Branch data Line data Source code
1 : : /*-------------------------------------------------------------------------
2 : : *
3 : : * s_lock.c
4 : : * 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) 1996-2026, 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 "common/pg_prng.h"
54 : : #include "storage/s_lock.h"
55 : : #include "utils/wait_event.h"
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 : : #ifdef S_LOCK_TEST
64 : : /*
65 : : * These are needed by pgstat_report_wait_start in the standalone compile of
66 : : * s_lock_test.
67 : : */
68 : : static uint32 local_my_wait_event_info;
69 : : uint32 *my_wait_event_info = &local_my_wait_event_info;
70 : : #endif
71 : :
72 : : static int spins_per_delay = DEFAULT_SPINS_PER_DELAY;
73 : :
74 : :
75 : : /*
76 : : * s_lock_stuck() - complain about a stuck spinlock
77 : : */
78 : : static void
79 : 0 : s_lock_stuck(const char *file, int line, const char *func)
80 : : {
81 [ # # ]: 0 : if (!func)
82 : 0 : func = "(unknown)";
83 : : #if defined(S_LOCK_TEST)
84 : : fprintf(stderr,
85 : : "\nStuck spinlock detected at %s, %s:%d.\n",
86 : : func, file, line);
87 : : exit(1);
88 : : #else
89 [ # # # # ]: 0 : elog(PANIC, "stuck spinlock detected at %s, %s:%d",
90 : : func, file, line);
91 : : #endif
92 : 0 : }
93 : :
94 : : /*
95 : : * s_lock(lock) - platform-independent portion of waiting for a spinlock.
96 : : */
97 : : int
98 : 133 : s_lock(volatile slock_t *lock, const char *file, int line, const char *func)
99 : : {
100 : 133 : SpinDelayStatus delayStatus;
101 : :
102 : 133 : init_spin_delay(&delayStatus, file, line, func);
103 : :
104 [ + + + + ]: 21122 : while (TAS_SPIN(lock))
105 : : {
106 : 20989 : perform_spin_delay(&delayStatus);
107 : : }
108 : :
109 : 133 : finish_spin_delay(&delayStatus);
110 : :
111 : 266 : return delayStatus.delays;
112 : 133 : }
113 : :
114 : : #ifdef USE_DEFAULT_S_UNLOCK
115 : : void
116 : : s_unlock(volatile slock_t *lock)
117 : : {
118 : : *lock = 0;
119 : : }
120 : : #endif
121 : :
122 : : /*
123 : : * Wait while spinning on a contended spinlock.
124 : : */
125 : : void
126 : 24410 : perform_spin_delay(SpinDelayStatus *status)
127 : : {
128 : : /* CPU-specific delay each time through the loop */
129 : 24410 : SPIN_DELAY();
130 : :
131 : : /* Block the process every spins_per_delay tries */
132 [ + + ]: 24410 : if (++(status->spins) >= spins_per_delay)
133 : : {
134 [ + - ]: 20 : if (++(status->delays) > NUM_DELAYS)
135 : 0 : s_lock_stuck(status->file, status->line, status->func);
136 : :
137 [ - + ]: 20 : if (status->cur_delay == 0) /* first time to delay? */
138 : 20 : status->cur_delay = MIN_DELAY_USEC;
139 : :
140 : : /*
141 : : * Once we start sleeping, the overhead of reporting a wait event is
142 : : * justified. Actively spinning easily stands out in profilers, but
143 : : * sleeping with an exponential backoff is harder to spot...
144 : : *
145 : : * We might want to report something more granular at some point, but
146 : : * this is better than nothing.
147 : : */
148 : 20 : pgstat_report_wait_start(WAIT_EVENT_SPIN_DELAY);
149 : 20 : pg_usleep(status->cur_delay);
150 : 20 : pgstat_report_wait_end();
151 : :
152 : : #if defined(S_LOCK_TEST)
153 : : fprintf(stdout, "*");
154 : : fflush(stdout);
155 : : #endif
156 : :
157 : : /* increase delay by a random fraction between 1X and 2X */
158 : 40 : status->cur_delay += (int) (status->cur_delay *
159 : 20 : pg_prng_double(&pg_global_prng_state) + 0.5);
160 : : /* wrap back to minimum delay when max is exceeded */
161 [ + - ]: 20 : if (status->cur_delay > MAX_DELAY_USEC)
162 : 0 : status->cur_delay = MIN_DELAY_USEC;
163 : :
164 : 20 : status->spins = 0;
165 : 20 : }
166 : 24410 : }
167 : :
168 : : /*
169 : : * After acquiring a spinlock, update estimates about how long to loop.
170 : : *
171 : : * If we were able to acquire the lock without delaying, it's a good
172 : : * indication we are in a multiprocessor. If we had to delay, it's a sign
173 : : * (but not a sure thing) that we are in a uniprocessor. Hence, we
174 : : * decrement spins_per_delay slowly when we had to delay, and increase it
175 : : * rapidly when we didn't. It's expected that spins_per_delay will
176 : : * converge to the minimum value on a uniprocessor and to the maximum
177 : : * value on a multiprocessor.
178 : : *
179 : : * Note: spins_per_delay is local within our current process. We want to
180 : : * average these observations across multiple backends, since it's
181 : : * relatively rare for this function to even get entered, and so a single
182 : : * backend might not live long enough to converge on a good value. That
183 : : * is handled by the two routines below.
184 : : */
185 : : void
186 : 1017 : finish_spin_delay(SpinDelayStatus *status)
187 : : {
188 [ + + ]: 1017 : if (status->cur_delay == 0)
189 : : {
190 : : /* we never had to delay */
191 [ + + ]: 997 : if (spins_per_delay < MAX_SPINS_PER_DELAY)
192 [ + + ]: 366 : spins_per_delay = Min(spins_per_delay + 100, MAX_SPINS_PER_DELAY);
193 : 997 : }
194 : : else
195 : : {
196 [ - + ]: 20 : if (spins_per_delay > MIN_SPINS_PER_DELAY)
197 [ + - ]: 20 : spins_per_delay = Max(spins_per_delay - 1, MIN_SPINS_PER_DELAY);
198 : : }
199 : 1017 : }
200 : :
201 : : /*
202 : : * Set local copy of spins_per_delay during backend startup.
203 : : *
204 : : * NB: this has to be pretty fast as it is called while holding a spinlock
205 : : */
206 : : void
207 : 806 : set_spins_per_delay(int shared_spins_per_delay)
208 : : {
209 : 806 : spins_per_delay = shared_spins_per_delay;
210 : 806 : }
211 : :
212 : : /*
213 : : * Update shared estimate of spins_per_delay during backend exit.
214 : : *
215 : : * NB: this has to be pretty fast as it is called while holding a spinlock
216 : : */
217 : : int
218 : 806 : update_spins_per_delay(int shared_spins_per_delay)
219 : : {
220 : : /*
221 : : * We use an exponential moving average with a relatively slow adaption
222 : : * rate, so that noise in any one backend's result won't affect the shared
223 : : * value too much. As long as both inputs are within the allowed range,
224 : : * the result must be too, so we need not worry about clamping the result.
225 : : *
226 : : * We deliberately truncate rather than rounding; this is so that single
227 : : * adjustments inside a backend can affect the shared estimate (see the
228 : : * asymmetric adjustment rules above).
229 : : */
230 : 806 : return (shared_spins_per_delay * 15 + spins_per_delay) / 16;
231 : : }
232 : :
233 : :
234 : : /*****************************************************************************/
235 : : #if defined(S_LOCK_TEST)
236 : :
237 : : /*
238 : : * test program for verifying a port's spinlock support.
239 : : */
240 : :
241 : : struct test_lock_struct
242 : : {
243 : : char pad1;
244 : : slock_t lock;
245 : : char pad2;
246 : : };
247 : :
248 : : volatile struct test_lock_struct test_lock;
249 : :
250 : : int
251 : : main()
252 : : {
253 : : pg_prng_seed(&pg_global_prng_state, (uint64) time(NULL));
254 : :
255 : : test_lock.pad1 = test_lock.pad2 = 0x44;
256 : :
257 : : S_INIT_LOCK(&test_lock.lock);
258 : :
259 : : if (test_lock.pad1 != 0x44 || test_lock.pad2 != 0x44)
260 : : {
261 : : printf("S_LOCK_TEST: failed, declared datatype is wrong size\n");
262 : : return 1;
263 : : }
264 : :
265 : : if (!S_LOCK_FREE(&test_lock.lock))
266 : : {
267 : : printf("S_LOCK_TEST: failed, lock not initialized\n");
268 : : return 1;
269 : : }
270 : :
271 : : S_LOCK(&test_lock.lock);
272 : :
273 : : if (test_lock.pad1 != 0x44 || test_lock.pad2 != 0x44)
274 : : {
275 : : printf("S_LOCK_TEST: failed, declared datatype is wrong size\n");
276 : : return 1;
277 : : }
278 : :
279 : : if (S_LOCK_FREE(&test_lock.lock))
280 : : {
281 : : printf("S_LOCK_TEST: failed, lock not locked\n");
282 : : return 1;
283 : : }
284 : :
285 : : S_UNLOCK(&test_lock.lock);
286 : :
287 : : if (test_lock.pad1 != 0x44 || test_lock.pad2 != 0x44)
288 : : {
289 : : printf("S_LOCK_TEST: failed, declared datatype is wrong size\n");
290 : : return 1;
291 : : }
292 : :
293 : : if (!S_LOCK_FREE(&test_lock.lock))
294 : : {
295 : : printf("S_LOCK_TEST: failed, lock not unlocked\n");
296 : : return 1;
297 : : }
298 : :
299 : : S_LOCK(&test_lock.lock);
300 : :
301 : : if (test_lock.pad1 != 0x44 || test_lock.pad2 != 0x44)
302 : : {
303 : : printf("S_LOCK_TEST: failed, declared datatype is wrong size\n");
304 : : return 1;
305 : : }
306 : :
307 : : if (S_LOCK_FREE(&test_lock.lock))
308 : : {
309 : : printf("S_LOCK_TEST: failed, lock not re-locked\n");
310 : : return 1;
311 : : }
312 : :
313 : : printf("S_LOCK_TEST: this will print %d stars and then\n", NUM_DELAYS);
314 : : printf(" exit with a 'stuck spinlock' message\n");
315 : : printf(" if S_LOCK() and TAS() are working.\n");
316 : : fflush(stdout);
317 : :
318 : : s_lock(&test_lock.lock, __FILE__, __LINE__, __func__);
319 : :
320 : : printf("S_LOCK_TEST: failed, lock not locked\n");
321 : : return 1;
322 : : }
323 : :
324 : : #endif /* S_LOCK_TEST */
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