1 /*-------------------------------------------------------------------------
4 * Hardware-dependent implementation of spinlocks.
7 * Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
8 * Portions Copyright (c) 1994, Regents of the University of California
12 * $PostgreSQL: pgsql/src/backend/storage/lmgr/s_lock.c,v 1.35 2004/12/31 22:01:05 pgsql Exp $
14 *-------------------------------------------------------------------------
21 #include "storage/s_lock.h"
22 #include "miscadmin.h"
25 * s_lock_stuck() - complain about a stuck spinlock
28 s_lock_stuck(volatile slock_t *lock, const char *file, int line)
30 #if defined(S_LOCK_TEST)
32 "\nStuck spinlock (%p) detected at %s:%d.\n",
36 elog(PANIC, "stuck spinlock (%p) detected at %s:%d",
43 * s_lock(lock) - platform-independent portion of waiting for a spinlock.
46 s_lock(volatile slock_t *lock, const char *file, int line)
49 * We loop tightly for awhile, then delay using pg_usleep() and try
50 * again. Preferably, "awhile" should be a small multiple of the
51 * maximum time we expect a spinlock to be held. 100 iterations seems
52 * about right. In most multi-CPU scenarios, the spinlock is probably
53 * held by a process on another CPU and will be released before we
54 * finish 100 iterations. However, on a uniprocessor, the tight loop
55 * is just a waste of cycles, so don't iterate thousands of times.
57 * Once we do decide to block, we use randomly increasing pg_usleep()
58 * delays. The first delay is 10 msec, then the delay randomly
59 * increases to about one second, after which we reset to 10 msec and
60 * start again. The idea here is that in the presence of heavy
61 * contention we need to increase the delay, else the spinlock holder
62 * may never get to run and release the lock. (Consider situation
63 * where spinlock holder has been nice'd down in priority by the
64 * scheduler --- it will not get scheduled until all would-be
65 * acquirers are sleeping, so if we always use a 10-msec sleep, there
66 * is a real possibility of starvation.) But we can't just clamp the
67 * delay to an upper bound, else it would take a long time to make a
68 * reasonable number of tries.
70 * We time out and declare error after NUM_DELAYS delays (thus, exactly
71 * that many tries). With the given settings, this will usually take
72 * 3 or so minutes. It seems better to fix the total number of tries
73 * (and thus the probability of unintended failure) than to fix the
76 * The pg_usleep() delays are measured in centiseconds (0.01 sec) because
77 * 10 msec is a common resolution limit at the OS level.
79 #define SPINS_PER_DELAY 100
80 #define NUM_DELAYS 1000
81 #define MIN_DELAY_CSEC 1
82 #define MAX_DELAY_CSEC 100
86 int cur_delay = MIN_DELAY_CSEC;
90 /* CPU-specific delay each time through the loop */
93 /* Block the process every SPINS_PER_DELAY tries */
94 if (++spins > SPINS_PER_DELAY)
96 if (++delays > NUM_DELAYS)
97 s_lock_stuck(lock, file, line);
99 pg_usleep(cur_delay * 10000L);
101 #if defined(S_LOCK_TEST)
102 fprintf(stdout, "*");
106 /* increase delay by a random fraction between 1X and 2X */
107 cur_delay += (int) (cur_delay *
108 (((double) random()) / ((double) MAX_RANDOM_VALUE)) + 0.5);
109 /* wrap back to minimum delay when max is exceeded */
110 if (cur_delay > MAX_DELAY_CSEC)
111 cur_delay = MIN_DELAY_CSEC;
119 * Various TAS implementations that cannot live in s_lock.h as no inline
120 * definition exists (yet).
121 * In the future, get rid of tas.[cso] and fold it into this file.
123 * If you change something here, you will likely need to modify s_lock.h too,
124 * because the definitions for these are split between this file and s_lock.h.
128 #ifdef HAVE_SPINLOCKS /* skip spinlocks if requested */
131 #if defined(__GNUC__)
134 * All the gcc flavors that are not inlined
138 #if defined(__m68k__)
139 /* really means: extern int tas(slock_t* **lock); */
143 __asm__ __volatile__(
144 #if defined(__NetBSD__) && defined(__ELF__)
145 /* no underscore for label and % for registers */
149 movel %sp@(0x4),%a0 \n\
161 movel sp@(0x4),a0 \n\
169 #endif /* __NetBSD__ && __ELF__ */
172 #endif /* __m68k__ */
175 #if defined(__mips__) && !defined(__sgi)
179 __asm__ __volatile__(
183 .frame $sp, 0, $31 \n\
193 .livereg 0x2000FF0E,0x00000FFF \n\
200 #endif /* __mips__ && !__sgi */
203 #else /* not __GNUC__ */
212 tas_dummy() /* really means: extern int tas(slock_t
221 asm("|#PROLOGUE# 1");
222 asm(" movel sp@(0x4),a0");
225 asm(" moveq #-128,d0");
235 #if defined(__sparc__) || defined(__sparc)
237 * sparc machines not using gcc
240 tas_dummy() /* really means: extern int tas(slock_t
245 asm(".seg \"data\"");
246 asm(".seg \"text\"");
248 asm(".section \"data\"");
249 asm(".section \"text\"");
255 * Sparc atomic test and set (sparc calls it "atomic load-store")
257 asm("ldstub [%r8], %r8");
261 #endif /* __sparc || __sparc__ */
262 #endif /* not __GNUC__ */
263 #endif /* HAVE_SPINLOCKS */
267 /*****************************************************************************/
268 #if defined(S_LOCK_TEST)
271 * test program for verifying a port's spinlock support.
274 struct test_lock_struct
281 volatile struct test_lock_struct test_lock;
286 srandom((unsigned int) time(NULL));
288 test_lock.pad1 = test_lock.pad2 = 0x44;
290 S_INIT_LOCK(&test_lock.lock);
292 if (test_lock.pad1 != 0x44 || test_lock.pad2 != 0x44)
294 printf("S_LOCK_TEST: failed, declared datatype is wrong size\n");
298 if (!S_LOCK_FREE(&test_lock.lock))
300 printf("S_LOCK_TEST: failed, lock not initialized\n");
304 S_LOCK(&test_lock.lock);
306 if (test_lock.pad1 != 0x44 || test_lock.pad2 != 0x44)
308 printf("S_LOCK_TEST: failed, declared datatype is wrong size\n");
312 if (S_LOCK_FREE(&test_lock.lock))
314 printf("S_LOCK_TEST: failed, lock not locked\n");
318 S_UNLOCK(&test_lock.lock);
320 if (test_lock.pad1 != 0x44 || test_lock.pad2 != 0x44)
322 printf("S_LOCK_TEST: failed, declared datatype is wrong size\n");
326 if (!S_LOCK_FREE(&test_lock.lock))
328 printf("S_LOCK_TEST: failed, lock not unlocked\n");
332 S_LOCK(&test_lock.lock);
334 if (test_lock.pad1 != 0x44 || test_lock.pad2 != 0x44)
336 printf("S_LOCK_TEST: failed, declared datatype is wrong size\n");
340 if (S_LOCK_FREE(&test_lock.lock))
342 printf("S_LOCK_TEST: failed, lock not re-locked\n");
346 printf("S_LOCK_TEST: this will print %d stars and then\n", NUM_DELAYS);
347 printf(" exit with a 'stuck spinlock' message\n");
348 printf(" if S_LOCK() and TAS() are working.\n");
351 s_lock(&test_lock.lock, __FILE__, __LINE__);
353 printf("S_LOCK_TEST: failed, lock not locked\n");
357 #endif /* S_LOCK_TEST */