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Commitbe7b284

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Make the different Unix-y semaphore implementations ABI-compatible.
Previously, the "sem" field of PGPROC varied in size depending on whichkernel semaphore API we were using. That was okay as long as there wasonly one likely choice per platform, but in the wake of commitecb0d20,that assumption seems rather shaky. It doesn't seem out of the questionanymore that an extension compiled against one API choice might be loadedinto a postmaster built with another choice. Moreover, this prevents anypossibility of selecting the semaphore API at postmaster startup, whichmight be something we want to do in future.Hence, change PGPROC.sem to be PGSemaphore (i.e. a pointer) for all Unixsemaphore APIs, and turn the pointed-to data into an opaque struct whosecontents are only known within the responsible modules.For the SysV and unnamed-POSIX APIs, the pointed-to data has to beallocated elsewhere in shared memory, which takes a little bit ofrejiggering of the InitShmemAllocation code sequence. (I invented aShmemAllocUnlocked() function to make that a little cleaner than it usedto be. That function is not meant for any uses other than the ones ithas now, but it beats having InitShmemAllocation() know explicitly aboutallocation of space for semaphores and spinlocks.) This change means anextra indirection to access the semaphore data, but since we only touchthat when blocking or awakening a process, there shouldn't be anymeaningful performance penalty. Moreover, at least for the unnamed-POSIXcase on Linux, the sem_t type is quite a bit wider than a pointer, so thisreduces sizeof(PGPROC) which seems like a good thing.For the named-POSIX API, there's effectively no change: the PGPROC.semfield was and still is a pointer to something returned by sem_open() inthe postmaster's memory space. Document and check the pre-existinglimitation that this case can't work in EXEC_BACKEND mode.It did not seem worth unifying the Windows semaphore ABI with the Unixcases, since there's no likelihood of needing ABI compatibility much lessruntime switching across those cases. However, we can simplify the Windowscode a bit if we define PGSemaphore as being directly a HANDLE, rather thanpointer to HANDLE, so let's do that while we're here. (This also ends upbeing no change in what's physically stored in PGPROC.sem. We're justmoving the HANDLE fetch from callees to callers.)It would take a bunch of additional code shuffling to get to the point ofactually choosing a semaphore API at postmaster start, but the effectsof that would now be localized in the port/XXX_sema.c files, so it seemslike fit material for a separate patch. The need for it is unproven asyet, anyhow, whereas the ABI risk to extensions seems real enough.Discussion:https://postgr.es/m/4029.1481413370@sss.pgh.pa.us
1 parent06e1848 commitbe7b284

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14 files changed

+268
-123
lines changed

14 files changed

+268
-123
lines changed

‎src/backend/port/posix_sema.c

Lines changed: 81 additions & 17 deletions
Original file line numberDiff line numberDiff line change
@@ -6,6 +6,14 @@
66
* We prefer the unnamed style of POSIX semaphore (the kind made with
77
* sem_init). We can cope with the kind made with sem_open, however.
88
*
9+
* In either implementation, typedef PGSemaphore is equivalent to "sem_t *".
10+
* With unnamed semaphores, the sem_t structs live in an array in shared
11+
* memory. With named semaphores, that's not true because we cannot persuade
12+
* sem_open to do its allocation there. Therefore, the named-semaphore code
13+
* *does not cope with EXEC_BACKEND*. The sem_t structs will just be in the
14+
* postmaster's private memory, where they are successfully inherited by
15+
* forked backends, but they could not be accessed by exec'd backends.
16+
*
917
*
1018
* Portions Copyright (c) 1996-2016, PostgreSQL Global Development Group
1119
* Portions Copyright (c) 1994, Regents of the University of California
@@ -18,28 +26,38 @@
1826
#include"postgres.h"
1927

2028
#include<fcntl.h>
29+
#include<semaphore.h>
2130
#include<signal.h>
2231
#include<unistd.h>
2332

2433
#include"miscadmin.h"
2534
#include"storage/ipc.h"
2635
#include"storage/pg_sema.h"
36+
#include"storage/shmem.h"
2737

2838

29-
#ifdefUSE_NAMED_POSIX_SEMAPHORES
30-
/* PGSemaphore is pointer to pointer to sem_t */
31-
#definePG_SEM_REF(x)(*(x))
32-
#else
33-
/* PGSemaphore is pointer to sem_t */
34-
#definePG_SEM_REF(x)(x)
39+
/* see file header comment */
40+
#if defined(USE_NAMED_POSIX_SEMAPHORES)&& defined(EXEC_BACKEND)
41+
#error cannot use named POSIX semaphores with EXEC_BACKEND
3542
#endif
3643

44+
/* typedef PGSemaphore is equivalent to pointer to sem_t */
45+
typedefstructPGSemaphoreData
46+
{
47+
sem_tpgsem;
48+
}PGSemaphoreData;
49+
50+
#definePG_SEM_REF(x)(&(x)->pgsem)
3751

3852
#defineIPCProtection(0600)/* access/modify by user only */
3953

54+
#ifdefUSE_NAMED_POSIX_SEMAPHORES
4055
staticsem_t**mySemPointers;/* keep track of created semaphores */
56+
#else
57+
staticPGSemaphoresharedSemas;/* array of PGSemaphoreData in shared memory */
58+
#endif
4159
staticintnumSems;/* number of semas acquired so far */
42-
staticintmaxSems;/* allocated size ofmySemaPointers array */
60+
staticintmaxSems;/* allocated size ofabove arrays */
4361
staticintnextSemKey;/* next name to try */
4462

4563

@@ -133,6 +151,21 @@ PosixSemaphoreKill(sem_t * sem)
133151
}
134152

135153

154+
/*
155+
* Report amount of shared memory needed for semaphores
156+
*/
157+
Size
158+
PGSemaphoreShmemSize(intmaxSemas)
159+
{
160+
#ifdefUSE_NAMED_POSIX_SEMAPHORES
161+
/* No shared memory needed in this case */
162+
return0;
163+
#else
164+
/* Need a PGSemaphoreData per semaphore */
165+
returnmul_size(maxSemas,sizeof(PGSemaphoreData));
166+
#endif
167+
}
168+
136169
/*
137170
* PGReserveSemaphores --- initialize semaphore support
138171
*
@@ -147,15 +180,33 @@ PosixSemaphoreKill(sem_t * sem)
147180
* zero will be passed.
148181
*
149182
* In the Posix implementation, we acquire semaphores on-demand; the
150-
* maxSemas parameter is just used to size the array that keeps track of
151-
* acquired semas for subsequent releasing.
183+
* maxSemas parameter is just used to size the arrays. For unnamed
184+
* semaphores, there is an array of PGSemaphoreData structs in shared memory.
185+
* For named semaphores, we keep a postmaster-local array of sem_t pointers,
186+
* which we use for releasing the semphores when done.
187+
* (This design minimizes the dependency of postmaster shutdown on the
188+
* contents of shared memory, which a failed backend might have clobbered.
189+
* We can't do much about the possibility of sem_destroy() crashing, but
190+
* we don't have to expose the counters to other processes.)
152191
*/
153192
void
154193
PGReserveSemaphores(intmaxSemas,intport)
155194
{
195+
#ifdefUSE_NAMED_POSIX_SEMAPHORES
156196
mySemPointers= (sem_t**)malloc(maxSemas*sizeof(sem_t*));
157197
if (mySemPointers==NULL)
158198
elog(PANIC,"out of memory");
199+
#else
200+
201+
/*
202+
* We must use ShmemAllocUnlocked(), since the spinlock protecting
203+
* ShmemAlloc() won't be ready yet. (This ordering is necessary when we
204+
* are emulating spinlocks with semaphores.)
205+
*/
206+
sharedSemas= (PGSemaphore)
207+
ShmemAllocUnlocked(PGSemaphoreShmemSize(maxSemas));
208+
#endif
209+
159210
numSems=0;
160211
maxSems=maxSemas;
161212
nextSemKey=port*1000;
@@ -173,19 +224,27 @@ ReleaseSemaphores(int status, Datum arg)
173224
{
174225
inti;
175226

227+
#ifdefUSE_NAMED_POSIX_SEMAPHORES
176228
for (i=0;i<numSems;i++)
177229
PosixSemaphoreKill(mySemPointers[i]);
178230
free(mySemPointers);
231+
#endif
232+
233+
#ifdefUSE_UNNAMED_POSIX_SEMAPHORES
234+
for (i=0;i<numSems;i++)
235+
PosixSemaphoreKill(PG_SEM_REF(sharedSemas+i));
236+
#endif
179237
}
180238

181239
/*
182240
* PGSemaphoreCreate
183241
*
184-
*Initialize a PGSemaphore structureto represent a semawith count 1
242+
*Allocate a PGSemaphore structure with initial count 1
185243
*/
186-
void
187-
PGSemaphoreCreate(PGSemaphoresema)
244+
PGSemaphore
245+
PGSemaphoreCreate(void)
188246
{
247+
PGSemaphoresema;
189248
sem_t*newsem;
190249

191250
/* Can't do this in a backend, because static state is postmaster's */
@@ -195,14 +254,19 @@ PGSemaphoreCreate(PGSemaphore sema)
195254
elog(PANIC,"too many semaphores created");
196255

197256
#ifdefUSE_NAMED_POSIX_SEMAPHORES
198-
*sema=newsem=PosixSemaphoreCreate();
257+
newsem=PosixSemaphoreCreate();
258+
/* Remember new sema for ReleaseSemaphores */
259+
mySemPointers[numSems]=newsem;
260+
sema= (PGSemaphore)newsem;
199261
#else
200-
PosixSemaphoreCreate(sema);
201-
newsem=sema;
262+
sema=&sharedSemas[numSems];
263+
newsem=PG_SEM_REF(sema);
264+
PosixSemaphoreCreate(newsem);
202265
#endif
203266

204-
/* Remember new sema for ReleaseSemaphores */
205-
mySemPointers[numSems++]=newsem;
267+
numSems++;
268+
269+
returnsema;
206270
}
207271

208272
/*

‎src/backend/port/sysv_sema.c

Lines changed: 46 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -27,8 +27,15 @@
2727
#include"miscadmin.h"
2828
#include"storage/ipc.h"
2929
#include"storage/pg_sema.h"
30+
#include"storage/shmem.h"
3031

3132

33+
typedefstructPGSemaphoreData
34+
{
35+
intsemId;/* semaphore set identifier */
36+
intsemNum;/* semaphore number within set */
37+
}PGSemaphoreData;
38+
3239
#ifndefHAVE_UNION_SEMUN
3340
unionsemun
3441
{
@@ -54,6 +61,9 @@ typedef int IpcSemaphoreId;/* semaphore ID returned by semget(2) */
5461
#definePGSemaMagic537/* must be less than SEMVMX */
5562

5663

64+
staticPGSemaphoresharedSemas;/* array of PGSemaphoreData in shared memory */
65+
staticintnumSharedSemas;/* number of PGSemaphoreDatas used so far */
66+
staticintmaxSharedSemas;/* allocated size of PGSemaphoreData array */
5767
staticIpcSemaphoreId*mySemaSets;/* IDs of sema sets acquired so far */
5868
staticintnumSemaSets;/* number of sema sets acquired so far */
5969
staticintmaxSemaSets;/* allocated size of mySemaSets array */
@@ -273,6 +283,15 @@ IpcSemaphoreCreate(int numSems)
273283
}
274284

275285

286+
/*
287+
* Report amount of shared memory needed for semaphores
288+
*/
289+
Size
290+
PGSemaphoreShmemSize(intmaxSemas)
291+
{
292+
returnmul_size(maxSemas,sizeof(PGSemaphoreData));
293+
}
294+
276295
/*
277296
* PGReserveSemaphores --- initialize semaphore support
278297
*
@@ -287,12 +306,26 @@ IpcSemaphoreCreate(int numSems)
287306
* zero will be passed.
288307
*
289308
* In the SysV implementation, we acquire semaphore sets on-demand; the
290-
* maxSemas parameter is just used to size the array that keeps track of
291-
* acquired sets for subsequent releasing.
309+
* maxSemas parameter is just used to size the arrays. There is an array
310+
* of PGSemaphoreData structs in shared memory, and a postmaster-local array
311+
* with one entry per SysV semaphore set, which we use for releasing the
312+
* semaphore sets when done. (This design ensures that postmaster shutdown
313+
* doesn't rely on the contents of shared memory, which a failed backend might
314+
* have clobbered.)
292315
*/
293316
void
294317
PGReserveSemaphores(intmaxSemas,intport)
295318
{
319+
/*
320+
* We must use ShmemAllocUnlocked(), since the spinlock protecting
321+
* ShmemAlloc() won't be ready yet. (This ordering is necessary when we
322+
* are emulating spinlocks with semaphores.)
323+
*/
324+
sharedSemas= (PGSemaphore)
325+
ShmemAllocUnlocked(PGSemaphoreShmemSize(maxSemas));
326+
numSharedSemas=0;
327+
maxSharedSemas=maxSemas;
328+
296329
maxSemaSets= (maxSemas+SEMAS_PER_SET-1) /SEMAS_PER_SET;
297330
mySemaSets= (IpcSemaphoreId*)
298331
malloc(maxSemaSets*sizeof(IpcSemaphoreId));
@@ -323,11 +356,13 @@ ReleaseSemaphores(int status, Datum arg)
323356
/*
324357
* PGSemaphoreCreate
325358
*
326-
*Initialize a PGSemaphore structureto represent a semawith count 1
359+
*Allocate a PGSemaphore structure with initial count 1
327360
*/
328-
void
329-
PGSemaphoreCreate(PGSemaphoresema)
361+
PGSemaphore
362+
PGSemaphoreCreate(void)
330363
{
364+
PGSemaphoresema;
365+
331366
/* Can't do this in a backend, because static state is postmaster's */
332367
Assert(!IsUnderPostmaster);
333368

@@ -340,11 +375,17 @@ PGSemaphoreCreate(PGSemaphore sema)
340375
numSemaSets++;
341376
nextSemaNumber=0;
342377
}
378+
/* Use the next shared PGSemaphoreData */
379+
if (numSharedSemas >=maxSharedSemas)
380+
elog(PANIC,"too many semaphores created");
381+
sema=&sharedSemas[numSharedSemas++];
343382
/* Assign the next free semaphore in the current set */
344383
sema->semId=mySemaSets[numSemaSets-1];
345384
sema->semNum=nextSemaNumber++;
346385
/* Initialize it to count 1 */
347386
IpcSemaphoreInitialize(sema->semId,sema->semNum,1);
387+
388+
returnsema;
348389
}
349390

350391
/*

‎src/backend/port/win32_sema.c

Lines changed: 27 additions & 11 deletions
Original file line numberDiff line numberDiff line change
@@ -23,6 +23,17 @@ static intmaxSems;/* allocated size of mySemaSet array */
2323

2424
staticvoidReleaseSemaphores(intcode,Datumarg);
2525

26+
27+
/*
28+
* Report amount of shared memory needed for semaphores
29+
*/
30+
Size
31+
PGSemaphoreShmemSize(intmaxSemas)
32+
{
33+
/* No shared memory needed on Windows */
34+
return0;
35+
}
36+
2637
/*
2738
* PGReserveSemaphores --- initialize semaphore support
2839
*
@@ -62,10 +73,10 @@ ReleaseSemaphores(int code, Datum arg)
6273
/*
6374
* PGSemaphoreCreate
6475
*
65-
*Initialize a PGSemaphore structureto represent a semawith count 1
76+
*Allocate a PGSemaphore structure with initial count 1
6677
*/
67-
void
68-
PGSemaphoreCreate(PGSemaphoresema)
78+
PGSemaphore
79+
PGSemaphoreCreate(void)
6980
{
7081
HANDLEcur_handle;
7182
SECURITY_ATTRIBUTESsec_attrs;
@@ -86,12 +97,14 @@ PGSemaphoreCreate(PGSemaphore sema)
8697
if (cur_handle)
8798
{
8899
/* Successfully done */
89-
*sema=cur_handle;
90100
mySemSet[numSems++]=cur_handle;
91101
}
92102
else
93103
ereport(PANIC,
94-
(errmsg("could not create semaphore: error code %lu",GetLastError())));
104+
(errmsg("could not create semaphore: error code %lu",
105+
GetLastError())));
106+
107+
return (PGSemaphore)cur_handle;
95108
}
96109

97110
/*
@@ -106,7 +119,8 @@ PGSemaphoreReset(PGSemaphore sema)
106119
* There's no direct API for this in Win32, so we have to ratchet the
107120
* semaphore down to 0 with repeated trylock's.
108121
*/
109-
while (PGSemaphoreTryLock(sema));
122+
while (PGSemaphoreTryLock(sema))
123+
/* loop */ ;
110124
}
111125

112126
/*
@@ -127,7 +141,7 @@ PGSemaphoreLock(PGSemaphore sema)
127141
* pending signals are serviced.
128142
*/
129143
wh[0]=pgwin32_signal_event;
130-
wh[1]=*sema;
144+
wh[1]=sema;
131145

132146
/*
133147
* As in other implementations of PGSemaphoreLock, we need to check for
@@ -182,9 +196,10 @@ PGSemaphoreLock(PGSemaphore sema)
182196
void
183197
PGSemaphoreUnlock(PGSemaphoresema)
184198
{
185-
if (!ReleaseSemaphore(*sema,1,NULL))
199+
if (!ReleaseSemaphore(sema,1,NULL))
186200
ereport(FATAL,
187-
(errmsg("could not unlock semaphore: error code %lu",GetLastError())));
201+
(errmsg("could not unlock semaphore: error code %lu",
202+
GetLastError())));
188203
}
189204

190205
/*
@@ -197,7 +212,7 @@ PGSemaphoreTryLock(PGSemaphore sema)
197212
{
198213
DWORDret;
199214

200-
ret=WaitForSingleObject(*sema,0);
215+
ret=WaitForSingleObject(sema,0);
201216

202217
if (ret==WAIT_OBJECT_0)
203218
{
@@ -213,7 +228,8 @@ PGSemaphoreTryLock(PGSemaphore sema)
213228

214229
/* Otherwise we are in trouble */
215230
ereport(FATAL,
216-
(errmsg("could not try-lock semaphore: error code %lu",GetLastError())));
231+
(errmsg("could not try-lock semaphore: error code %lu",
232+
GetLastError())));
217233

218234
/* keep compiler quiet */
219235
return false;

‎src/backend/postmaster/postmaster.c

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -484,7 +484,7 @@ typedef struct
484484
VariableCacheShmemVariableCache;
485485
Backend*ShmemBackendArray;
486486
#ifndefHAVE_SPINLOCKS
487-
PGSemaphoreSpinlockSemaArray;
487+
PGSemaphore*SpinlockSemaArray;
488488
#endif
489489
intNamedLWLockTrancheRequests;
490490
NamedLWLockTranche*NamedLWLockTrancheArray;

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