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Sourcecode: postgresql-8.4 version File versions

s_lock.c

/*-------------------------------------------------------------------------
 *
 * s_lock.c
 *       Hardware-dependent implementation of spinlocks.
 *
 *
 * Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *      $PostgreSQL: pgsql/src/backend/storage/lmgr/s_lock.c,v 1.50 2009/01/01 17:23:48 momjian Exp $
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include <time.h>
#include <unistd.h>

#include "storage/s_lock.h"


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.
 */
void
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(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);
      }
}


/*
 * 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 */

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