greenplumn lwlock 源码
greenplumn lwlock 代码
文件路径：/src/backend/storage/lmgr/lwlock.c
/*-------------------------------------------------------------------------
*
* 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) 1996-2019, 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 "pgstat.h"
#include "pg_trace.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 *lockid);
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 releaseOk 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;
}
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