granicus.if.org Git - postgresql/blob - src/backend/storage/lmgr/s_lock.c

   1 /*-------------------------------------------------------------------------
   2  *
   3  * s_lock.c
   4  *         Hardware-dependent implementation of spinlocks.
   5  *
   6  *
   7  * Portions Copyright (c) 1996-2003, PostgreSQL Global Development Group
   8  * Portions Copyright (c) 1994, Regents of the University of California
   9  *
  10  *
  11  * IDENTIFICATION
  12  *        $PostgreSQL: pgsql/src/backend/storage/lmgr/s_lock.c,v 1.18 2003/12/22 23:39:53 momjian Exp $
  13  *
  14  *-------------------------------------------------------------------------
  15  */
  16 #include "postgres.h"
  17
  18 #include <sys/time.h>
  19 #include <unistd.h>
  20
  21 #include "storage/s_lock.h"
  22
  23
  24 /*
  25  * s_lock_stuck() - complain about a stuck spinlock
  26  */
  27 static void
  28 s_lock_stuck(volatile slock_t *lock, const char *file, int line)
  29 {
  30 #if defined(S_LOCK_TEST)
  31         fprintf(stderr,
  32                         "\nStuck spinlock (%p) detected at %s:%d.\n",
  33                         lock, file, line);
  34         exit(1);
  35 #else
  36         elog(PANIC, "stuck spinlock (%p) detected at %s:%d",
  37                  lock, file, line);
  38 #endif
  39 }
  40
  41
  42 /*
  43  * s_lock(lock) - platform-independent portion of waiting for a spinlock.
  44  */
  45 void
  46 s_lock(volatile slock_t *lock, const char *file, int line)
  47 {
  48         /*
  49          * We loop tightly for awhile, then delay using select() 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.
  56          *
  57          * Once we do decide to block, we use randomly increasing select()
  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.
  69          *
  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
  74          * total time spent.
  75          *
  76          * The select() delays are measured in centiseconds (0.01 sec) because 10
  77          * msec is a common resolution limit at the OS level.
  78          */
  79 #define SPINS_PER_DELAY         100
  80 #define NUM_DELAYS                      1000
  81 #define MIN_DELAY_CSEC          1
  82 #define MAX_DELAY_CSEC          100
  83
  84         int                     spins = 0;
  85         int                     delays = 0;
  86         int                     cur_delay = MIN_DELAY_CSEC;
  87         struct timeval delay;
  88
  89         while (TAS(lock))
  90         {
  91                 if (++spins > SPINS_PER_DELAY)
  92                 {
  93                         if (++delays > NUM_DELAYS)
  94                                 s_lock_stuck(lock, file, line);
  95
  96                         delay.tv_sec = cur_delay / 100;
  97                         delay.tv_usec = (cur_delay % 100) * 10000;
  98                         (void) select(0, NULL, NULL, NULL, &delay);
  99
 100 #if defined(S_LOCK_TEST)
 101                         fprintf(stdout, "*");
 102                         fflush(stdout);
 103 #endif
 104
 105                         /* increase delay by a random fraction between 1X and 2X */
 106                         cur_delay += (int) (cur_delay *
 107                           (((double) random()) / ((double) MAX_RANDOM_VALUE)) + 0.5);
 108                         /* wrap back to minimum delay when max is exceeded */
 109                         if (cur_delay > MAX_DELAY_CSEC)
 110                                 cur_delay = MIN_DELAY_CSEC;
 111
 112                         spins = 0;
 113                 }
 114         }
 115 }
 116
 117 /*
 118  * Various TAS implementations that cannot live in s_lock.h as no inline
 119  * definition exists (yet).
 120  * In the future, get rid of tas.[cso] and fold it into this file.
 121  */
 122
 123
 124 #if defined(__GNUC__)
 125 /*************************************************************************
 126  * All the gcc flavors that are not inlined
 127  */
 128
 129
 130 #if defined(__m68k__)
 131 static void
 132 tas_dummy()                                             /* really means: extern int tas(slock_t
 133                                                                  * **lock); */
 134 {
 135         __asm__         __volatile__(
 136                                                                                  "\
 137 .global         _tas                            \n\
 138 _tas:                                                   \n\
 139                         movel   sp@(0x4),a0     \n\
 140                         tas     a0@                     \n\
 141                         beq     _success        \n\
 142                         moveq   #-128,d0        \n\
 143                         rts                                     \n\
 144 _success:                                               \n\
 145                         moveq   #0,d0           \n\
 146                         rts                                     \n\
 147 ");
 148 }
 149 #endif   /* __m68k__ */
 150
 151 #if defined(__mips__) && !defined(__sgi)
 152 static void
 153 tas_dummy()
 154 {
 155         __asm__         __volatile__(
 156                                                                                  "\
 157 .global tas                                             \n\
 158 tas:                                                    \n\
 159                         .frame  $sp, 0, $31     \n\
 160                         .set push               \n\
 161                         .set mips2              \n\
 162                         ll              $14, 0($4)      \n\
 163                         or              $15, $14, 1     \n\
 164                         sc              $15, 0($4)      \n\
 165                         .set pop                        \n\
 166                         beq             $15, 0, fail\n\
 167                         bne             $14, 0, fail\n\
 168                         li              $2, 0           \n\
 169                         .livereg 0x2000FF0E,0x00000FFF  \n\
 170                         j               $31                     \n\
 171 fail:                                                   \n\
 172                         li              $2, 1           \n\
 173                         j       $31                     \n\
 174 ");
 175 }
 176 #endif   /* __mips__ && !__sgi */
 177
 178 #else                                                   /* not __GNUC__ */
 179 /***************************************************************************
 180  * All non gcc
 181  */
 182
 183
 184
 185 #if defined(sun3)
 186 static void
 187 tas_dummy()                                             /* really means: extern int tas(slock_t
 188                                                                  * *lock); */
 189 {
 190         asm("LLA0:");
 191         asm("   .data");
 192         asm("   .text");
 193         asm("|#PROC# 04");
 194         asm("   .globl  _tas");
 195         asm("_tas:");
 196         asm("|#PROLOGUE# 1");
 197         asm("   movel   sp@(0x4),a0");
 198         asm("   tas a0@");
 199         asm("   beq LLA1");
 200         asm("   moveq   #-128,d0");
 201         asm("   rts");
 202         asm("LLA1:");
 203         asm("   moveq   #0,d0");
 204         asm("   rts");
 205         asm("   .data");
 206 }
 207 #endif   /* sun3 */
 208
 209
 210
 211 #if defined(__sparc__) || defined(__sparc)
 212 /*
 213  * sparc machines not using gcc
 214  */
 215 static void
 216 tas_dummy()                                             /* really means: extern int tas(slock_t
 217                                                                  * *lock); */
 218 {
 219         asm(".seg \"data\"");
 220         asm(".seg \"text\"");
 221         asm("_tas:");
 222
 223         /*
 224          * Sparc atomic test and set (sparc calls it "atomic load-store")
 225          */
 226         asm("ldstub [%r8], %r8");
 227         asm("retl");
 228         asm("nop");
 229 }
 230 #endif   /* __sparc || __sparc__ */
 231
 232
 233
 234
 235 #if defined(NEED_I386_TAS_ASM)
 236 /* non gcc i386 based things */
 237 #endif   /* NEED_I386_TAS_ASM */
 238 #endif   /* not __GNUC__ */
 239
 240
 241
 242
 243 /*****************************************************************************/
 244 #if defined(S_LOCK_TEST)
 245
 246 /*
 247  * test program for verifying a port's spinlock support.
 248  */
 249
 250 volatile slock_t test_lock;
 251
 252 int
 253 main()
 254 {
 255         srandom((unsigned int) time(NULL));
 256
 257         S_INIT_LOCK(&test_lock);
 258
 259         if (!S_LOCK_FREE(&test_lock))
 260         {
 261                 printf("S_LOCK_TEST: failed, lock not initialized\n");
 262                 return 1;
 263         }
 264
 265         S_LOCK(&test_lock);
 266
 267         if (S_LOCK_FREE(&test_lock))
 268         {
 269                 printf("S_LOCK_TEST: failed, lock not locked\n");
 270                 return 1;
 271         }
 272
 273         S_UNLOCK(&test_lock);
 274
 275         if (!S_LOCK_FREE(&test_lock))
 276         {
 277                 printf("S_LOCK_TEST: failed, lock not unlocked\n");
 278                 return 1;
 279         }
 280
 281         S_LOCK(&test_lock);
 282
 283         if (S_LOCK_FREE(&test_lock))
 284         {
 285                 printf("S_LOCK_TEST: failed, lock not re-locked\n");
 286                 return 1;
 287         }
 288
 289         printf("S_LOCK_TEST: this will print %d stars and then\n", NUM_DELAYS);
 290         printf("             exit with a 'stuck spinlock' message\n");
 291         printf("             if S_LOCK() and TAS() are working.\n");
 292         fflush(stdout);
 293
 294         s_lock(&test_lock, __FILE__, __LINE__);
 295
 296         printf("S_LOCK_TEST: failed, lock not locked\n");
 297         return 1;
 298 }
 299
 300 #endif   /* S_LOCK_TEST */