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Commit6f901b6

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Oops, only wanted datetime.c changes in there. lock stuff reversed out.
1 parent9e7b9c6 commit6f901b6

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1 file changed

+58
-63
lines changed

1 file changed

+58
-63
lines changed

‎src/backend/storage/lmgr/lwlock.c

Lines changed: 58 additions & 63 deletions
Original file line numberDiff line numberDiff line change
@@ -15,7 +15,7 @@
1515
* Portions Copyright (c) 1994, Regents of the University of California
1616
*
1717
* IDENTIFICATION
18-
* $Header: /cvsroot/pgsql/src/backend/storage/lmgr/lwlock.c,v 1.6 2001/12/29 21:28:18 momjian Exp $
18+
* $Header: /cvsroot/pgsql/src/backend/storage/lmgr/lwlock.c,v 1.7 2001/12/29 21:30:32 momjian Exp $
1919
*
2020
*-------------------------------------------------------------------------
2121
*/
@@ -195,8 +195,7 @@ void
195195
LWLockAcquire(LWLockIdlockid,LWLockModemode)
196196
{
197197
volatileLWLock*lock=LWLockArray+lockid;
198-
PROC*proc=MyProc;
199-
intextraWaits=0;
198+
boolmustwait;
200199

201200
PRINT_LWDEBUG("LWLockAcquire",lockid,lock);
202201

@@ -207,57 +206,43 @@ LWLockAcquire(LWLockId lockid, LWLockMode mode)
207206
*/
208207
HOLD_INTERRUPTS();
209208

210-
/*
211-
* Loop here to try to acquire lock after each time we are signaled
212-
* by LWLockRelease.
213-
*
214-
* NOTE: it might seem better to have LWLockRelease actually grant us
215-
* the lock, rather than retrying and possibly having to go back to
216-
* sleep. But in practice that is no good because it means a process
217-
* swap for every lock acquisition when two or more processes are
218-
* contending for the same lock. Since LWLocks are normally used to
219-
* protect not-very-long sections of computation, a process needs to
220-
* be able to acquire and release the same lock many times during a
221-
* single process dispatch cycle, even in the presence of contention.
222-
* The efficiency of being able to do that outweighs the inefficiency of
223-
* sometimes wasting a dispatch cycle because the lock is not free when a
224-
* released waiter gets to run. See pgsql-hackers archives for 29-Dec-01.
225-
*/
226-
for (;;)
227-
{
228-
boolmustwait;
229-
230-
/* Acquire mutex. Time spent holding mutex should be short! */
231-
SpinLockAcquire_NoHoldoff(&lock->mutex);
209+
/* Acquire mutex. Time spent holding mutex should be short! */
210+
SpinLockAcquire_NoHoldoff(&lock->mutex);
232211

233-
/* If I can get the lock, do so quickly. */
234-
if (mode==LW_EXCLUSIVE)
212+
/* If I can get the lock, do so quickly. */
213+
if (mode==LW_EXCLUSIVE)
214+
{
215+
if (lock->exclusive==0&&lock->shared==0)
235216
{
236-
if (lock->exclusive==0&&lock->shared==0)
237-
{
238-
lock->exclusive++;
239-
mustwait= false;
240-
}
241-
else
242-
mustwait= true;
217+
lock->exclusive++;
218+
mustwait= false;
243219
}
244220
else
221+
mustwait= true;
222+
}
223+
else
224+
{
225+
/*
226+
* If there is someone waiting (presumably for exclusive access),
227+
* queue up behind him even though I could get the lock. This
228+
* prevents a stream of read locks from starving a writer.
229+
*/
230+
if (lock->exclusive==0&&lock->head==NULL)
245231
{
246-
if (lock->exclusive==0)
247-
{
248-
lock->shared++;
249-
mustwait= false;
250-
}
251-
else
252-
mustwait= true;
232+
lock->shared++;
233+
mustwait= false;
253234
}
235+
else
236+
mustwait= true;
237+
}
254238

255-
if (!mustwait)
256-
break;/* got the lock */
239+
if (mustwait)
240+
{
241+
/* Add myself to wait queue */
242+
PROC*proc=MyProc;
243+
intextraWaits=0;
257244

258245
/*
259-
* Add myself to wait queue.
260-
*
261246
* If we don't have a PROC structure, there's no way to wait. This
262247
* should never occur, since MyProc should only be null during
263248
* shared memory initialization.
@@ -282,9 +267,9 @@ LWLockAcquire(LWLockId lockid, LWLockMode mode)
282267
*
283268
* Since we share the process wait semaphore with the regular lock
284269
* manager and ProcWaitForSignal, and we may need to acquire an
285-
* LWLock while one of those is pending, it is possible that we get
286-
* awakened for a reason other than beingsignaled by LWLockRelease.
287-
* If so, loop back and wait again. Once we've gotten theLWLock,
270+
* LWLock while one of those is pending, it is possible that we
271+
*getawakened for a reason other than beinggranted the LWLock.
272+
* If so, loop back and wait again. Once we've gotten thelock,
288273
* re-increment the sema by the number of additional signals
289274
* received, so that the lock manager or signal manager will see
290275
* the received signal when it next waits.
@@ -302,21 +287,23 @@ LWLockAcquire(LWLockId lockid, LWLockMode mode)
302287

303288
LOG_LWDEBUG("LWLockAcquire",lockid,"awakened");
304289

305-
/* Now loop back and try to acquire lock again. */
290+
/*
291+
* The awakener already updated the lock struct's state, so we
292+
* don't need to do anything more to it. Just need to fix the
293+
* semaphore count.
294+
*/
295+
while (extraWaits-->0)
296+
IpcSemaphoreUnlock(proc->sem.semId,proc->sem.semNum);
297+
}
298+
else
299+
{
300+
/* Got the lock without waiting */
301+
SpinLockRelease_NoHoldoff(&lock->mutex);
306302
}
307-
308-
/* We are done updating shared state of the lock itself. */
309-
SpinLockRelease_NoHoldoff(&lock->mutex);
310303

311304
/* Add lock to list of locks held by this backend */
312305
Assert(num_held_lwlocks<MAX_SIMUL_LWLOCKS);
313306
held_lwlocks[num_held_lwlocks++]=lockid;
314-
315-
/*
316-
* Fix the process wait semaphore's count for any absorbed wakeups.
317-
*/
318-
while (extraWaits-->0)
319-
IpcSemaphoreUnlock(proc->sem.semId,proc->sem.semNum);
320307
}
321308

322309
/*
@@ -357,7 +344,12 @@ LWLockConditionalAcquire(LWLockId lockid, LWLockMode mode)
357344
}
358345
else
359346
{
360-
if (lock->exclusive==0)
347+
/*
348+
* If there is someone waiting (presumably for exclusive access),
349+
* queue up behind him even though I could get the lock. This
350+
* prevents a stream of read locks from starving a writer.
351+
*/
352+
if (lock->exclusive==0&&lock->head==NULL)
361353
{
362354
lock->shared++;
363355
mustwait= false;
@@ -435,17 +427,20 @@ LWLockRelease(LWLockId lockid)
435427
if (lock->exclusive==0&&lock->shared==0)
436428
{
437429
/*
438-
* Remove the to-be-awakened PROCs from the queue. If the
439-
* front waiter wants exclusive lock, awaken him only.
440-
* Otherwise awaken as many waiters as want shared access.
430+
* Remove the to-be-awakened PROCs from the queue, and update
431+
* the lock state to show them as holding the lock.
441432
*/
442433
proc=head;
443-
if (!proc->lwExclusive)
434+
if (proc->lwExclusive)
435+
lock->exclusive++;
436+
else
444437
{
438+
lock->shared++;
445439
while (proc->lwWaitLink!=NULL&&
446440
!proc->lwWaitLink->lwExclusive)
447441
{
448442
proc=proc->lwWaitLink;
443+
lock->shared++;
449444
}
450445
}
451446
/* proc is now the last PROC to be released */

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