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) 1996-2019, 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
64slock_t dummy_spinlock;
65
66static int spins_per_delay = DEFAULT_SPINS_PER_DELAY;
67
68
69/*
70 * s_lock_stuck() - complain about a stuck spinlock
71 */
72static void
73s_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 */
91int
92s_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
109void
110s_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 */
124void
125perform_spin_delay(SpinDelayStatus *status)
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 */
174void
175finish_spin_delay(SpinDelayStatus *status)
176{
177 if (status->cur_delay == 0)
178 {
179 /* we never had to delay */
180 if (spins_per_delay < MAX_SPINS_PER_DELAY)
181 spins_per_delay = Min(spins_per_delay + 100, MAX_SPINS_PER_DELAY);
182 }
183 else
184 {
185 if (spins_per_delay > MIN_SPINS_PER_DELAY)
186 spins_per_delay = Max(spins_per_delay - 1, MIN_SPINS_PER_DELAY);
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 */
195void
196set_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 */
206int
207update_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); */
250static void
251tas_dummy()
252{
253 __asm__ __volatile__(
254#if (defined(__NetBSD__) || defined(__OpenBSD__)) && defined(__ELF__)
255/* no underscore for label and % for registers */
256 "\
257.global tas \n\
258tas: \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__ || __OpenBSD__) && __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
295struct test_lock_struct
296{
297 char pad1;
298 slock_t lock;
299 char pad2;
300};
301
302volatile struct test_lock_struct test_lock;
303
304int
305main()
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 */
379