#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"

Include dependency graph for condition_variable.c:

Functions
void	ConditionVariableInit (ConditionVariable *cv)

void	ConditionVariablePrepareToSleep (ConditionVariable *cv)

void	ConditionVariableSleep (ConditionVariable *cv, uint32 wait_event_info)

bool	ConditionVariableTimedSleep (ConditionVariable *cv, long timeout, uint32 wait_event_info)

void	ConditionVariableCancelSleep (void)

void	ConditionVariableSignal (ConditionVariable *cv)

void	ConditionVariableBroadcast (ConditionVariable *cv)

Variables
static ConditionVariable *	cv_sleep_target = NULL

Function Documentation

◆ ConditionVariableBroadcast()

void ConditionVariableBroadcast ( ConditionVariable * cv )

Definition at line 286 of file condition_variable.c.

 {
     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);
     }
 }

References Assert(), ConditionVariableCancelSleep(), cv_sleep_target, ConditionVariable::mutex, MyProc, PGPROC::pgprocno, PGPROC::procLatch, proclist_contains, proclist_is_empty(), proclist_pop_head_node, proclist_push_tail, SetLatch(), SpinLockAcquire, SpinLockRelease, and ConditionVariable::wakeup.

Referenced by _bt_parallel_done(), BarrierArriveAndWait(), BarrierDetachImpl(), BitmapDoneInitializingSharedState(), CheckpointerMain(), CleanupProcSignalState(), ProcessProcSignalBarrier(), ReplicationOriginExitCleanup(), ReplicationSlotAcquire(), ReplicationSlotCleanup(), ReplicationSlotCreate(), ReplicationSlotDropPtr(), ReplicationSlotRelease(), replorigin_session_reset(), replorigin_session_setup(), SetRecoveryPause(), ShutdownWalRcv(), TerminateBufferIO(), WalRcvDie(), WalRcvRunning(), WalRcvStreaming(), WalReceiverMain(), and WalSndWakeup().

◆ ConditionVariableCancelSleep()

void ConditionVariableCancelSleep ( void )

Definition at line 228 of file condition_variable.c.

 {
     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;
 }

References ConditionVariableSignal(), cv_sleep_target, ConditionVariable::mutex, MyProc, PGPROC::pgprocno, proclist_contains, proclist_delete, SpinLockAcquire, SpinLockRelease, and ConditionVariable::wakeup.

Referenced by _bt_parallel_heapscan(), _bt_parallel_seize(), AbortSubTransaction(), AbortTransaction(), AuxiliaryProcKill(), BackgroundWriterMain(), BarrierArriveAndWait(), BitmapShouldInitializeSharedState(), CheckpointerMain(), ConditionVariableBroadcast(), ConditionVariablePrepareToSleep(), ProcKill(), recoveryPausesHere(), RecoveryRequiresIntParameter(), ReplicationSlotAcquire(), replorigin_state_clear(), RequestCheckpoint(), ShutdownAuxiliaryProcess(), ShutdownWalRcv(), WaitForProcSignalBarrier(), WaitIO(), WalSndErrorCleanup(), WalSndWait(), and WalWriterMain().

◆ ConditionVariableInit()

void ConditionVariableInit ( ConditionVariable * cv )

Definition at line 37 of file condition_variable.c.

 {
     SpinLockInit(&cv->mutex);
     proclist_init(&cv->wakeup);
 }

References ConditionVariable::mutex, proclist_init(), SpinLockInit, and ConditionVariable::wakeup.

Referenced by _bt_begin_parallel(), BarrierInit(), btinitparallelscan(), CheckpointerShmemInit(), ExecBitmapHeapInitializeDSM(), InitBufferPool(), ProcSignalShmemInit(), ReplicationOriginShmemInit(), ReplicationSlotsShmemInit(), WalRcvShmemInit(), WalSndShmemInit(), and XLogRecoveryShmemInit().

◆ ConditionVariablePrepareToSleep()

void ConditionVariablePrepareToSleep ( ConditionVariable * cv )

Definition at line 58 of file condition_variable.c.

 {
     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);
 }

References ConditionVariableCancelSleep(), cv_sleep_target, ConditionVariable::mutex, MyProc, PGPROC::pgprocno, proclist_push_tail, SpinLockAcquire, SpinLockRelease, and ConditionVariable::wakeup.

Referenced by BarrierArriveAndWait(), ConditionVariableTimedSleep(), InvalidatePossiblyObsoleteSlot(), ReplicationSlotAcquire(), RequestCheckpoint(), ShutdownWalRcv(), WaitIO(), and WalSndWait().

◆ ConditionVariableSignal()

void ConditionVariableSignal ( ConditionVariable * cv )

Definition at line 263 of file condition_variable.c.

 {
     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);
 }

References ConditionVariable::mutex, PGPROC::procLatch, proclist_is_empty(), proclist_pop_head_node, SetLatch(), SpinLockAcquire, SpinLockRelease, and ConditionVariable::wakeup.

Referenced by _bt_parallel_release(), _bt_parallel_scan_and_sort(), and ConditionVariableCancelSleep().

◆ ConditionVariableSleep()

void ConditionVariableSleep	(	ConditionVariable *	cv,
		uint32	wait_event_info
	)

Definition at line 98 of file condition_variable.c.

 {
     (void) ConditionVariableTimedSleep(cv, -1 /* no timeout */ ,
                                        wait_event_info);
 }

References ConditionVariableTimedSleep().

Referenced by _bt_parallel_heapscan(), _bt_parallel_seize(), BarrierArriveAndWait(), BitmapShouldInitializeSharedState(), InvalidatePossiblyObsoleteSlot(), ReplicationSlotAcquire(), replorigin_state_clear(), RequestCheckpoint(), ShutdownWalRcv(), and WaitIO().

◆ ConditionVariableTimedSleep()

bool ConditionVariableTimedSleep	(	ConditionVariable *	cv,
		long	timeout,
		uint32	wait_event_info
	)

Definition at line 112 of file condition_variable.c.

 {
     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;
         }
     }
 }