s_lock.c

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/*-------------------------------------------------------------------------
 *
 * s_lock.c
 *     Implementation of spinlocks.
 *
 * When waiting for a contended spinlock 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.
 *
 * Portions Copyright (c) 1996-2025, 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 "common/pg_prng.h"
#include "storage/s_lock.h"
#include "utils/wait_event.h"
 
#define MIN_SPINS_PER_DELAY 10
#define MAX_SPINS_PER_DELAY 1000
#define NUM_DELAYS          1000
#define MIN_DELAY_USEC      1000L
#define MAX_DELAY_USEC      1000000L
 
#ifdef S_LOCK_TEST
/*
 * These are needed by pgstat_report_wait_start in the standalone compile of
 * s_lock_test.
 */
static uint32 local_my_wait_event_info;
uint32     *my_wait_event_info = &local_my_wait_event_info;
#endif
 
static int  spins_per_delay = DEFAULT_SPINS_PER_DELAY;
 
 
/*
 * s_lock_stuck() - complain about a stuck spinlock
 */
static void
s_lock_stuck(const char *file, int line, const char *func)
{
    if (!func)
        func = "(unknown)";
#if defined(S_LOCK_TEST)
    fprintf(stderr,
            "\nStuck spinlock detected at %s, %s:%d.\n",
            func, file, line);
    exit(1);
#else
    elog(PANIC, "stuck spinlock detected at %s, %s:%d",
         func, 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, const char *func)
{
    SpinDelayStatus delayStatus;
 
    init_spin_delay(&delayStatus, file, line, func);
 
    while (TAS_SPIN(lock))
    {
        perform_spin_delay(&delayStatus);
    }
 
    finish_spin_delay(&delayStatus);
 
    return delayStatus.delays;
}
 
#ifdef USE_DEFAULT_S_UNLOCK
void
s_unlock(volatile slock_t *lock)
{
    *lock = 0;
}
#endif
 
/*
 * Wait while spinning on a contended spinlock.
 */
void
perform_spin_delay(SpinDelayStatus *status)
{
    /* CPU-specific delay each time through the loop */
    SPIN_DELAY();
 
    /* Block the process every spins_per_delay tries */
    if (++(status->spins) >= spins_per_delay)
    {
        if (++(status->delays) > NUM_DELAYS)
            s_lock_stuck(status->file, status->line, status->func);
 
        if (status->cur_delay == 0) /* first time to delay? */
            status->cur_delay = MIN_DELAY_USEC;
 
        /*
         * Once we start sleeping, the overhead of reporting a wait event is
         * justified. Actively spinning easily stands out in profilers, but
         * sleeping with an exponential backoff is harder to spot...
         *
         * We might want to report something more granular at some point, but
         * this is better than nothing.
         */
        pgstat_report_wait_start(WAIT_EVENT_SPIN_DELAY);
        pg_usleep(status->cur_delay);
        pgstat_report_wait_end();
 
#if defined(S_LOCK_TEST)
        fprintf(stdout, "*");
        fflush(stdout);
#endif
 
        /* increase delay by a random fraction between 1X and 2X */
        status->cur_delay += (int) (status->cur_delay *
                                    pg_prng_double(&pg_global_prng_state) + 0.5);
        /* wrap back to minimum delay when max is exceeded */
        if (status->cur_delay > MAX_DELAY_USEC)
            status->cur_delay = MIN_DELAY_USEC;
 
        status->spins = 0;
    }
}
 
/*
 * After acquiring a spinlock, update estimates about how long to loop.
 *
 * 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.
 */
void
finish_spin_delay(SpinDelayStatus *status)
{
    if (status->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;
}
 
 
/*****************************************************************************/
#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()
{
    pg_prng_seed(&pg_global_prng_state, (uint64) 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__, __func__);
 
    printf("S_LOCK_TEST: failed, lock not locked\n");
    return 1;
}
 
#endif                          /* S_LOCK_TEST */