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s_lock.c
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1 /*-------------------------------------------------------------------------
2  *
3  * s_lock.c
4  * Hardware-dependent implementation of spinlocks.
5  *
6  * When waiting for a contended spinlock we loop tightly for awhile, then
7  * delay using pg_usleep() and try again. Preferably, "awhile" should be a
8  * small multiple of the maximum time we expect a spinlock to be held. 100
9  * iterations seems about right as an initial guess. However, on a
10  * uniprocessor the loop is a waste of cycles, while in a multi-CPU scenario
11  * it's usually better to spin a bit longer than to call the kernel, so we try
12  * to adapt the spin loop count depending on whether we seem to be in a
13  * uniprocessor or multiprocessor.
14  *
15  * Note: you might think MIN_SPINS_PER_DELAY should be just 1, but you'd
16  * be wrong; there are platforms where that can result in a "stuck
17  * spinlock" failure. This has been seen particularly on Alphas; it seems
18  * that the first TAS after returning from kernel space will always fail
19  * on that hardware.
20  *
21  * Once we do decide to block, we use randomly increasing pg_usleep()
22  * delays. The first delay is 1 msec, then the delay randomly increases to
23  * about one second, after which we reset to 1 msec and start again. The
24  * idea here is that in the presence of heavy contention we need to
25  * increase the delay, else the spinlock holder may never get to run and
26  * release the lock. (Consider situation where spinlock holder has been
27  * nice'd down in priority by the scheduler --- it will not get scheduled
28  * until all would-be acquirers are sleeping, so if we always use a 1-msec
29  * sleep, there is a real possibility of starvation.) But we can't just
30  * clamp the delay to an upper bound, else it would take a long time to
31  * make a reasonable number of tries.
32  *
33  * We time out and declare error after NUM_DELAYS delays (thus, exactly
34  * that many tries). With the given settings, this will usually take 2 or
35  * so minutes. It seems better to fix the total number of tries (and thus
36  * the probability of unintended failure) than to fix the total time
37  * spent.
38  *
39  * Portions Copyright (c) 1996-2017, PostgreSQL Global Development Group
40  * Portions Copyright (c) 1994, Regents of the University of California
41  *
42  *
43  * IDENTIFICATION
44  * src/backend/storage/lmgr/s_lock.c
45  *
46  *-------------------------------------------------------------------------
47  */
48 #include "postgres.h"
49 
50 #include <time.h>
51 #include <unistd.h>
52 
53 #include "storage/s_lock.h"
54 #include "port/atomics.h"
55 
56 
57 #define MIN_SPINS_PER_DELAY 10
58 #define MAX_SPINS_PER_DELAY 1000
59 #define NUM_DELAYS 1000
60 #define MIN_DELAY_USEC 1000L
61 #define MAX_DELAY_USEC 1000000L
62 
63 
65 
67 
68 
69 /*
70  * s_lock_stuck() - complain about a stuck spinlock
71  */
72 static void
73 s_lock_stuck(const char *file, int line, const char *func)
74 {
75  if (!func)
76  func = "(unknown)";
77 #if defined(S_LOCK_TEST)
78  fprintf(stderr,
79  "\nStuck spinlock detected at %s, %s:%d.\n",
80  func, file, line);
81  exit(1);
82 #else
83  elog(PANIC, "stuck spinlock detected at %s, %s:%d",
84  func, file, line);
85 #endif
86 }
87 
88 /*
89  * s_lock(lock) - platform-independent portion of waiting for a spinlock.
90  */
91 int
92 s_lock(volatile slock_t *lock, const char *file, int line, const char *func)
93 {
94  SpinDelayStatus delayStatus;
95 
96  init_spin_delay(&delayStatus, file, line, func);
97 
98  while (TAS_SPIN(lock))
99  {
100  perform_spin_delay(&delayStatus);
101  }
102 
103  finish_spin_delay(&delayStatus);
104 
105  return delayStatus.delays;
106 }
107 
108 #ifdef USE_DEFAULT_S_UNLOCK
109 void
110 s_unlock(volatile slock_t *lock)
111 {
112 #ifdef TAS_ACTIVE_WORD
113  /* HP's PA-RISC */
114  *TAS_ACTIVE_WORD(lock) = -1;
115 #else
116  *lock = 0;
117 #endif
118 }
119 #endif
120 
121 /*
122  * Wait while spinning on a contended spinlock.
123  */
124 void
126 {
127  /* CPU-specific delay each time through the loop */
128  SPIN_DELAY();
129 
130  /* Block the process every spins_per_delay tries */
131  if (++(status->spins) >= spins_per_delay)
132  {
133  if (++(status->delays) > NUM_DELAYS)
134  s_lock_stuck(status->file, status->line, status->func);
135 
136  if (status->cur_delay == 0) /* first time to delay? */
137  status->cur_delay = MIN_DELAY_USEC;
138 
139  pg_usleep(status->cur_delay);
140 
141 #if defined(S_LOCK_TEST)
142  fprintf(stdout, "*");
143  fflush(stdout);
144 #endif
145 
146  /* increase delay by a random fraction between 1X and 2X */
147  status->cur_delay += (int) (status->cur_delay *
148  ((double) random() / (double) MAX_RANDOM_VALUE) + 0.5);
149  /* wrap back to minimum delay when max is exceeded */
150  if (status->cur_delay > MAX_DELAY_USEC)
151  status->cur_delay = MIN_DELAY_USEC;
152 
153  status->spins = 0;
154  }
155 }
156 
157 /*
158  * After acquiring a spinlock, update estimates about how long to loop.
159  *
160  * If we were able to acquire the lock without delaying, it's a good
161  * indication we are in a multiprocessor. If we had to delay, it's a sign
162  * (but not a sure thing) that we are in a uniprocessor. Hence, we
163  * decrement spins_per_delay slowly when we had to delay, and increase it
164  * rapidly when we didn't. It's expected that spins_per_delay will
165  * converge to the minimum value on a uniprocessor and to the maximum
166  * value on a multiprocessor.
167  *
168  * Note: spins_per_delay is local within our current process. We want to
169  * average these observations across multiple backends, since it's
170  * relatively rare for this function to even get entered, and so a single
171  * backend might not live long enough to converge on a good value. That
172  * is handled by the two routines below.
173  */
174 void
176 {
177  if (status->cur_delay == 0)
178  {
179  /* we never had to delay */
182  }
183  else
184  {
187  }
188 }
189 
190 /*
191  * Set local copy of spins_per_delay during backend startup.
192  *
193  * NB: this has to be pretty fast as it is called while holding a spinlock
194  */
195 void
196 set_spins_per_delay(int shared_spins_per_delay)
197 {
198  spins_per_delay = shared_spins_per_delay;
199 }
200 
201 /*
202  * Update shared estimate of spins_per_delay during backend exit.
203  *
204  * NB: this has to be pretty fast as it is called while holding a spinlock
205  */
206 int
207 update_spins_per_delay(int shared_spins_per_delay)
208 {
209  /*
210  * We use an exponential moving average with a relatively slow adaption
211  * rate, so that noise in any one backend's result won't affect the shared
212  * value too much. As long as both inputs are within the allowed range,
213  * the result must be too, so we need not worry about clamping the result.
214  *
215  * We deliberately truncate rather than rounding; this is so that single
216  * adjustments inside a backend can affect the shared estimate (see the
217  * asymmetric adjustment rules above).
218  */
219  return (shared_spins_per_delay * 15 + spins_per_delay) / 16;
220 }
221 
222 
223 /*
224  * Various TAS implementations that cannot live in s_lock.h as no inline
225  * definition exists (yet).
226  * In the future, get rid of tas.[cso] and fold it into this file.
227  *
228  * If you change something here, you will likely need to modify s_lock.h too,
229  * because the definitions for these are split between this file and s_lock.h.
230  */
231 
232 
233 #ifdef HAVE_SPINLOCKS /* skip spinlocks if requested */
234 
235 
236 #if defined(__GNUC__)
237 
238 /*
239  * All the gcc flavors that are not inlined
240  */
241 
242 
243 /*
244  * Note: all the if-tests here probably ought to be testing gcc version
245  * rather than platform, but I don't have adequate info to know what to
246  * write. Ideally we'd flush all this in favor of the inline version.
247  */
248 #if defined(__m68k__) && !defined(__linux__)
249 /* really means: extern int tas(slock_t* **lock); */
250 static void
251 tas_dummy()
252 {
253  __asm__ __volatile__(
254 #if defined(__NetBSD__) && defined(__ELF__)
255 /* no underscore for label and % for registers */
256  "\
257 .global tas \n\
258 tas: \n\
259  movel %sp@(0x4),%a0 \n\
260  tas %a0@ \n\
261  beq _success \n\
262  moveq #-128,%d0 \n\
263  rts \n\
264 _success: \n\
265  moveq #0,%d0 \n\
266  rts \n"
267 #else
268  "\
269 .global _tas \n\
270 _tas: \n\
271  movel sp@(0x4),a0 \n\
272  tas a0@ \n\
273  beq _success \n\
274  moveq #-128,d0 \n\
275  rts \n\
276 _success: \n\
277  moveq #0,d0 \n\
278  rts \n"
279 #endif /* __NetBSD__ && __ELF__ */
280  );
281 }
282 #endif /* __m68k__ && !__linux__ */
283 #endif /* not __GNUC__ */
284 #endif /* HAVE_SPINLOCKS */
285 
286 
287 
288 /*****************************************************************************/
289 #if defined(S_LOCK_TEST)
290 
291 /*
292  * test program for verifying a port's spinlock support.
293  */
294 
295 struct test_lock_struct
296 {
297  char pad1;
298  slock_t lock;
299  char pad2;
300 };
301 
302 volatile struct test_lock_struct test_lock;
303 
304 int
305 main()
306 {
307  srandom((unsigned int) time(NULL));
308 
309  test_lock.pad1 = test_lock.pad2 = 0x44;
310 
311  S_INIT_LOCK(&test_lock.lock);
312 
313  if (test_lock.pad1 != 0x44 || test_lock.pad2 != 0x44)
314  {
315  printf("S_LOCK_TEST: failed, declared datatype is wrong size\n");
316  return 1;
317  }
318 
319  if (!S_LOCK_FREE(&test_lock.lock))
320  {
321  printf("S_LOCK_TEST: failed, lock not initialized\n");
322  return 1;
323  }
324 
325  S_LOCK(&test_lock.lock);
326 
327  if (test_lock.pad1 != 0x44 || test_lock.pad2 != 0x44)
328  {
329  printf("S_LOCK_TEST: failed, declared datatype is wrong size\n");
330  return 1;
331  }
332 
333  if (S_LOCK_FREE(&test_lock.lock))
334  {
335  printf("S_LOCK_TEST: failed, lock not locked\n");
336  return 1;
337  }
338 
339  S_UNLOCK(&test_lock.lock);
340 
341  if (test_lock.pad1 != 0x44 || test_lock.pad2 != 0x44)
342  {
343  printf("S_LOCK_TEST: failed, declared datatype is wrong size\n");
344  return 1;
345  }
346 
347  if (!S_LOCK_FREE(&test_lock.lock))
348  {
349  printf("S_LOCK_TEST: failed, lock not unlocked\n");
350  return 1;
351  }
352 
353  S_LOCK(&test_lock.lock);
354 
355  if (test_lock.pad1 != 0x44 || test_lock.pad2 != 0x44)
356  {
357  printf("S_LOCK_TEST: failed, declared datatype is wrong size\n");
358  return 1;
359  }
360 
361  if (S_LOCK_FREE(&test_lock.lock))
362  {
363  printf("S_LOCK_TEST: failed, lock not re-locked\n");
364  return 1;
365  }
366 
367  printf("S_LOCK_TEST: this will print %d stars and then\n", NUM_DELAYS);
368  printf(" exit with a 'stuck spinlock' message\n");
369  printf(" if S_LOCK() and TAS() are working.\n");
370  fflush(stdout);
371 
372  s_lock(&test_lock.lock, __FILE__, __LINE__);
373 
374  printf("S_LOCK_TEST: failed, lock not locked\n");
375  return 1;
376 }
377 
378 #endif /* S_LOCK_TEST */
#define NUM_DELAYS
Definition: s_lock.c:59
int slock_t
Definition: s_lock.h:888
#define S_INIT_LOCK(lock)
Definition: s_lock.h:897
#define SPIN_DELAY()
Definition: s_lock.h:944
void set_spins_per_delay(int shared_spins_per_delay)
Definition: s_lock.c:196
long random(void)
Definition: random.c:22
#define S_LOCK_FREE(lock)
Definition: s_lock.h:895
#define Min(x, y)
Definition: c.h:812
const char * func
Definition: s_lock.h:982
#define MAX_DELAY_USEC
Definition: s_lock.c:61
static void s_lock_stuck(const char *file, int line, const char *func)
Definition: s_lock.c:73
#define PANIC
Definition: elog.h:53
int main(int argc, char **argv)
Definition: oid2name.c:545
static void init_spin_delay(SpinDelayStatus *status, const char *file, int line, const char *func)
Definition: s_lock.h:986
#define MIN_DELAY_USEC
Definition: s_lock.c:60
void pg_usleep(long microsec)
Definition: signal.c:53
void finish_spin_delay(SpinDelayStatus *status)
Definition: s_lock.c:175
#define MAX_SPINS_PER_DELAY
Definition: s_lock.c:58
#define MAX_RANDOM_VALUE
static int spins_per_delay
Definition: s_lock.c:66
int update_spins_per_delay(int shared_spins_per_delay)
Definition: s_lock.c:207
const char * file
Definition: s_lock.h:980
#define TAS_SPIN(lock)
Definition: s_lock.h:955
#define Max(x, y)
Definition: c.h:806
#define S_UNLOCK(lock)
Definition: s_lock.h:896
slock_t dummy_spinlock
Definition: s_lock.c:64
void srandom(unsigned int seed)
Definition: srandom.c:22
#define elog
Definition: elog.h:219
static void static void status(const char *fmt,...) pg_attribute_printf(1
Definition: pg_regress.c:225
#define S_LOCK(lock)
Definition: s_lock.h:909
void perform_spin_delay(SpinDelayStatus *status)
Definition: s_lock.c:125
int s_lock(volatile slock_t *lock, const char *file, int line, const char *func)
Definition: s_lock.c:92
#define DEFAULT_SPINS_PER_DELAY
Definition: s_lock.h:966
#define MIN_SPINS_PER_DELAY
Definition: s_lock.c:57