condition_variable.c

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/*-------------------------------------------------------------------------
 *
 * condition_variable.c
 *    Implementation of condition variables.  Condition variables provide
 *    a way for one process to wait until a specific condition occurs,
 *    without needing to know the specific identity of the process for
 *    which they are waiting.  Waits for condition variables can be
 *    interrupted, unlike LWLock waits.  Condition variables are safe
 *    to use within dynamic shared memory segments.
 *
 * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/backend/storage/lmgr/condition_variable.c
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "miscadmin.h"
#include "portability/instr_time.h"
#include "storage/condition_variable.h"
#include "storage/ipc.h"
#include "storage/proc.h"
#include "storage/proclist.h"
#include "storage/spin.h"
#include "utils/memutils.h"

/* Initially, we are not prepared to sleep on any condition variable. */
static ConditionVariable *cv_sleep_target = NULL;

/*
 * Initialize a condition variable.
 */
void
ConditionVariableInit(ConditionVariable *cv)
{
    SpinLockInit(&cv->mutex);
    proclist_init(&cv->wakeup);
}

/*
 * Prepare to wait on a given condition variable.
 *
 * This can optionally be called before entering a test/sleep loop.
 * Doing so is more efficient if we'll need to sleep at least once.
 * However, if the first test of the exit condition is likely to succeed,
 * it's more efficient to omit the ConditionVariablePrepareToSleep call.
 * See comments in ConditionVariableSleep for more detail.
 *
 * Caution: "before entering the loop" means you *must* test the exit
 * condition between calling ConditionVariablePrepareToSleep and calling
 * ConditionVariableSleep.  If that is inconvenient, omit calling
 * ConditionVariablePrepareToSleep.
 */
void
ConditionVariablePrepareToSleep(ConditionVariable *cv)
{
    int         pgprocno = MyProc->pgprocno;

    /*
     * If some other sleep is already prepared, cancel it; this is necessary
     * because we have just one static variable tracking the prepared sleep,
     * and also only one cvWaitLink in our PGPROC.  It's okay to do this
     * because whenever control does return to the other test-and-sleep loop,
     * its ConditionVariableSleep call will just re-establish that sleep as
     * the prepared one.
     */
    if (cv_sleep_target != NULL)
        ConditionVariableCancelSleep();

    /* Record the condition variable on which we will sleep. */
    cv_sleep_target = cv;

    /* Add myself to the wait queue. */
    SpinLockAcquire(&cv->mutex);
    proclist_push_tail(&cv->wakeup, pgprocno, cvWaitLink);
    SpinLockRelease(&cv->mutex);
}

/*
 * Wait for the given condition variable to be signaled.
 *
 * This should be called in a predicate loop that tests for a specific exit
 * condition and otherwise sleeps, like so:
 *
 *   ConditionVariablePrepareToSleep(cv);  // optional
 *   while (condition for which we are waiting is not true)
 *       ConditionVariableSleep(cv, wait_event_info);
 *   ConditionVariableCancelSleep();
 *
 * wait_event_info should be a value from one of the WaitEventXXX enums
 * defined in pgstat.h.  This controls the contents of pg_stat_activity's
 * wait_event_type and wait_event columns while waiting.
 */
void
ConditionVariableSleep(ConditionVariable *cv, uint32 wait_event_info)
{
    (void) ConditionVariableTimedSleep(cv, -1 /* no timeout */ ,
                                       wait_event_info);
}

/*
 * Wait for a condition variable to be signaled or a timeout to be reached.
 *
 * Returns true when timeout expires, otherwise returns false.
 *
 * See ConditionVariableSleep() for general usage.
 */
bool
ConditionVariableTimedSleep(ConditionVariable *cv, long timeout,
                            uint32 wait_event_info)
{
    long        cur_timeout = -1;
    instr_time  start_time;
    instr_time  cur_time;
    int         wait_events;

    /*
     * If the caller didn't prepare to sleep explicitly, then do so now and
     * return immediately.  The caller's predicate loop should immediately
     * call again if its exit condition is not yet met.  This will result in
     * the exit condition being tested twice before we first sleep.  The extra
     * test can be prevented by calling ConditionVariablePrepareToSleep(cv)
     * first.  Whether it's worth doing that depends on whether you expect the
     * exit condition to be met initially, in which case skipping the prepare
     * is recommended because it avoids manipulations of the wait list, or not
     * met initially, in which case preparing first is better because it
     * avoids one extra test of the exit condition.
     *
     * If we are currently prepared to sleep on some other CV, we just cancel
     * that and prepare this one; see ConditionVariablePrepareToSleep.
     */
    if (cv_sleep_target != cv)
    {
        ConditionVariablePrepareToSleep(cv);
        return false;
    }

    /*
     * Record the current time so that we can calculate the remaining timeout
     * if we are woken up spuriously.
     */
    if (timeout >= 0)
    {
        INSTR_TIME_SET_CURRENT(start_time);
        Assert(timeout >= 0 && timeout <= INT_MAX);
        cur_timeout = timeout;
        wait_events = WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH;
    }
    else
        wait_events = WL_LATCH_SET | WL_EXIT_ON_PM_DEATH;

    while (true)
    {
        bool        done = false;

        /*
         * Wait for latch to be set.  (If we're awakened for some other
         * reason, the code below will cope anyway.)
         */
        (void) WaitLatch(MyLatch, wait_events, cur_timeout, wait_event_info);

        /* Reset latch before examining the state of the wait list. */
        ResetLatch(MyLatch);

        /*
         * If this process has been taken out of the wait list, then we know
         * that it has been signaled by ConditionVariableSignal (or
         * ConditionVariableBroadcast), so we should return to the caller. But
         * that doesn't guarantee that the exit condition is met, only that we
         * ought to check it.  So we must put the process back into the wait
         * list, to ensure we don't miss any additional wakeup occurring while
         * the caller checks its exit condition.  We can take ourselves out of
         * the wait list only when the caller calls
         * ConditionVariableCancelSleep.
         *
         * If we're still in the wait list, then the latch must have been set
         * by something other than ConditionVariableSignal; though we don't
         * guarantee not to return spuriously, we'll avoid this obvious case.
         */
        SpinLockAcquire(&cv->mutex);
        if (!proclist_contains(&cv->wakeup, MyProc->pgprocno, cvWaitLink))
        {
            done = true;
            proclist_push_tail(&cv->wakeup, MyProc->pgprocno, cvWaitLink);
        }
        SpinLockRelease(&cv->mutex);

        /*
         * Check for interrupts, and return spuriously if that caused the
         * current sleep target to change (meaning that interrupt handler code
         * waited for a different condition variable).
         */
        CHECK_FOR_INTERRUPTS();
        if (cv != cv_sleep_target)
            done = true;

        /* We were signaled, so return */
        if (done)
            return false;

        /* If we're not done, update cur_timeout for next iteration */
        if (timeout >= 0)
        {
            INSTR_TIME_SET_CURRENT(cur_time);
            INSTR_TIME_SUBTRACT(cur_time, start_time);
            cur_timeout = timeout - (long) INSTR_TIME_GET_MILLISEC(cur_time);

            /* Have we crossed the timeout threshold? */
            if (cur_timeout <= 0)
                return true;
        }
    }
}

/*
 * Cancel any pending sleep operation.
 *
 * We just need to remove ourselves from the wait queue of any condition
 * variable for which we have previously prepared a sleep.
 *
 * Do nothing if nothing is pending; this allows this function to be called
 * during transaction abort to clean up any unfinished CV sleep.
 */
void
ConditionVariableCancelSleep(void)
{
    ConditionVariable *cv = cv_sleep_target;
    bool        signaled = false;

    if (cv == NULL)
        return;

    SpinLockAcquire(&cv->mutex);
    if (proclist_contains(&cv->wakeup, MyProc->pgprocno, cvWaitLink))
        proclist_delete(&cv->wakeup, MyProc->pgprocno, cvWaitLink);
    else
        signaled = true;
    SpinLockRelease(&cv->mutex);

    /*
     * If we've received a signal, pass it on to another waiting process, if
     * there is one.  Otherwise a call to ConditionVariableSignal() might get
     * lost, despite there being another process ready to handle it.
     */
    if (signaled)
        ConditionVariableSignal(cv);

    cv_sleep_target = NULL;
}

/*
 * Wake up the oldest process sleeping on the CV, if there is any.
 *
 * Note: it's difficult to tell whether this has any real effect: we know
 * whether we took an entry off the list, but the entry might only be a
 * sentinel.  Hence, think twice before proposing that this should return
 * a flag telling whether it woke somebody.
 */
void
ConditionVariableSignal(ConditionVariable *cv)
{
    PGPROC     *proc = NULL;

    /* Remove the first process from the wakeup queue (if any). */
    SpinLockAcquire(&cv->mutex);
    if (!proclist_is_empty(&cv->wakeup))
        proc = proclist_pop_head_node(&cv->wakeup, cvWaitLink);
    SpinLockRelease(&cv->mutex);

    /* If we found someone sleeping, set their latch to wake them up. */
    if (proc != NULL)
        SetLatch(&proc->procLatch);
}

/*
 * Wake up all processes sleeping on the given CV.
 *
 * This guarantees to wake all processes that were sleeping on the CV
 * at time of call, but processes that add themselves to the list mid-call
 * will typically not get awakened.
 */
void
ConditionVariableBroadcast(ConditionVariable *cv)
{
    int         pgprocno = MyProc->pgprocno;
    PGPROC     *proc = NULL;
    bool        have_sentinel = false;

    /*
     * In some use-cases, it is common for awakened processes to immediately
     * re-queue themselves.  If we just naively try to reduce the wakeup list
     * to empty, we'll get into a potentially-indefinite loop against such a
     * process.  The semantics we really want are just to be sure that we have
     * wakened all processes that were in the list at entry.  We can use our
     * own cvWaitLink as a sentinel to detect when we've finished.
     *
     * A seeming flaw in this approach is that someone else might signal the
     * CV and in doing so remove our sentinel entry.  But that's fine: since
     * CV waiters are always added and removed in order, that must mean that
     * every previous waiter has been wakened, so we're done.  We'll get an
     * extra "set" on our latch from the someone else's signal, which is
     * slightly inefficient but harmless.
     *
     * We can't insert our cvWaitLink as a sentinel if it's already in use in
     * some other proclist.  While that's not expected to be true for typical
     * uses of this function, we can deal with it by simply canceling any
     * prepared CV sleep.  The next call to ConditionVariableSleep will take
     * care of re-establishing the lost state.
     */
    if (cv_sleep_target != NULL)
        ConditionVariableCancelSleep();

    /*
     * Inspect the state of the queue.  If it's empty, we have nothing to do.
     * If there's exactly one entry, we need only remove and signal that
     * entry.  Otherwise, remove the first entry and insert our sentinel.
     */
    SpinLockAcquire(&cv->mutex);
    /* While we're here, let's assert we're not in the list. */
    Assert(!proclist_contains(&cv->wakeup, pgprocno, cvWaitLink));

    if (!proclist_is_empty(&cv->wakeup))
    {
        proc = proclist_pop_head_node(&cv->wakeup, cvWaitLink);
        if (!proclist_is_empty(&cv->wakeup))
        {
            proclist_push_tail(&cv->wakeup, pgprocno, cvWaitLink);
            have_sentinel = true;
        }
    }
    SpinLockRelease(&cv->mutex);

    /* Awaken first waiter, if there was one. */
    if (proc != NULL)
        SetLatch(&proc->procLatch);

    while (have_sentinel)
    {
        /*
         * Each time through the loop, remove the first wakeup list entry, and
         * signal it unless it's our sentinel.  Repeat as long as the sentinel
         * remains in the list.
         *
         * Notice that if someone else removes our sentinel, we will waken one
         * additional process before exiting.  That's intentional, because if
         * someone else signals the CV, they may be intending to waken some
         * third process that added itself to the list after we added the
         * sentinel.  Better to give a spurious wakeup (which should be
         * harmless beyond wasting some cycles) than to lose a wakeup.
         */
        proc = NULL;
        SpinLockAcquire(&cv->mutex);
        if (!proclist_is_empty(&cv->wakeup))
            proc = proclist_pop_head_node(&cv->wakeup, cvWaitLink);
        have_sentinel = proclist_contains(&cv->wakeup, pgprocno, cvWaitLink);
        SpinLockRelease(&cv->mutex);

        if (proc != NULL && proc != MyProc)
            SetLatch(&proc->procLatch);
    }
}