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 1.11 2007/07/02 16:51:58 dillon 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/clock.h> 47 #include <sys/spinlock.h> 48 #include <sys/spinlock2.h> 49 #include <sys/ktr.h> 50 51 #define BACKOFF_INITIAL 1 52 #define BACKOFF_LIMIT 256 53 54 #ifdef SMP 55 56 /* 57 * Kernal Trace 58 */ 59 #if !defined(KTR_SPIN_CONTENTION) 60 #define KTR_SPIN_CONTENTION KTR_ALL 61 #endif 62 #define SPIN_STRING "spin=%p type=%c" 63 #define SPIN_ARG_SIZE (sizeof(void *) + sizeof(int)) 64 65 KTR_INFO_MASTER(spin); 66 KTR_INFO(KTR_SPIN_CONTENTION, spin, beg, 0, SPIN_STRING, SPIN_ARG_SIZE); 67 KTR_INFO(KTR_SPIN_CONTENTION, spin, end, 1, SPIN_STRING, SPIN_ARG_SIZE); 68 69 #define logspin(name, mtx, type) \ 70 KTR_LOG(spin_ ## name, mtx, type) 71 72 #ifdef INVARIANTS 73 static int spin_lock_test_mode; 74 #endif 75 76 static int64_t spinlocks_contested1; 77 SYSCTL_QUAD(_debug, OID_AUTO, spinlocks_contested1, CTLFLAG_RD, &spinlocks_contested1, 0, ""); 78 static int64_t spinlocks_contested2; 79 SYSCTL_QUAD(_debug, OID_AUTO, spinlocks_contested2, CTLFLAG_RD, &spinlocks_contested2, 0, ""); 80 81 struct exponential_backoff { 82 int backoff; 83 int nsec; 84 struct spinlock *mtx; 85 sysclock_t base; 86 }; 87 static int exponential_backoff(struct exponential_backoff *bo); 88 89 static __inline 90 void 91 exponential_init(struct exponential_backoff *bo, struct spinlock *mtx) 92 { 93 bo->backoff = BACKOFF_INITIAL; 94 bo->nsec = 0; 95 bo->mtx = mtx; 96 } 97 98 /* 99 * We were either contested due to another exclusive lock holder, 100 * or due to the presence of shared locks. We have to undo the mess 101 * we created by returning the shared locks. 102 * 103 * If there was another exclusive lock holder only the exclusive bit 104 * in value will be the only bit set. We don't have to do anything since 105 * restoration does not involve any work. 106 * 107 * Otherwise we successfully obtained the exclusive bit. Attempt to 108 * clear the shared bits. If we are able to clear the shared bits 109 * we win. Otherwise we lose and we have to restore the shared bits 110 * we couldn't clear (and also clear our exclusive bit). 111 */ 112 int 113 spin_trylock_wr_contested(struct spinlock *mtx, int value) 114 { 115 int bit; 116 117 ++spinlocks_contested1; 118 if ((value & SPINLOCK_EXCLUSIVE) == 0) { 119 while (value) { 120 bit = bsfl(value); 121 if (globaldata_find(bit)->gd_spinlock_rd != mtx) { 122 atomic_swap_int(&mtx->lock, value); 123 return (FALSE); 124 } 125 value &= ~(1 << bit); 126 } 127 return (TRUE); 128 } 129 return (FALSE); 130 } 131 132 /* 133 * We were either contested due to another exclusive lock holder, 134 * or due to the presence of shared locks 135 * 136 * NOTE: If value indicates an exclusively held mutex, no shared bits 137 * would have been set and we can throw away value. 138 */ 139 void 140 spin_lock_wr_contested(struct spinlock *mtx, int value) 141 { 142 struct exponential_backoff backoff; 143 globaldata_t gd = mycpu; 144 int bit; 145 int mask; 146 147 /* 148 * Wait until we can gain exclusive access vs another exclusive 149 * holder. 150 */ 151 exponential_init(&backoff, mtx); 152 ++spinlocks_contested1; 153 logspin(beg, mtx, 'w'); 154 155 while (value & SPINLOCK_EXCLUSIVE) { 156 value = atomic_swap_int(&mtx->lock, SPINLOCK_EXCLUSIVE); 157 if (exponential_backoff(&backoff)) { 158 value &= ~SPINLOCK_EXCLUSIVE; 159 break; 160 } 161 } 162 163 /* 164 * Kill the cached shared bit for our own cpu. This is the most 165 * common case and there's no sense wasting cpu on it. Since 166 * spinlocks aren't recursive, we can't own a shared ref on the 167 * spinlock while trying to get an exclusive one. 168 * 169 * If multiple bits are set do not stall on any single cpu. Check 170 * all cpus that have the cache bit set, then loop and check again, 171 * until we've cleaned all the bits. 172 */ 173 value &= ~gd->gd_cpumask; 174 175 while ((mask = value) != 0) { 176 while (mask) { 177 bit = bsfl(value); 178 if (globaldata_find(bit)->gd_spinlock_rd != mtx) { 179 value &= ~(1 << bit); 180 } else if (exponential_backoff(&backoff)) { 181 value = 0; 182 break; 183 } 184 mask &= ~(1 << bit); 185 } 186 } 187 logspin(end, mtx, 'w'); 188 } 189 190 /* 191 * The cache bit wasn't set for our cpu. Loop until we can set the bit. 192 * As with the spin_lock_rd() inline we need a memory fence after setting 193 * gd_spinlock_rd to interlock against exclusive spinlocks waiting for 194 * that field to clear. 195 */ 196 void 197 spin_lock_rd_contested(struct spinlock *mtx) 198 { 199 struct exponential_backoff backoff; 200 globaldata_t gd = mycpu; 201 int value = mtx->lock; 202 203 /* 204 * Shortcut the op if we can just set the cache bit. This case 205 * occurs when the last lock was an exclusive lock. 206 */ 207 while ((value & SPINLOCK_EXCLUSIVE) == 0) { 208 if (atomic_cmpset_int(&mtx->lock, value, value|gd->gd_cpumask)) 209 return; 210 value = mtx->lock; 211 } 212 213 exponential_init(&backoff, mtx); 214 ++spinlocks_contested1; 215 216 logspin(beg, mtx, 'r'); 217 218 while ((value & gd->gd_cpumask) == 0) { 219 if (value & SPINLOCK_EXCLUSIVE) { 220 gd->gd_spinlock_rd = NULL; 221 if (exponential_backoff(&backoff)) { 222 gd->gd_spinlock_rd = mtx; 223 break; 224 } 225 gd->gd_spinlock_rd = mtx; 226 cpu_mfence(); 227 } else { 228 if (atomic_cmpset_int(&mtx->lock, value, value|gd->gd_cpumask)) 229 break; 230 } 231 value = mtx->lock; 232 } 233 logspin(end, mtx, 'r'); 234 } 235 236 /* 237 * Handle exponential backoff and indefinite waits. 238 * 239 * If the system is handling a panic we hand the spinlock over to the caller 240 * after 1 second. After 10 seconds we attempt to print a debugger 241 * backtrace. We also run pending interrupts in order to allow a console 242 * break into DDB. 243 */ 244 static 245 int 246 exponential_backoff(struct exponential_backoff *bo) 247 { 248 sysclock_t count; 249 int i; 250 251 /* 252 * Quick backoff 253 */ 254 for (i = 0; i < bo->backoff; ++i) 255 cpu_nop(); 256 if (bo->backoff < BACKOFF_LIMIT) { 257 bo->backoff <<= 1; 258 return (FALSE); 259 } 260 261 /* 262 * Indefinite 263 */ 264 ++spinlocks_contested2; 265 cpu_spinlock_contested(); 266 if (bo->nsec == 0) { 267 bo->base = sys_cputimer->count(); 268 bo->nsec = 1; 269 } 270 271 count = sys_cputimer->count(); 272 if (count - bo->base > sys_cputimer->freq) { 273 kprintf("spin_lock: %p, indefinite wait!\n", bo->mtx); 274 if (panicstr) 275 return (TRUE); 276 #if defined(INVARIANTS) && defined(DDB) 277 if (spin_lock_test_mode) { 278 db_print_backtrace(); 279 return (TRUE); 280 } 281 #endif 282 ++bo->nsec; 283 #if defined(INVARIANTS) && defined(DDB) 284 if (bo->nsec == 11) 285 db_print_backtrace(); 286 #endif 287 if (bo->nsec == 60) 288 panic("spin_lock: %p, indefinite wait!\n", bo->mtx); 289 splz(); 290 bo->base = count; 291 } 292 return (FALSE); 293 } 294 295 /* 296 * If INVARIANTS is enabled various spinlock timing tests can be run 297 * by setting debug.spin_lock_test: 298 * 299 * 1 Test the indefinite wait code 300 * 2 Time the best-case exclusive lock overhead (spin_test_count) 301 * 3 Time the best-case shared lock overhead (spin_test_count) 302 */ 303 304 #ifdef INVARIANTS 305 306 static int spin_test_count = 10000000; 307 SYSCTL_INT(_debug, OID_AUTO, spin_test_count, CTLFLAG_RW, &spin_test_count, 0, ""); 308 309 static int 310 sysctl_spin_lock_test(SYSCTL_HANDLER_ARGS) 311 { 312 struct spinlock mtx; 313 int error; 314 int value = 0; 315 int i; 316 317 if ((error = suser(curthread)) != 0) 318 return (error); 319 if ((error = SYSCTL_IN(req, &value, sizeof(value))) != 0) 320 return (error); 321 322 /* 323 * Indefinite wait test 324 */ 325 if (value == 1) { 326 spin_init(&mtx); 327 spin_lock_wr(&mtx); /* force an indefinite wait */ 328 spin_lock_test_mode = 1; 329 spin_lock_wr(&mtx); 330 spin_unlock_wr(&mtx); /* Clean up the spinlock count */ 331 spin_unlock_wr(&mtx); 332 spin_lock_test_mode = 0; 333 } 334 335 /* 336 * Time best-case exclusive spinlocks 337 */ 338 if (value == 2) { 339 globaldata_t gd = mycpu; 340 341 spin_init(&mtx); 342 for (i = spin_test_count; i > 0; --i) { 343 spin_lock_wr_quick(gd, &mtx); 344 spin_unlock_wr_quick(gd, &mtx); 345 } 346 } 347 348 /* 349 * Time best-case shared spinlocks 350 */ 351 if (value == 3) { 352 globaldata_t gd = mycpu; 353 354 spin_init(&mtx); 355 for (i = spin_test_count; i > 0; --i) { 356 spin_lock_rd_quick(gd, &mtx); 357 spin_unlock_rd_quick(gd, &mtx); 358 } 359 } 360 return (0); 361 } 362 363 SYSCTL_PROC(_debug, KERN_PROC_ALL, spin_lock_test, CTLFLAG_RW|CTLTYPE_INT, 364 0, 0, sysctl_spin_lock_test, "I", "Test spinlock wait code"); 365 366 #endif /* INVARIANTS */ 367 #endif /* SMP */ 368