text: /*-------------------------------------------------------------------------
   *
   * s_lock.c
   *	   Hardware-dependent implementation of spinlocks.
   *
   *
   * Portions Copyright (c) 1996-2013, PostgreSQL Global Development Group
   * Portions Copyright (c) 1994, Regents of the University of California
   *
   *
   * IDENTIFICATION
   *	  src/backend/storage/lmgr/s_lock.c
   *
   *-------------------------------------------------------------------------
   */
  #include "postgres.h"
  
  #include <time.h>
  #include <unistd.h>
  
  #include "storage/s_lock.h"
  
  slock_t		dummy_spinlock;
  
  static int	spins_per_delay = DEFAULT_SPINS_PER_DELAY;
  
  
  /*
   * s_lock_stuck() - complain about a stuck spinlock
   */
  static void
  s_lock_stuck(volatile slock_t *lock, const char *file, int line)
  {
  #if defined(S_LOCK_TEST)
  	fprintf(stderr,
  			"\nStuck spinlock (%p) detected at %s:%d.\n",
  			lock, file, line);
  	exit(1);
  #else
  	elog(PANIC, "stuck spinlock (%p) detected at %s:%d",
  		 lock, file, line);
  #endif
  }
  
  
  /*
   * s_lock(lock) - platform-independent portion of waiting for a spinlock.
   */
  int
  s_lock(volatile slock_t *lock, const char *file, int line)
  {
  	/*
  	 * We loop tightly for awhile, then delay using pg_usleep() and try again.
  	 * Preferably, "awhile" should be a small multiple of the maximum time we
  	 * expect a spinlock to be held.  100 iterations seems about right as an
  	 * initial guess.  However, on a uniprocessor the loop is a waste of
  	 * cycles, while in a multi-CPU scenario it's usually better to spin a bit
  	 * longer than to call the kernel, so we try to adapt the spin loop count
  	 * depending on whether we seem to be in a uniprocessor or multiprocessor.
  	 *
  	 * Note: you might think MIN_SPINS_PER_DELAY should be just 1, but you'd
  	 * be wrong; there are platforms where that can result in a "stuck
  	 * spinlock" failure.  This has been seen particularly on Alphas; it seems
  	 * that the first TAS after returning from kernel space will always fail
  	 * on that hardware.
  	 *
  	 * Once we do decide to block, we use randomly increasing pg_usleep()
  	 * delays. The first delay is 1 msec, then the delay randomly increases to
  	 * about one second, after which we reset to 1 msec and start again.  The
  	 * idea here is that in the presence of heavy contention we need to
  	 * increase the delay, else the spinlock holder may never get to run and
  	 * release the lock.  (Consider situation where spinlock holder has been
  	 * nice'd down in priority by the scheduler --- it will not get scheduled
  	 * until all would-be acquirers are sleeping, so if we always use a 1-msec
  	 * sleep, there is a real possibility of starvation.)  But we can't just
  	 * clamp the delay to an upper bound, else it would take a long time to
  	 * make a reasonable number of tries.
  	 *
  	 * We time out and declare error after NUM_DELAYS delays (thus, exactly
  	 * that many tries).  With the given settings, this will usually take 2 or
  	 * so minutes.  It seems better to fix the total number of tries (and thus
  	 * the probability of unintended failure) than to fix the total time
  	 * spent.
  	 *
  	 * The pg_usleep() delays are measured in milliseconds because 1 msec is a
  	 * common resolution limit at the OS level for newer platforms. On older
  	 * platforms the resolution limit is usually 10 msec, in which case the
  	 * total delay before timeout will be a bit more.
  	 */
  #define MIN_SPINS_PER_DELAY 10
  #define MAX_SPINS_PER_DELAY 1000
  #define NUM_DELAYS			1000
  #define MIN_DELAY_MSEC		1
  #define MAX_DELAY_MSEC		1000
  
  	int			spins = 0;
  	int			delays = 0;
  	int			cur_delay = 0;
  
  	while (TAS_SPIN(lock))
  	{
  		/* CPU-specific delay each time through the loop */
  		SPIN_DELAY();
  
  		/* Block the process every spins_per_delay tries */
  		if (++spins >= spins_per_delay)
  		{
  			if (++delays > NUM_DELAYS)
  				s_lock_stuck(lock, file, line);
  
  			if (cur_delay == 0) /* first time to delay? */
  				cur_delay = MIN_DELAY_MSEC;
  
  			pg_usleep(cur_delay * 1000L);
  
  #if defined(S_LOCK_TEST)
  			fprintf(stdout, "*");
  			fflush(stdout);
  #endif
  
  			/* increase delay by a random fraction between 1X and 2X */
  			cur_delay += (int) (cur_delay *
  					  ((double) random() / (double) MAX_RANDOM_VALUE) + 0.5);
  			/* wrap back to minimum delay when max is exceeded */
  			if (cur_delay > MAX_DELAY_MSEC)
  				cur_delay = MIN_DELAY_MSEC;
  
  			spins = 0;
  		}
  	}
  
  	/*
  	 * If we were able to acquire the lock without delaying, it's a good
  	 * indication we are in a multiprocessor.  If we had to delay, it's a sign
  	 * (but not a sure thing) that we are in a uniprocessor. Hence, we
  	 * decrement spins_per_delay slowly when we had to delay, and increase it
  	 * rapidly when we didn't.  It's expected that spins_per_delay will
  	 * converge to the minimum value on a uniprocessor and to the maximum
  	 * value on a multiprocessor.
  	 *
  	 * Note: spins_per_delay is local within our current process. We want to
  	 * average these observations across multiple backends, since it's
  	 * relatively rare for this function to even get entered, and so a single
  	 * backend might not live long enough to converge on a good value.  That
  	 * is handled by the two routines below.
  	 */
  	if (cur_delay == 0)
  	{
  		/* we never had to delay */
  		if (spins_per_delay < MAX_SPINS_PER_DELAY)
  			spins_per_delay = Min(spins_per_delay + 100, MAX_SPINS_PER_DELAY);
  	}
  	else
  	{
  		if (spins_per_delay > MIN_SPINS_PER_DELAY)
  			spins_per_delay = Max(spins_per_delay - 1, MIN_SPINS_PER_DELAY);
  	}
  	return delays;
  }
  
  
  /*
   * Set local copy of spins_per_delay during backend startup.
   *
   * NB: this has to be pretty fast as it is called while holding a spinlock
   */
  void
  set_spins_per_delay(int shared_spins_per_delay)
  {
  	spins_per_delay = shared_spins_per_delay;
  }
  
  /*
   * Update shared estimate of spins_per_delay during backend exit.
   *
   * NB: this has to be pretty fast as it is called while holding a spinlock
   */
  int
  update_spins_per_delay(int shared_spins_per_delay)
  {
  	/*
  	 * We use an exponential moving average with a relatively slow adaption
  	 * rate, so that noise in any one backend's result won't affect the shared
  	 * value too much.  As long as both inputs are within the allowed range,
  	 * the result must be too, so we need not worry about clamping the result.
  	 *
  	 * We deliberately truncate rather than rounding; this is so that single
  	 * adjustments inside a backend can affect the shared estimate (see the
  	 * asymmetric adjustment rules above).
  	 */
  	return (shared_spins_per_delay * 15 + spins_per_delay) / 16;
  }
  
  
  /*
   * Various TAS implementations that cannot live in s_lock.h as no inline
   * definition exists (yet).
   * In the future, get rid of tas.[cso] and fold it into this file.
   *
   * If you change something here, you will likely need to modify s_lock.h too,
   * because the definitions for these are split between this file and s_lock.h.
   */
  
  
  #ifdef HAVE_SPINLOCKS			/* skip spinlocks if requested */
  
  
  #if defined(__GNUC__)
  
  /*
   * All the gcc flavors that are not inlined
   */
  
  
  /*
   * Note: all the if-tests here probably ought to be testing gcc version
   * rather than platform, but I don't have adequate info to know what to
   * write.  Ideally we'd flush all this in favor of the inline version.
   */
  #if defined(__m68k__) && !defined(__linux__)
  /* really means: extern int tas(slock_t* **lock); */
  static void
  tas_dummy()
  {
  	__asm__		__volatile__(
  #if defined(__NetBSD__) && defined(__ELF__)
  /* no underscore for label and % for registers */
  										 "\
  .global		tas 				\n\
  tas:							\n\
  			movel	%sp@(0x4),%a0	\n\
  			tas 	%a0@		\n\
  			beq 	_success	\n\
  			moveq	#-128,%d0	\n\
  			rts 				\n\
  _success:						\n\
  			moveq	#0,%d0		\n\
  			rts 				\n"
  #else
  										 "\
  .global		_tas				\n\
  _tas:							\n\
  			movel	sp@(0x4),a0	\n\
  			tas 	a0@			\n\
  			beq 	_success	\n\
  			moveq 	#-128,d0	\n\
  			rts					\n\
  _success:						\n\
  			moveq 	#0,d0		\n\
  			rts					\n"
  #endif   /* __NetBSD__ && __ELF__ */
  	);
  }
  #endif   /* __m68k__ && !__linux__ */
  #else							/* not __GNUC__ */
  
  /*
   * All non gcc
   */
  
  
  #if defined(sun3)
  static void
  tas_dummy()						/* really means: extern int tas(slock_t
  								 * *lock); */
  {
  	asm("LLA0:");
  	asm("   .data");
  	asm("   .text");
  	asm("|#PROC# 04");
  	asm("   .globl  _tas");
  	asm("_tas:");
  	asm("|#PROLOGUE# 1");
  	asm("   movel   sp@(0x4),a0");
  	asm("   tas a0@");
  	asm("   beq LLA1");
  	asm("   moveq   #-128,d0");
  	asm("   rts");
  	asm("LLA1:");
  	asm("   moveq   #0,d0");
  	asm("   rts");
  	asm("   .data");
  }
  #endif   /* sun3 */
  #endif   /* not __GNUC__ */
  #endif   /* HAVE_SPINLOCKS */
  
  
  
  /*****************************************************************************/
  #if defined(S_LOCK_TEST)
  
  /*
   * test program for verifying a port's spinlock support.
   */
  
  struct test_lock_struct
  {
  	char		pad1;
  	slock_t		lock;
  	char		pad2;
  };
  
  volatile struct test_lock_struct test_lock;
  
  int
  main()
  {
  	srandom((unsigned int) time(NULL));
  
  	test_lock.pad1 = test_lock.pad2 = 0x44;
  
  	S_INIT_LOCK(&test_lock.lock);
  
  	if (test_lock.pad1 != 0x44 || test_lock.pad2 != 0x44)
  	{
  		printf("S_LOCK_TEST: failed, declared datatype is wrong size\n");
  		return 1;
  	}
  
  	if (!S_LOCK_FREE(&test_lock.lock))
  	{
  		printf("S_LOCK_TEST: failed, lock not initialized\n");
  		return 1;
  	}
  
  	S_LOCK(&test_lock.lock);
  
  	if (test_lock.pad1 != 0x44 || test_lock.pad2 != 0x44)
  	{
  		printf("S_LOCK_TEST: failed, declared datatype is wrong size\n");
  		return 1;
  	}
  
  	if (S_LOCK_FREE(&test_lock.lock))
  	{
  		printf("S_LOCK_TEST: failed, lock not locked\n");
  		return 1;
  	}
  
  	S_UNLOCK(&test_lock.lock);
  
  	if (test_lock.pad1 != 0x44 || test_lock.pad2 != 0x44)
  	{
  		printf("S_LOCK_TEST: failed, declared datatype is wrong size\n");
  		return 1;
  	}
  
  	if (!S_LOCK_FREE(&test_lock.lock))
  	{
  		printf("S_LOCK_TEST: failed, lock not unlocked\n");
  		return 1;
  	}
  
  	S_LOCK(&test_lock.lock);
  
  	if (test_lock.pad1 != 0x44 || test_lock.pad2 != 0x44)
  	{
  		printf("S_LOCK_TEST: failed, declared datatype is wrong size\n");
  		return 1;
  	}
  
  	if (S_LOCK_FREE(&test_lock.lock))
  	{
  		printf("S_LOCK_TEST: failed, lock not re-locked\n");
  		return 1;
  	}
  
  	printf("S_LOCK_TEST: this will print %d stars and then\n", NUM_DELAYS);
  	printf("             exit with a 'stuck spinlock' message\n");
  	printf("             if S_LOCK() and TAS() are working.\n");
  	fflush(stdout);
  
  	s_lock(&test_lock.lock, __FILE__, __LINE__);
  
  	printf("S_LOCK_TEST: failed, lock not locked\n");
  	return 1;
  }
  
  #endif   /* S_LOCK_TEST */
  