spin.c

Go to the documentation of this file.

/*-------------------------------------------------------------------------
 *
 * spin.c
 *     Hardware-independent implementation of spinlocks.
 *
 *
 * For machines that have test-and-set (TAS) instructions, s_lock.h/.c
 * define the spinlock implementation.  This file contains only a stub
 * implementation for spinlocks using PGSemaphores.  Unless semaphores
 * are implemented in a way that doesn't involve a kernel call, this
 * is too slow to be very useful :-(
 *
 *
 * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/storage/lmgr/spin.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "storage/pg_sema.h"
#include "storage/shmem.h"
#include "storage/spin.h"


#ifndef HAVE_SPINLOCKS
PGSemaphore *SpinlockSemaArray;
#endif

/*
 * Report the amount of shared memory needed to store semaphores for spinlock
 * support.
 */
Size
SpinlockSemaSize(void)
{
    return SpinlockSemas() * sizeof(PGSemaphore);
}

#ifdef HAVE_SPINLOCKS

/*
 * Report number of semaphores needed to support spinlocks.
 */
int
SpinlockSemas(void)
{
    return 0;
}
#else                           /* !HAVE_SPINLOCKS */

/*
 * No TAS, so spinlocks are implemented as PGSemaphores.
 */


/*
 * Report number of semaphores needed to support spinlocks.
 */
int
SpinlockSemas(void)
{
    return NUM_SPINLOCK_SEMAPHORES + NUM_ATOMICS_SEMAPHORES;
}

/*
 * Initialize spinlock emulation.
 *
 * This must be called after PGReserveSemaphores().
 */
void
SpinlockSemaInit(void)
{
    PGSemaphore *spinsemas;
    int         nsemas = SpinlockSemas();
    int         i;

    /*
     * We must use ShmemAllocUnlocked(), since the spinlock protecting
     * ShmemAlloc() obviously can't be ready yet.
     */
    spinsemas = (PGSemaphore *) ShmemAllocUnlocked(SpinlockSemaSize());
    for (i = 0; i < nsemas; ++i)
        spinsemas[i] = PGSemaphoreCreate();
    SpinlockSemaArray = spinsemas;
}

/*
 * s_lock.h hardware-spinlock emulation using semaphores
 *
 * We map all spinlocks onto a set of NUM_SPINLOCK_SEMAPHORES semaphores.
 * It's okay to map multiple spinlocks onto one semaphore because no process
 * should ever hold more than one at a time.  We just need enough semaphores
 * so that we aren't adding too much extra contention from that.
 *
 * slock_t is just an int for this implementation; it holds the spinlock
 * number from 1..NUM_SPINLOCK_SEMAPHORES.  We intentionally ensure that 0
 * is not a valid value, so that testing with this code can help find
 * failures to initialize spinlocks.
 */

void
s_init_lock_sema(volatile slock_t *lock, bool nested)
{
    static int  counter = 0;

    *lock = ((++counter) % NUM_SPINLOCK_SEMAPHORES) + 1;
}

void
s_unlock_sema(volatile slock_t *lock)
{
    int         lockndx = *lock;

    if (lockndx <= 0 || lockndx > NUM_SPINLOCK_SEMAPHORES)
        elog(ERROR, "invalid spinlock number: %d", lockndx);
    PGSemaphoreUnlock(SpinlockSemaArray[lockndx - 1]);
}

bool
s_lock_free_sema(volatile slock_t *lock)
{
    /* We don't currently use S_LOCK_FREE anyway */
    elog(ERROR, "spin.c does not support S_LOCK_FREE()");
    return false;
}

int
tas_sema(volatile slock_t *lock)
{
    int         lockndx = *lock;

    if (lockndx <= 0 || lockndx > NUM_SPINLOCK_SEMAPHORES)
        elog(ERROR, "invalid spinlock number: %d", lockndx);
    /* Note that TAS macros return 0 if *success* */
    return !PGSemaphoreTryLock(SpinlockSemaArray[lockndx - 1]);
}

#endif                          /* !HAVE_SPINLOCKS */