s_lock.c

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