/*-------------------------------------------------------------------------
 *
 * 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 */
