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condition_variable.c
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1 /*-------------------------------------------------------------------------
2  *
3  * condition_variable.c
4  * Implementation of condition variables. Condition variables provide
5  * a way for one process to wait until a specific condition occurs,
6  * without needing to know the specific identity of the process for
7  * which they are waiting. Waits for condition variables can be
8  * interrupted, unlike LWLock waits. Condition variables are safe
9  * to use within dynamic shared memory segments.
10  *
11  * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
12  * Portions Copyright (c) 1994, Regents of the University of California
13  *
14  * src/backend/storage/lmgr/condition_variable.c
15  *
16  *-------------------------------------------------------------------------
17  */
18 
19 #include "postgres.h"
20 
21 #include "miscadmin.h"
22 #include "portability/instr_time.h"
24 #include "storage/ipc.h"
25 #include "storage/proc.h"
26 #include "storage/proclist.h"
27 #include "storage/spin.h"
28 #include "utils/memutils.h"
29 
30 /* Initially, we are not prepared to sleep on any condition variable. */
32 
33 /*
34  * Initialize a condition variable.
35  */
36 void
38 {
39  SpinLockInit(&cv->mutex);
40  proclist_init(&cv->wakeup);
41 }
42 
43 /*
44  * Prepare to wait on a given condition variable.
45  *
46  * This can optionally be called before entering a test/sleep loop.
47  * Doing so is more efficient if we'll need to sleep at least once.
48  * However, if the first test of the exit condition is likely to succeed,
49  * it's more efficient to omit the ConditionVariablePrepareToSleep call.
50  * See comments in ConditionVariableSleep for more detail.
51  *
52  * Caution: "before entering the loop" means you *must* test the exit
53  * condition between calling ConditionVariablePrepareToSleep and calling
54  * ConditionVariableSleep. If that is inconvenient, omit calling
55  * ConditionVariablePrepareToSleep.
56  */
57 void
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 }
81 
82 /*
83  * Wait for the given condition variable to be signaled.
84  *
85  * This should be called in a predicate loop that tests for a specific exit
86  * condition and otherwise sleeps, like so:
87  *
88  * ConditionVariablePrepareToSleep(cv); // optional
89  * while (condition for which we are waiting is not true)
90  * ConditionVariableSleep(cv, wait_event_info);
91  * ConditionVariableCancelSleep();
92  *
93  * wait_event_info should be a value from one of the WaitEventXXX enums
94  * defined in pgstat.h. This controls the contents of pg_stat_activity's
95  * wait_event_type and wait_event columns while waiting.
96  */
97 void
99 {
100  (void) ConditionVariableTimedSleep(cv, -1 /* no timeout */ ,
101  wait_event_info);
102 }
103 
104 /*
105  * Wait for a condition variable to be signaled or a timeout to be reached.
106  *
107  * Returns true when timeout expires, otherwise returns false.
108  *
109  * See ConditionVariableSleep() for general usage.
110  */
111 bool
113  uint32 wait_event_info)
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  {
147  INSTR_TIME_SET_CURRENT(start_time);
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 }
217 
218 /*
219  * Cancel any pending sleep operation.
220  *
221  * We just need to remove ourselves from the wait queue of any condition
222  * variable for which we have previously prepared a sleep.
223  *
224  * Do nothing if nothing is pending; this allows this function to be called
225  * during transaction abort to clean up any unfinished CV sleep.
226  */
227 void
229 {
231  bool signaled = false;
232 
233  if (cv == NULL)
234  return;
235 
236  SpinLockAcquire(&cv->mutex);
237  if (proclist_contains(&cv->wakeup, MyProc->pgprocno, cvWaitLink))
238  proclist_delete(&cv->wakeup, MyProc->pgprocno, cvWaitLink);
239  else
240  signaled = true;
241  SpinLockRelease(&cv->mutex);
242 
243  /*
244  * If we've received a signal, pass it on to another waiting process, if
245  * there is one. Otherwise a call to ConditionVariableSignal() might get
246  * lost, despite there being another process ready to handle it.
247  */
248  if (signaled)
250 
251  cv_sleep_target = NULL;
252 }
253 
254 /*
255  * Wake up the oldest process sleeping on the CV, if there is any.
256  *
257  * Note: it's difficult to tell whether this has any real effect: we know
258  * whether we took an entry off the list, but the entry might only be a
259  * sentinel. Hence, think twice before proposing that this should return
260  * a flag telling whether it woke somebody.
261  */
262 void
264 {
265  PGPROC *proc = NULL;
266 
267  /* Remove the first process from the wakeup queue (if any). */
268  SpinLockAcquire(&cv->mutex);
269  if (!proclist_is_empty(&cv->wakeup))
270  proc = proclist_pop_head_node(&cv->wakeup, cvWaitLink);
271  SpinLockRelease(&cv->mutex);
272 
273  /* If we found someone sleeping, set their latch to wake them up. */
274  if (proc != NULL)
275  SetLatch(&proc->procLatch);
276 }
277 
278 /*
279  * Wake up all processes sleeping on the given CV.
280  *
281  * This guarantees to wake all processes that were sleeping on the CV
282  * at time of call, but processes that add themselves to the list mid-call
283  * will typically not get awakened.
284  */
285 void
287 {
288  int pgprocno = MyProc->pgprocno;
289  PGPROC *proc = NULL;
290  bool have_sentinel = false;
291 
292  /*
293  * In some use-cases, it is common for awakened processes to immediately
294  * re-queue themselves. If we just naively try to reduce the wakeup list
295  * to empty, we'll get into a potentially-indefinite loop against such a
296  * process. The semantics we really want are just to be sure that we have
297  * wakened all processes that were in the list at entry. We can use our
298  * own cvWaitLink as a sentinel to detect when we've finished.
299  *
300  * A seeming flaw in this approach is that someone else might signal the
301  * CV and in doing so remove our sentinel entry. But that's fine: since
302  * CV waiters are always added and removed in order, that must mean that
303  * every previous waiter has been wakened, so we're done. We'll get an
304  * extra "set" on our latch from the someone else's signal, which is
305  * slightly inefficient but harmless.
306  *
307  * We can't insert our cvWaitLink as a sentinel if it's already in use in
308  * some other proclist. While that's not expected to be true for typical
309  * uses of this function, we can deal with it by simply canceling any
310  * prepared CV sleep. The next call to ConditionVariableSleep will take
311  * care of re-establishing the lost state.
312  */
313  if (cv_sleep_target != NULL)
315 
316  /*
317  * Inspect the state of the queue. If it's empty, we have nothing to do.
318  * If there's exactly one entry, we need only remove and signal that
319  * entry. Otherwise, remove the first entry and insert our sentinel.
320  */
321  SpinLockAcquire(&cv->mutex);
322  /* While we're here, let's assert we're not in the list. */
323  Assert(!proclist_contains(&cv->wakeup, pgprocno, cvWaitLink));
324 
325  if (!proclist_is_empty(&cv->wakeup))
326  {
327  proc = proclist_pop_head_node(&cv->wakeup, cvWaitLink);
328  if (!proclist_is_empty(&cv->wakeup))
329  {
330  proclist_push_tail(&cv->wakeup, pgprocno, cvWaitLink);
331  have_sentinel = true;
332  }
333  }
334  SpinLockRelease(&cv->mutex);
335 
336  /* Awaken first waiter, if there was one. */
337  if (proc != NULL)
338  SetLatch(&proc->procLatch);
339 
340  while (have_sentinel)
341  {
342  /*
343  * Each time through the loop, remove the first wakeup list entry, and
344  * signal it unless it's our sentinel. Repeat as long as the sentinel
345  * remains in the list.
346  *
347  * Notice that if someone else removes our sentinel, we will waken one
348  * additional process before exiting. That's intentional, because if
349  * someone else signals the CV, they may be intending to waken some
350  * third process that added itself to the list after we added the
351  * sentinel. Better to give a spurious wakeup (which should be
352  * harmless beyond wasting some cycles) than to lose a wakeup.
353  */
354  proc = NULL;
355  SpinLockAcquire(&cv->mutex);
356  if (!proclist_is_empty(&cv->wakeup))
357  proc = proclist_pop_head_node(&cv->wakeup, cvWaitLink);
358  have_sentinel = proclist_contains(&cv->wakeup, pgprocno, cvWaitLink);
359  SpinLockRelease(&cv->mutex);
360 
361  if (proc != NULL && proc != MyProc)
362  SetLatch(&proc->procLatch);
363  }
364 }
#define WL_TIMEOUT
Definition: latch.h:128
proclist_head wakeup
PGPROC * MyProc
Definition: proc.c:68
#define SpinLockInit(lock)
Definition: spin.h:60
#define INSTR_TIME_GET_MILLISEC(t)
Definition: instr_time.h:202
struct timeval instr_time
Definition: instr_time.h:150
void ConditionVariableBroadcast(ConditionVariable *cv)
void SetLatch(Latch *latch)
Definition: latch.c:567
#define proclist_delete(list, procno, link_member)
Definition: proclist.h:187
void ResetLatch(Latch *latch)
Definition: latch.c:660
Latch procLatch
Definition: proc.h:130
static time_t start_time
Definition: pg_ctl.c:99
int WaitLatch(Latch *latch, int wakeEvents, long timeout, uint32 wait_event_info)
Definition: latch.c:452
void ConditionVariablePrepareToSleep(ConditionVariable *cv)
#define SpinLockAcquire(lock)
Definition: spin.h:62
void ConditionVariableInit(ConditionVariable *cv)
void ConditionVariableCancelSleep(void)
bool ConditionVariableTimedSleep(ConditionVariable *cv, long timeout, uint32 wait_event_info)
#define INSTR_TIME_SUBTRACT(x, y)
Definition: instr_time.h:170
void ConditionVariableSignal(ConditionVariable *cv)
unsigned int uint32
Definition: c.h:441
#define SpinLockRelease(lock)
Definition: spin.h:64
static ConditionVariable * cv_sleep_target
#define proclist_pop_head_node(list, link_member)
Definition: proclist.h:193
void ConditionVariableSleep(ConditionVariable *cv, uint32 wait_event_info)
#define Assert(condition)
Definition: c.h:804
static bool proclist_is_empty(proclist_head *list)
Definition: proclist.h:38
#define INSTR_TIME_SET_CURRENT(t)
Definition: instr_time.h:156
#define proclist_push_tail(list, procno, link_member)
Definition: proclist.h:191
int pgprocno
Definition: proc.h:150
static void proclist_init(proclist_head *list)
Definition: proclist.h:29
struct Latch * MyLatch
Definition: globals.c:57
#define CHECK_FOR_INTERRUPTS()
Definition: miscadmin.h:102
Definition: proc.h:121
#define WL_LATCH_SET
Definition: latch.h:125
#define WL_EXIT_ON_PM_DEATH
Definition: latch.h:130
#define proclist_contains(list, procno, link_member)
Definition: proclist.h:195