, uint16 eventId)
{
    Assert(classId == PG_WAIT_LWLOCK);

    /*
     * It is quite possible that user has registered tranche in one of the
     * backends (e.g. by allocating lwlocks in dynamic shared memory) but not
     * all of them, so we can't assume the tranche is registered here.
     */
    if (eventId >= LWLockTranchesAllocated ||
        LWLockTrancheArray[eventId] == NULL)
        return "extension";

    return LWLockTrancheArray[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
 * callers job.
 *
 * Returns true if the lock isn't free and we need to wait.
 */
static bool
LWLockAttemptLock(LWLock *lock, LWLockMode mode)
{
    uint32      old_state;

    AssertArg(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 lock directly */
        old_state = pg_atomic_fetch_or_u32(&lock->state, LW_FLAG_LOCKED);
        if (!(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_release_ok;
    bool        wokeup_somebody = false;
    proclist_head wakeup;
    proclist_mutable_iter iter;

    proclist_init(&wakeup);

    new_release_ok = true;

    /* 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_release_ok = false;

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

        /*
         * 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_release_ok)
                desired_state |= LW_FLAG_RELEASE_OK;
            else
                desired_state &= ~LW_FLAG_RELEASE_OK;

            if (proclist_is_empty(&wakeup))
                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 = false;
        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)
        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 = true;
    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, MyProc->pgprocno, lwWaitLink);
    else
        proclist_push_tail(&lock->waiters, MyProc->pgprocno, 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        found = false;
    proclist_mutable_iter iter;

#ifdef LWLOCK_STATS
    lwlock_stats *lwstats;

    lwstats = get_lwlock_stats_entry(lock);

    lwstats->dequeue_self_count++;
#endif

    LWLockWaitListLock(lock);

    /*
     * Can't just remove ourselves from the list, but we need to iterate over
     * all entries as somebody else could have dequeued us.
     */
    proclist_foreach_modify(iter, &lock->waiters, lwWaitLink)
    {
        if (iter.cur == MyProc->pgprocno)
        {
            found = true;
            proclist_delete(&lock->waiters, iter.cur, lwWaitLink);
            break;
        }
    }

    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 (found)
        MyProc->lwWaiting = false;
    else
    {
        int         extraWaits = 0;

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

        /*
         * Reset RELEASE_OK flag if somebody woke us before we removed
         * ourselves - they'll have set it to false.
         */
        pg_atomic_fetch_or_u32(&lock->state, LW_FLAG_RELEASE_OK);

        /*
         * 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)
                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

    AssertArg(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.
         *
         * Since we share the process wait semaphore with the regular lock
         * manager and ProcWaitForSignal, and we may need to acquire an LWLock
         * while one of those is pending, 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,
         * so that the lock manager or signal manager will see the received
         * signal when it next waits.
         */
        LOG_LWDEBUG("LWLockAcquire", lock, "waiting");

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

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

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

        /* Retrying, allow LWLockRelease to release waiters again. */
        pg_atomic_fetch_or_u32(&lock->state, LW_FLAG_RELEASE_OK);

#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

        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;
    }

    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;

    AssertArg(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");
        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;
        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, because we share the semaphore with
             * ProcWaitForSignal.
             */
            LOG_LWDEBUG("LWLockAcquireOrWait", lock, "waiting");

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

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

            for (;;)
            {
                PGSemaphoreLock(proc->sem);
                if (!proc->lwWaiting)
                    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
            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");
        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;
        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,
                       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 caller uses 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;

    /*
     * Read value using the lwlock's wait list lock, as we can't generally
     * rely on atomic 64 bit reads/stores.  TODO: On platforms with a way to
     * do atomic 64 bit reads/writes the spinlock should be optimized away.
     */
    LWLockWaitListLock(lock);
    value = *valptr;
    LWLockWaitListUnlock(lock);

    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'.
 */
bool
LWLockWaitForVar(LWLock *lock, 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);

        /*
         * Set RELEASE_OK flag, to make sure we get woken up as soon as the
         * lock is released.
         */
        pg_atomic_fetch_or_u32(&lock->state, LW_FLAG_RELEASE_OK);

        /*
         * 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.
         *
         * Since we share the process wait semaphore with the regular lock
         * manager and ProcWaitForSignal, and we may need to acquire an LWLock
         * while one of those is pending, 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,
         * so that the lock manager or signal manager will see the received
         * signal when it next waits.
         */
        LOG_LWDEBUG("LWLockWaitForVar", lock, "waiting");

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

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

        for (;;)
        {
            PGSemaphoreLock(proc->sem);
            if (!proc->lwWaiting)
                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

        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. */
    }

    TRACE_POSTGRESQL_LWLOCK_ACQUIRE(T_NAME(lock), LW_EXCLUSIVE);

    /*
     * 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().  Setting the value and waking up the processes happen
 * atomically 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, uint64 *valptr, uint64 val)
{
    proclist_head wakeup;
    proclist_mutable_iter iter;

    PRINT_LWDEBUG("LWLockUpdateVar", lock, LW_EXCLUSIVE);

    proclist_init(&wakeup);

    LWLockWaitListLock(lock);

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

    /* Update the lock's value */
    *valptr = val;

    /*
     * 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);
    }

    /* 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 = false;
        PGSemaphoreUnlock(waiter->sem);
    }
}


/*
 * LWLockRelease - release a previously acquired lock
 */
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));


    /*
     * We're still waiting for backends to get scheduled, don't wake them up
     * again.
     */
    if ((oldstate & (LW_FLAG_HAS_WAITERS | LW_FLAG_RELEASE_OK)) ==
        (LW_FLAG_HAS_WAITERS | LW_FLAG_RELEASE_OK) &&
        (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);
    }

    TRACE_POSTGRESQL_LWLOCK_RELEASE(T_NAME(lock));

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

/*
 * LWLockReleaseClearVar - release a previously acquired lock, reset variable
 */
void
LWLockReleaseClearVar(LWLock *lock, uint64 *valptr, uint64 val)
{
    LWLockWaitListLock(lock);

    /*
     * Set the variable's value before releasing the lock, that prevents race
     * a race condition wherein a new locker acquires the lock, but hasn't yet
     * set the variables value.
     */
    *valptr = val;
    LWLockWaitListUnlock(lock);

    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!
 */
void
LWLockReleaseAll(void)
{
    while (num_held_lwlocks > 0)
    {
        HOLD_INTERRUPTS();      /* match the upcoming RESUME_INTERRUPTS */

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


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

    for (i = 0; i < num_held_lwlocks; i++)
    {
        if (held_lwlocks[i].lock == l)
            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 *l, LWLockMode mode)
{
    int         i;

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