Line data Source code
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 :
64 : slock_t dummy_spinlock;
65 :
66 : static int spins_per_delay = DEFAULT_SPINS_PER_DELAY;
67 :
68 :
69 : /*
70 : * s_lock_stuck() - complain about a stuck spinlock
71 : */
72 : static void
73 0 : s_lock_stuck(const char *file, int line, const char *func)
74 : {
75 0 : if (!func)
76 0 : 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 0 : 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 2936 : s_lock(volatile slock_t *lock, const char *file, int line, const char *func)
93 : {
94 : SpinDelayStatus delayStatus;
95 :
96 2936 : init_spin_delay(&delayStatus, file, line, func);
97 :
98 29824 : while (TAS_SPIN(lock))
99 : {
100 23952 : perform_spin_delay(&delayStatus);
101 : }
102 :
103 2936 : finish_spin_delay(&delayStatus);
104 :
105 2936 : 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
125 32227 : perform_spin_delay(SpinDelayStatus *status)
126 : {
127 : /* CPU-specific delay each time through the loop */
128 32227 : SPIN_DELAY();
129 :
130 : /* Block the process every spins_per_delay tries */
131 32227 : if (++(status->spins) >= spins_per_delay)
132 : {
133 32 : if (++(status->delays) > NUM_DELAYS)
134 0 : s_lock_stuck(status->file, status->line, status->func);
135 :
136 32 : if (status->cur_delay == 0) /* first time to delay? */
137 16 : status->cur_delay = MIN_DELAY_USEC;
138 :
139 32 : 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 64 : status->cur_delay += (int) (status->cur_delay *
148 32 : ((double) random() / (double) MAX_RANDOM_VALUE) + 0.5);
149 : /* wrap back to minimum delay when max is exceeded */
150 32 : if (status->cur_delay > MAX_DELAY_USEC)
151 0 : status->cur_delay = MIN_DELAY_USEC;
152 :
153 32 : status->spins = 0;
154 : }
155 32227 : }
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
175 322565 : finish_spin_delay(SpinDelayStatus *status)
176 : {
177 322565 : if (status->cur_delay == 0)
178 : {
179 : /* we never had to delay */
180 322549 : if (spins_per_delay < MAX_SPINS_PER_DELAY)
181 392 : spins_per_delay = Min(spins_per_delay + 100, MAX_SPINS_PER_DELAY);
182 : }
183 : else
184 : {
185 16 : if (spins_per_delay > MIN_SPINS_PER_DELAY)
186 16 : spins_per_delay = Max(spins_per_delay - 1, MIN_SPINS_PER_DELAY);
187 : }
188 322565 : }
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
196 342 : set_spins_per_delay(int shared_spins_per_delay)
197 : {
198 342 : spins_per_delay = shared_spins_per_delay;
199 342 : }
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
207 342 : 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 : */
219 342 : 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 */
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