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 1.12 2008/05/04 04:48:47 sephe 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(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                                 return (FALSE);
 139                         }
 140                         value &= ~(1 << bit);
 141                 }
 142                 return (TRUE);
 143         }
 144         return (FALSE);
 145 }
 146
 147 /*
 148  * We were either contested due to another exclusive lock holder,
 149  * or due to the presence of shared locks
 150  *
 151  * NOTE: If value indicates an exclusively held mutex, no shared bits
 152  * would have been set and we can throw away value.
 153  */
 154 void
 155 spin_lock_wr_contested(struct spinlock *mtx, int value)
 156 {
 157         struct exponential_backoff backoff;
 158         globaldata_t gd = mycpu;
 159         int bit;
 160         int mask;
 161
 162         /*
 163          * Wait until we can gain exclusive access vs another exclusive
 164          * holder.
 165          */
 166         exponential_init(&backoff, mtx);
 167         ++spinlocks_contested1;
 168         logspin(beg, mtx, 'w');
 169
 170         while (value & SPINLOCK_EXCLUSIVE) {
 171                 value = atomic_swap_int(&mtx->lock, SPINLOCK_EXCLUSIVE);
 172                 if (exponential_backoff(&backoff)) {
 173                         value &= ~SPINLOCK_EXCLUSIVE;
 174                         break;
 175                 }
 176         }
 177
 178         /*
 179          * Kill the cached shared bit for our own cpu.  This is the most
 180          * common case and there's no sense wasting cpu on it.  Since
 181          * spinlocks aren't recursive, we can't own a shared ref on the
 182          * spinlock while trying to get an exclusive one.
 183          *
 184          * If multiple bits are set do not stall on any single cpu.  Check
 185          * all cpus that have the cache bit set, then loop and check again,
 186          * until we've cleaned all the bits.
 187          */
 188         value &= ~gd->gd_cpumask;
 189
 190         while ((mask = value) != 0) {
 191                 while (mask) {
 192                         bit = bsfl(value);
 193                         if (globaldata_find(bit)->gd_spinlock_rd != mtx) {
 194                                 value &= ~(1 << bit);
 195                         } else if (exponential_backoff(&backoff)) {
 196                                 value = 0;
 197                                 break;
 198                         }
 199                         mask &= ~(1 << bit);
 200                 }
 201         }
 202         logspin(end, mtx, 'w');
 203 }
 204
 205 /*
 206  * The cache bit wasn't set for our cpu.  Loop until we can set the bit.
 207  * As with the spin_lock_rd() inline we need a memory fence after setting
 208  * gd_spinlock_rd to interlock against exclusive spinlocks waiting for
 209  * that field to clear.
 210  */
 211 void
 212 spin_lock_rd_contested(struct spinlock *mtx)
 213 {
 214         struct exponential_backoff backoff;
 215         globaldata_t gd = mycpu;
 216         int value = mtx->lock;
 217
 218         /*
 219          * Shortcut the op if we can just set the cache bit.  This case
 220          * occurs when the last lock was an exclusive lock.
 221          */
 222         while ((value & SPINLOCK_EXCLUSIVE) == 0) {
 223                 if (atomic_cmpset_int(&mtx->lock, value, value|gd->gd_cpumask))
 224                         return;
 225                 value = mtx->lock;
 226         }
 227
 228         exponential_init(&backoff, mtx);
 229         ++spinlocks_contested1;
 230
 231         logspin(beg, mtx, 'r');
 232
 233         while ((value & gd->gd_cpumask) == 0) {
 234                 if (value & SPINLOCK_EXCLUSIVE) {
 235                         gd->gd_spinlock_rd = NULL;
 236                         if (exponential_backoff(&backoff)) {
 237                                 gd->gd_spinlock_rd = mtx;
 238                                 break;
 239                         }
 240                         gd->gd_spinlock_rd = mtx;
 241                         cpu_mfence();
 242                 } else {
 243                         if (atomic_cmpset_int(&mtx->lock, value, value|gd->gd_cpumask))
 244                                 break;
 245                 }
 246                 value = mtx->lock;
 247         }
 248         logspin(end, mtx, 'r');
 249 }
 250
 251 /*
 252  * Handle exponential backoff and indefinite waits.
 253  *
 254  * If the system is handling a panic we hand the spinlock over to the caller
 255  * after 1 second.  After 10 seconds we attempt to print a debugger
 256  * backtrace.  We also run pending interrupts in order to allow a console
 257  * break into DDB.
 258  */
 259 static
 260 int
 261 exponential_backoff(struct exponential_backoff *bo)
 262 {
 263         sysclock_t count;
 264         int backoff;
 265
 266 #ifdef _RDTSC_SUPPORTED_
 267         if (cpu_feature & CPUID_TSC) {
 268                 backoff =
 269                 ((u_long)rdtsc() ^ (((u_long)curthread) >> 5)) % bo->backoff
 270                 + BACKOFF_INITIAL;
 271         } else
 272 #endif
 273                 backoff = bo->backoff;
 274         logspin_backoff(bo->mtx, bo->backoff, curthread, backoff);
 275
 276         /*
 277          * Quick backoff
 278          */
 279         for (; backoff; --backoff)
 280                 cpu_nop();
 281         if (bo->backoff < spinlocks_backoff_limit) {
 282                 bo->backoff <<= 1;
 283                 return (FALSE);
 284         } else {
 285                 bo->backoff = BACKOFF_INITIAL;
 286         }
 287
 288         logspin(bofail, bo->mtx, 'u');
 289
 290         /*
 291          * Indefinite
 292          */
 293         ++spinlocks_contested2;
 294         cpu_spinlock_contested();
 295         if (bo->nsec == 0) {
 296                 bo->base = sys_cputimer->count();
 297                 bo->nsec = 1;
 298         }
 299
 300         count = sys_cputimer->count();
 301         if (count - bo->base > sys_cputimer->freq) {
 302                 kprintf("spin_lock: %p, indefinite wait!\n", bo->mtx);
 303                 if (panicstr)
 304                         return (TRUE);
 305 #if defined(INVARIANTS) && defined(DDB)
 306                 if (spin_lock_test_mode) {
 307                         db_print_backtrace();
 308                         return (TRUE);
 309                 }
 310 #endif
 311                 ++bo->nsec;
 312 #if defined(INVARIANTS) && defined(DDB)
 313                 if (bo->nsec == 11)
 314                         db_print_backtrace();
 315 #endif
 316                 if (bo->nsec == 60)
 317                         panic("spin_lock: %p, indefinite wait!\n", bo->mtx);
 318                 splz();
 319                 bo->base = count;
 320         }
 321         return (FALSE);
 322 }
 323
 324 /*
 325  * If INVARIANTS is enabled various spinlock timing tests can be run
 326  * by setting debug.spin_lock_test:
 327  *
 328  *      1       Test the indefinite wait code
 329  *      2       Time the best-case exclusive lock overhead (spin_test_count)
 330  *      3       Time the best-case shared lock overhead (spin_test_count)
 331  */
 332
 333 #ifdef INVARIANTS
 334
 335 static int spin_test_count = 10000000;
 336 SYSCTL_INT(_debug, OID_AUTO, spin_test_count, CTLFLAG_RW, &spin_test_count, 0, "");
 337
 338 static int
 339 sysctl_spin_lock_test(SYSCTL_HANDLER_ARGS)
 340 {
 341         struct spinlock mtx;
 342         int error;
 343         int value = 0;
 344         int i;
 345
 346         if ((error = suser(curthread)) != 0)
 347                 return (error);
 348         if ((error = SYSCTL_IN(req, &value, sizeof(value))) != 0)
 349                 return (error);
 350
 351         /*
 352          * Indefinite wait test
 353          */
 354         if (value == 1) {
 355                 spin_init(&mtx);
 356                 spin_lock_wr(&mtx);     /* force an indefinite wait */
 357                 spin_lock_test_mode = 1;
 358                 spin_lock_wr(&mtx);
 359                 spin_unlock_wr(&mtx);   /* Clean up the spinlock count */
 360                 spin_unlock_wr(&mtx);
 361                 spin_lock_test_mode = 0;
 362         }
 363
 364         /*
 365          * Time best-case exclusive spinlocks
 366          */
 367         if (value == 2) {
 368                 globaldata_t gd = mycpu;
 369
 370                 spin_init(&mtx);
 371                 for (i = spin_test_count; i > 0; --i) {
 372                     spin_lock_wr_quick(gd, &mtx);
 373                     spin_unlock_wr_quick(gd, &mtx);
 374                 }
 375         }
 376
 377         /*
 378          * Time best-case shared spinlocks
 379          */
 380         if (value == 3) {
 381                 globaldata_t gd = mycpu;
 382
 383                 spin_init(&mtx);
 384                 for (i = spin_test_count; i > 0; --i) {
 385                     spin_lock_rd_quick(gd, &mtx);
 386                     spin_unlock_rd_quick(gd, &mtx);
 387                 }
 388         }
 389         return (0);
 390 }
 391
 392 SYSCTL_PROC(_debug, KERN_PROC_ALL, spin_lock_test, CTLFLAG_RW|CTLTYPE_INT,
 393         0, 0, sysctl_spin_lock_test, "I", "Test spinlock wait code");
 394
 395 #endif  /* INVARIANTS */
 396 #endif  /* SMP */