spin.c

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/*-------------------------------------------------------------------------
 *
 * 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) 1996-2013, 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 "miscadmin.h"
#include "replication/walsender.h"
#include "storage/lwlock.h"
#include "storage/spin.h"


#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)
{
    int     nsemas;

    /*
     * It would be cleaner to distribute this logic into the affected modules,
     * similar to the way shmem space estimation is handled.
     *
     * For now, though, there are few enough users of spinlocks that we just
     * keep the knowledge here.
     */
    nsemas = NumLWLocks();      /* one for each lwlock */
    nsemas += NBuffers;         /* one for each buffer header */
    nsemas += max_wal_senders;  /* one for each wal sender process */
    nsemas += 30;               /* plus a bunch for other small-scale use */

    return nsemas;
}

/*
 * s_lock.h hardware-spinlock emulation
 */

void
s_init_lock_sema(volatile slock_t *lock)
{
    PGSemaphoreCreate((PGSemaphore) lock);
}

void
s_unlock_sema(volatile slock_t *lock)
{
    PGSemaphoreUnlock((PGSemaphore) lock);
}

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)
{
    /* Note that TAS macros return 0 if *success* */
    return !PGSemaphoreTryLock((PGSemaphore) lock);
}

#endif   /* !HAVE_SPINLOCKS */