PostgreSQL Source Code  git master
condition_variable.c File Reference
#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:

Go to the source code of this file.

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)
 
bool ConditionVariableCancelSleep (void)
 
void ConditionVariableSignal (ConditionVariable *cv)
 
void ConditionVariableBroadcast (ConditionVariable *cv)
 

Variables

static ConditionVariablecv_sleep_target = NULL
 

Function Documentation

◆ ConditionVariableBroadcast()

void ConditionVariableBroadcast ( ConditionVariable cv)

Definition at line 282 of file condition_variable.c.

283 {
284  int pgprocno = MyProc->pgprocno;
285  PGPROC *proc = NULL;
286  bool have_sentinel = false;
287 
288  /*
289  * In some use-cases, it is common for awakened processes to immediately
290  * re-queue themselves. If we just naively try to reduce the wakeup list
291  * to empty, we'll get into a potentially-indefinite loop against such a
292  * process. The semantics we really want are just to be sure that we have
293  * wakened all processes that were in the list at entry. We can use our
294  * own cvWaitLink as a sentinel to detect when we've finished.
295  *
296  * A seeming flaw in this approach is that someone else might signal the
297  * CV and in doing so remove our sentinel entry. But that's fine: since
298  * CV waiters are always added and removed in order, that must mean that
299  * every previous waiter has been wakened, so we're done. We'll get an
300  * extra "set" on our latch from the someone else's signal, which is
301  * slightly inefficient but harmless.
302  *
303  * We can't insert our cvWaitLink as a sentinel if it's already in use in
304  * some other proclist. While that's not expected to be true for typical
305  * uses of this function, we can deal with it by simply canceling any
306  * prepared CV sleep. The next call to ConditionVariableSleep will take
307  * care of re-establishing the lost state.
308  */
309  if (cv_sleep_target != NULL)
311 
312  /*
313  * Inspect the state of the queue. If it's empty, we have nothing to do.
314  * If there's exactly one entry, we need only remove and signal that
315  * entry. Otherwise, remove the first entry and insert our sentinel.
316  */
317  SpinLockAcquire(&cv->mutex);
318  /* While we're here, let's assert we're not in the list. */
319  Assert(!proclist_contains(&cv->wakeup, pgprocno, cvWaitLink));
320 
321  if (!proclist_is_empty(&cv->wakeup))
322  {
323  proc = proclist_pop_head_node(&cv->wakeup, cvWaitLink);
324  if (!proclist_is_empty(&cv->wakeup))
325  {
326  proclist_push_tail(&cv->wakeup, pgprocno, cvWaitLink);
327  have_sentinel = true;
328  }
329  }
330  SpinLockRelease(&cv->mutex);
331 
332  /* Awaken first waiter, if there was one. */
333  if (proc != NULL)
334  SetLatch(&proc->procLatch);
335 
336  while (have_sentinel)
337  {
338  /*
339  * Each time through the loop, remove the first wakeup list entry, and
340  * signal it unless it's our sentinel. Repeat as long as the sentinel
341  * remains in the list.
342  *
343  * Notice that if someone else removes our sentinel, we will waken one
344  * additional process before exiting. That's intentional, because if
345  * someone else signals the CV, they may be intending to waken some
346  * third process that added itself to the list after we added the
347  * sentinel. Better to give a spurious wakeup (which should be
348  * harmless beyond wasting some cycles) than to lose a wakeup.
349  */
350  proc = NULL;
351  SpinLockAcquire(&cv->mutex);
352  if (!proclist_is_empty(&cv->wakeup))
353  proc = proclist_pop_head_node(&cv->wakeup, cvWaitLink);
354  have_sentinel = proclist_contains(&cv->wakeup, pgprocno, cvWaitLink);
355  SpinLockRelease(&cv->mutex);
356 
357  if (proc != NULL && proc != MyProc)
358  SetLatch(&proc->procLatch);
359  }
360 }
bool ConditionVariableCancelSleep(void)
static ConditionVariable * cv_sleep_target
void SetLatch(Latch *latch)
Definition: latch.c:605
Assert(fmt[strlen(fmt) - 1] !='\n')
#define proclist_pop_head_node(list, link_member)
Definition: proclist.h:193
#define proclist_push_tail(list, procno, link_member)
Definition: proclist.h:191
#define proclist_contains(list, procno, link_member)
Definition: proclist.h:195
static bool proclist_is_empty(const proclist_head *list)
Definition: proclist.h:38
#define SpinLockRelease(lock)
Definition: spin.h:64
#define SpinLockAcquire(lock)
Definition: spin.h:62
PGPROC * MyProc
Definition: proc.c:66
proclist_head wakeup
Definition: proc.h:162
int pgprocno
Definition: proc.h:191
Latch procLatch
Definition: proc.h:170

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()

◆ ConditionVariableInit()

◆ ConditionVariablePrepareToSleep()

void ConditionVariablePrepareToSleep ( ConditionVariable cv)

Definition at line 58 of file condition_variable.c.

59 {
60  int pgprocno = MyProc->pgprocno;
61 
62  /*
63  * If some other sleep is already prepared, cancel it; this is necessary
64  * because we have just one static variable tracking the prepared sleep,
65  * and also only one cvWaitLink in our PGPROC. It's okay to do this
66  * because whenever control does return to the other test-and-sleep loop,
67  * its ConditionVariableSleep call will just re-establish that sleep as
68  * the prepared one.
69  */
70  if (cv_sleep_target != NULL)
72 
73  /* Record the condition variable on which we will sleep. */
74  cv_sleep_target = cv;
75 
76  /* Add myself to the wait queue. */
77  SpinLockAcquire(&cv->mutex);
78  proclist_push_tail(&cv->wakeup, pgprocno, cvWaitLink);
79  SpinLockRelease(&cv->mutex);
80 }

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 259 of file condition_variable.c.

260 {
261  PGPROC *proc = NULL;
262 
263  /* Remove the first process from the wakeup queue (if any). */
264  SpinLockAcquire(&cv->mutex);
265  if (!proclist_is_empty(&cv->wakeup))
266  proc = proclist_pop_head_node(&cv->wakeup, cvWaitLink);
267  SpinLockRelease(&cv->mutex);
268 
269  /* If we found someone sleeping, set their latch to wake them up. */
270  if (proc != NULL)
271  SetLatch(&proc->procLatch);
272 }

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

Referenced by _bt_parallel_release(), and _bt_parallel_scan_and_sort().

◆ ConditionVariableSleep()

void ConditionVariableSleep ( ConditionVariable cv,
uint32  wait_event_info 
)

Definition at line 98 of file condition_variable.c.

99 {
100  (void) ConditionVariableTimedSleep(cv, -1 /* no timeout */ ,
101  wait_event_info);
102 }
bool ConditionVariableTimedSleep(ConditionVariable *cv, long timeout, uint32 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.

114 {
115  long cur_timeout = -1;
117  instr_time cur_time;
118  int wait_events;
119 
120  /*
121  * If the caller didn't prepare to sleep explicitly, then do so now and
122  * return immediately. The caller's predicate loop should immediately
123  * call again if its exit condition is not yet met. This will result in
124  * the exit condition being tested twice before we first sleep. The extra
125  * test can be prevented by calling ConditionVariablePrepareToSleep(cv)
126  * first. Whether it's worth doing that depends on whether you expect the
127  * exit condition to be met initially, in which case skipping the prepare
128  * is recommended because it avoids manipulations of the wait list, or not
129  * met initially, in which case preparing first is better because it
130  * avoids one extra test of the exit condition.
131  *
132  * If we are currently prepared to sleep on some other CV, we just cancel
133  * that and prepare this one; see ConditionVariablePrepareToSleep.
134  */
135  if (cv_sleep_target != cv)
136  {
138  return false;
139  }
140 
141  /*
142  * Record the current time so that we can calculate the remaining timeout
143  * if we are woken up spuriously.
144  */
145  if (timeout >= 0)
146  {
148  Assert(timeout >= 0 && timeout <= INT_MAX);
149  cur_timeout = timeout;
150  wait_events = WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH;
151  }
152  else
153  wait_events = WL_LATCH_SET | WL_EXIT_ON_PM_DEATH;
154 
155  while (true)
156  {
157  bool done = false;
158 
159  /*
160  * Wait for latch to be set. (If we're awakened for some other
161  * reason, the code below will cope anyway.)
162  */
163  (void) WaitLatch(MyLatch, wait_events, cur_timeout, wait_event_info);
164 
165  /* Reset latch before examining the state of the wait list. */
167 
168  /*
169  * If this process has been taken out of the wait list, then we know
170  * that it has been signaled by ConditionVariableSignal (or
171  * ConditionVariableBroadcast), so we should return to the caller. But
172  * that doesn't guarantee that the exit condition is met, only that we
173  * ought to check it. So we must put the process back into the wait
174  * list, to ensure we don't miss any additional wakeup occurring while
175  * the caller checks its exit condition. We can take ourselves out of
176  * the wait list only when the caller calls
177  * ConditionVariableCancelSleep.
178  *
179  * If we're still in the wait list, then the latch must have been set
180  * by something other than ConditionVariableSignal; though we don't
181  * guarantee not to return spuriously, we'll avoid this obvious case.
182  */
183  SpinLockAcquire(&cv->mutex);
184  if (!proclist_contains(&cv->wakeup, MyProc->pgprocno, cvWaitLink))
185  {
186  done = true;
187  proclist_push_tail(&cv->wakeup, MyProc->pgprocno, cvWaitLink);
188  }
189  SpinLockRelease(&cv->mutex);
190 
191  /*
192  * Check for interrupts, and return spuriously if that caused the
193  * current sleep target to change (meaning that interrupt handler code
194  * waited for a different condition variable).
195  */
197  if (cv != cv_sleep_target)
198  done = true;
199 
200  /* We were signaled, so return */
201  if (done)
202  return false;
203 
204  /* If we're not done, update cur_timeout for next iteration */
205  if (timeout >= 0)
206  {
207  INSTR_TIME_SET_CURRENT(cur_time);
208  INSTR_TIME_SUBTRACT(cur_time, start_time);
209  cur_timeout = timeout - (long) INSTR_TIME_GET_MILLISEC(cur_time);
210 
211  /* Have we crossed the timeout threshold? */
212  if (cur_timeout <= 0)
213  return true;
214  }
215  }
216 }
void ConditionVariablePrepareToSleep(ConditionVariable *cv)
struct Latch * MyLatch
Definition: globals.c:58
#define INSTR_TIME_SET_CURRENT(t)
Definition: instr_time.h:122
#define INSTR_TIME_SUBTRACT(x, y)
Definition: instr_time.h:181
#define INSTR_TIME_GET_MILLISEC(t)
Definition: instr_time.h:191
void ResetLatch(Latch *latch)
Definition: latch.c:697
int WaitLatch(Latch *latch, int wakeEvents, long timeout, uint32 wait_event_info)
Definition: latch.c:490
#define WL_TIMEOUT
Definition: latch.h:128
#define WL_EXIT_ON_PM_DEATH
Definition: latch.h:130
#define WL_LATCH_SET
Definition: latch.h:125
#define CHECK_FOR_INTERRUPTS()
Definition: miscadmin.h:121
static time_t start_time
Definition: pg_ctl.c:94

References Assert(), CHECK_FOR_INTERRUPTS, ConditionVariablePrepareToSleep(), cv_sleep_target, INSTR_TIME_GET_MILLISEC, INSTR_TIME_SET_CURRENT, INSTR_TIME_SUBTRACT, ConditionVariable::mutex, MyLatch, MyProc, PGPROC::pgprocno, proclist_contains, proclist_push_tail, ResetLatch(), SpinLockAcquire, SpinLockRelease, start_time, WaitLatch(), ConditionVariable::wakeup, WL_EXIT_ON_PM_DEATH, WL_LATCH_SET, and WL_TIMEOUT.

Referenced by ConditionVariableSleep(), recoveryPausesHere(), RecoveryRequiresIntParameter(), and WaitForProcSignalBarrier().

Variable Documentation

◆ cv_sleep_target