sys/kern/kern_spinlock.c

   1 /*
   2  * Copyright (c) 2005 Jeffrey M. Hsu.  All rights reserved.
   3  *
   4  * This code is derived from software contributed to The DragonFly Project
   5  * by Jeffrey M. Hsu. and Matthew Dillon
   6  *
   7  * Redistribution and use in source and binary forms, with or without
   8  * modification, are permitted provided that the following conditions
   9  * are met:
  10  * 1. Redistributions of source code must retain the above copyright
  11  *    notice, this list of conditions and the following disclaimer.
  12  * 2. Redistributions in binary form must reproduce the above copyright
  13  *    notice, this list of conditions and the following disclaimer in the
  14  *    documentation and/or other materials provided with the distribution.
  15  * 3. Neither the name of The DragonFly Project nor the names of its
  16  *    contributors may be used to endorse or promote products derived
  17  *    from this software without specific, prior written permission.
  18  *
  19  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
  20  * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
  21  * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
  22  * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
  23  * COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
  24  * INCIDENTAL, SPECIAL, EXEMPLARY OR CONSEQUENTIAL DAMAGES (INCLUDING,
  25  * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
  26  * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
  27  * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
  28  * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
  29  * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  30  * SUCH DAMAGE.
  31  *
  32  * $DragonFly: src/sys/kern/kern_spinlock.c,v 1.15 2008/06/04 04:34:54 nth Exp $
  33  */
  34
  35 #include <sys/param.h>
  36 #include <sys/systm.h>
  37 #include <sys/types.h>
  38 #include <sys/kernel.h>
  39 #include <sys/sysctl.h>
  40 #ifdef INVARIANTS
  41 #include <sys/proc.h>
  42 #endif
  43 #include <ddb/ddb.h>
  44 #include <machine/atomic.h>
  45 #include <machine/cpufunc.h>
  46 #include <machine/specialreg.h>
  47 #include <machine/clock.h>
  48 #include <sys/spinlock.h>
  49 #include <sys/spinlock2.h>
  50 #include <sys/ktr.h>
  51
  52 #define BACKOFF_INITIAL 1
  53 #define BACKOFF_LIMIT   256
  54
  55 #ifdef SMP
  56
  57 /*
  58  * Kernal Trace
  59  */
  60 #if !defined(KTR_SPIN_CONTENTION)
  61 #define KTR_SPIN_CONTENTION     KTR_ALL
  62 #endif
  63 #define SPIN_STRING     "spin=%p type=%c"
  64 #define SPIN_ARG_SIZE   (sizeof(void *) + sizeof(int))
  65
  66 KTR_INFO_MASTER(spin);
  67 KTR_INFO(KTR_SPIN_CONTENTION, spin, beg, 0, SPIN_STRING, SPIN_ARG_SIZE);
  68 KTR_INFO(KTR_SPIN_CONTENTION, spin, end, 1, SPIN_STRING, SPIN_ARG_SIZE);
  69 KTR_INFO(KTR_SPIN_CONTENTION, spin, backoff, 2,
  70          "spin=%p bo1=%d thr=%p bo=%d",
  71          ((2 * sizeof(void *)) + (2 * sizeof(int))));
  72 KTR_INFO(KTR_SPIN_CONTENTION, spin, bofail, 3, SPIN_STRING, SPIN_ARG_SIZE);
  73
  74 #define logspin(name, mtx, type)                        \
  75         KTR_LOG(spin_ ## name, mtx, type)
  76
  77 #define logspin_backoff(mtx, bo1, thr, bo)              \
  78         KTR_LOG(spin_backoff, mtx, bo1, thr, bo)
  79
  80 #ifdef INVARIANTS
  81 static int spin_lock_test_mode;
  82 #endif
  83
  84 static int64_t spinlocks_contested1;
  85 SYSCTL_QUAD(_debug, OID_AUTO, spinlocks_contested1, CTLFLAG_RD,
  86             &spinlocks_contested1, 0, "");
  87
  88 static int64_t spinlocks_contested2;
  89 SYSCTL_QUAD(_debug, OID_AUTO, spinlocks_contested2, CTLFLAG_RD,
  90             &spinlocks_contested2, 0, "");
  91
  92 static int spinlocks_backoff_limit = BACKOFF_LIMIT;
  93 SYSCTL_INT(_debug, OID_AUTO, spinlocks_bolim, CTLFLAG_RW,
  94            &spinlocks_backoff_limit, 0, "");
  95
  96 struct exponential_backoff {
  97         int backoff;
  98         int nsec;
  99         struct spinlock *mtx;
 100         sysclock_t base;
 101 };
 102 static int exponential_backoff(struct exponential_backoff *bo);
 103
 104 static __inline
 105 void
 106 exponential_init(struct exponential_backoff *bo, struct spinlock *mtx)
 107 {
 108         bo->backoff = BACKOFF_INITIAL;
 109         bo->nsec = 0;
 110         bo->mtx = mtx;
 111 }
 112
 113 /*
 114  * We were either contested due to another exclusive lock holder,
 115  * or due to the presence of shared locks.  We have to undo the mess
 116  * we created by returning the shared locks.
 117  *
 118  * If there was another exclusive lock holder only the exclusive bit
 119  * in value will be the only bit set.  We don't have to do anything since
 120  * restoration does not involve any work.
 121  *
 122  * Otherwise we successfully obtained the exclusive bit.  Attempt to
 123  * clear the shared bits.  If we are able to clear the shared bits
 124  * we win.  Otherwise we lose and we have to restore the shared bits
 125  * we couldn't clear (and also clear our exclusive bit).
 126  */
 127 int
 128 spin_trylock_wr_contested(globaldata_t gd, struct spinlock *mtx, int value)
 129 {
 130         int bit;
 131
 132         ++spinlocks_contested1;
 133         if ((value & SPINLOCK_EXCLUSIVE) == 0) {
 134                 while (value) {
 135                         bit = bsfl(value);
 136                         if (globaldata_find(bit)->gd_spinlock_rd == mtx) {
 137                                 atomic_swap_int(&mtx->lock, value);
 138                                 --gd->gd_spinlocks_wr;
 139                                 return (FALSE);
 140                         }
 141                         value &= ~(1 << bit);
 142                 }
 143                 return (TRUE);
 144         }
 145         --gd->gd_spinlocks_wr;
 146         return (FALSE);
 147 }
 148
 149 /*
 150  * We were either contested due to another exclusive lock holder,
 151  * or due to the presence of shared locks
 152  *
 153  * NOTE: If value indicates an exclusively held mutex, no shared bits
 154  * would have been set and we can throw away value.
 155  */
 156 void
 157 spin_lock_wr_contested(struct spinlock *mtx, int value)
 158 {
 159         struct exponential_backoff backoff;
 160         globaldata_t gd = mycpu;
 161         int bit;
 162         int mask;
 163
 164         /*
 165          * Wait until we can gain exclusive access vs another exclusive
 166          * holder.
 167          */
 168         exponential_init(&backoff, mtx);
 169         ++spinlocks_contested1;
 170         logspin(beg, mtx, 'w');
 171
 172         while (value & SPINLOCK_EXCLUSIVE) {
 173                 value = atomic_swap_int(&mtx->lock, SPINLOCK_EXCLUSIVE);
 174                 if (exponential_backoff(&backoff)) {
 175                         value &= ~SPINLOCK_EXCLUSIVE;
 176                         break;
 177                 }
 178         }
 179
 180         /*
 181          * Kill the cached shared bit for our own cpu.  This is the most
 182          * common case and there's no sense wasting cpu on it.  Since
 183          * spinlocks aren't recursive, we can't own a shared ref on the
 184          * spinlock while trying to get an exclusive one.
 185          *
 186          * If multiple bits are set do not stall on any single cpu.  Check
 187          * all cpus that have the cache bit set, then loop and check again,
 188          * until we've cleaned all the bits.
 189          */
 190         value &= ~gd->gd_cpumask;
 191
 192         while ((mask = value) != 0) {
 193                 while (mask) {
 194                         bit = bsfl(value);
 195                         if (globaldata_find(bit)->gd_spinlock_rd != mtx) {
 196                                 value &= ~(1 << bit);
 197                         } else if (exponential_backoff(&backoff)) {
 198                                 value = 0;
 199                                 break;
 200                         }
 201                         mask &= ~(1 << bit);
 202                 }
 203         }
 204         logspin(end, mtx, 'w');
 205 }
 206
 207 /*
 208  * The cache bit wasn't set for our cpu.  Loop until we can set the bit.
 209  * As with the spin_lock_rd() inline we need a memory fence after setting
 210  * gd_spinlock_rd to interlock against exclusive spinlocks waiting for
 211  * that field to clear.
 212  */
 213 void
 214 spin_lock_rd_contested(struct spinlock *mtx)
 215 {
 216         struct exponential_backoff backoff;
 217         globaldata_t gd = mycpu;
 218         int value = mtx->lock;
 219
 220         /*
 221          * Shortcut the op if we can just set the cache bit.  This case
 222          * occurs when the last lock was an exclusive lock.
 223          */
 224         while ((value & SPINLOCK_EXCLUSIVE) == 0) {
 225                 if (atomic_cmpset_int(&mtx->lock, value, value|gd->gd_cpumask))
 226                         return;
 227                 value = mtx->lock;
 228         }
 229
 230         exponential_init(&backoff, mtx);
 231         ++spinlocks_contested1;
 232
 233         logspin(beg, mtx, 'r');
 234
 235         while ((value & gd->gd_cpumask) == 0) {
 236                 if (value & SPINLOCK_EXCLUSIVE) {
 237                         gd->gd_spinlock_rd = NULL;
 238                         if (exponential_backoff(&backoff)) {
 239                                 gd->gd_spinlock_rd = mtx;
 240                                 break;
 241                         }
 242                         gd->gd_spinlock_rd = mtx;
 243                         cpu_mfence();
 244                 } else {
 245                         if (atomic_cmpset_int(&mtx->lock, value, value|gd->gd_cpumask))
 246                                 break;
 247                 }
 248                 value = mtx->lock;
 249         }
 250         logspin(end, mtx, 'r');
 251 }
 252
 253 /*
 254  * Handle exponential backoff and indefinite waits.
 255  *
 256  * If the system is handling a panic we hand the spinlock over to the caller
 257  * after 1 second.  After 10 seconds we attempt to print a debugger
 258  * backtrace.  We also run pending interrupts in order to allow a console
 259  * break into DDB.
 260  */
 261 static
 262 int
 263 exponential_backoff(struct exponential_backoff *bo)
 264 {
 265         sysclock_t count;
 266         int backoff;
 267
 268 #ifdef _RDTSC_SUPPORTED_
 269         if (cpu_feature & CPUID_TSC) {
 270                 backoff =
 271                 (((u_long)rdtsc() ^ (((u_long)curthread) >> 5)) &
 272                  (bo->backoff - 1)) + BACKOFF_INITIAL;
 273         } else
 274 #endif
 275                 backoff = bo->backoff;
 276         logspin_backoff(bo->mtx, bo->backoff, curthread, backoff);
 277
 278         /*
 279          * Quick backoff
 280          */
 281         for (; backoff; --backoff)
 282                 cpu_pause();
 283         if (bo->backoff < spinlocks_backoff_limit) {
 284                 bo->backoff <<= 1;
 285                 return (FALSE);
 286         } else {
 287                 bo->backoff = BACKOFF_INITIAL;
 288         }
 289
 290         logspin(bofail, bo->mtx, 'u');
 291
 292         /*
 293          * Indefinite
 294          */
 295         ++spinlocks_contested2;
 296         cpu_spinlock_contested();
 297         if (bo->nsec == 0) {
 298                 bo->base = sys_cputimer->count();
 299                 bo->nsec = 1;
 300         }
 301
 302         count = sys_cputimer->count();
 303         if (count - bo->base > sys_cputimer->freq) {
 304                 kprintf("spin_lock: %p, indefinite wait!\n", bo->mtx);
 305                 if (panicstr)
 306                         return (TRUE);
 307 #if defined(INVARIANTS) && defined(DDB)
 308                 if (spin_lock_test_mode) {
 309                         db_print_backtrace();
 310                         return (TRUE);
 311                 }
 312 #endif
 313                 ++bo->nsec;
 314 #if defined(INVARIANTS) && defined(DDB)
 315                 if (bo->nsec == 11)
 316                         db_print_backtrace();
 317 #endif
 318                 if (bo->nsec == 60)
 319                         panic("spin_lock: %p, indefinite wait!\n", bo->mtx);
 320                 splz();
 321                 bo->base = count;
 322         }
 323         return (FALSE);
 324 }
 325
 326 /*
 327  * If INVARIANTS is enabled various spinlock timing tests can be run
 328  * by setting debug.spin_lock_test:
 329  *
 330  *      1       Test the indefinite wait code
 331  *      2       Time the best-case exclusive lock overhead (spin_test_count)
 332  *      3       Time the best-case shared lock overhead (spin_test_count)
 333  */
 334
 335 #ifdef INVARIANTS
 336
 337 static int spin_test_count = 10000000;
 338 SYSCTL_INT(_debug, OID_AUTO, spin_test_count, CTLFLAG_RW, &spin_test_count, 0, "");
 339
 340 static int
 341 sysctl_spin_lock_test(SYSCTL_HANDLER_ARGS)
 342 {
 343         struct spinlock mtx;
 344         int error;
 345         int value = 0;
 346         int i;
 347
 348         if ((error = suser(curthread)) != 0)
 349                 return (error);
 350         if ((error = SYSCTL_IN(req, &value, sizeof(value))) != 0)
 351                 return (error);
 352
 353         /*
 354          * Indefinite wait test
 355          */
 356         if (value == 1) {
 357                 spin_init(&mtx);
 358                 spin_lock_wr(&mtx);     /* force an indefinite wait */
 359                 spin_lock_test_mode = 1;
 360                 spin_lock_wr(&mtx);
 361                 spin_unlock_wr(&mtx);   /* Clean up the spinlock count */
 362                 spin_unlock_wr(&mtx);
 363                 spin_lock_test_mode = 0;
 364         }
 365
 366         /*
 367          * Time best-case exclusive spinlocks
 368          */
 369         if (value == 2) {
 370                 globaldata_t gd = mycpu;
 371
 372                 spin_init(&mtx);
 373                 for (i = spin_test_count; i > 0; --i) {
 374                     spin_lock_wr_quick(gd, &mtx);
 375                     spin_unlock_wr_quick(gd, &mtx);
 376                 }
 377         }
 378
 379         /*
 380          * Time best-case shared spinlocks
 381          */
 382         if (value == 3) {
 383                 globaldata_t gd = mycpu;
 384
 385                 spin_init(&mtx);
 386                 for (i = spin_test_count; i > 0; --i) {
 387                     spin_lock_rd_quick(gd, &mtx);
 388                     spin_unlock_rd_quick(gd, &mtx);
 389                 }
 390         }
 391         return (0);
 392 }
 393
 394 SYSCTL_PROC(_debug, KERN_PROC_ALL, spin_lock_test, CTLFLAG_RW|CTLTYPE_INT,
 395         0, 0, sysctl_spin_lock_test, "I", "Test spinlock wait code");
 396
 397 #endif  /* INVARIANTS */
 398 #endif  /* SMP */