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 33 /* 34 * The implementation is designed to avoid looping when compatible operations 35 * are executed. 36 * 37 * To acquire a spinlock we first increment counta. Then we check if counta 38 * meets our requirements. For an exclusive spinlock it must be 1, of a 39 * shared spinlock it must either be 1 or the SHARED_SPINLOCK bit must be set. 40 * 41 * Shared spinlock failure case: Decrement the count, loop until we can 42 * transition from 0 to SHARED_SPINLOCK|1, or until we find SHARED_SPINLOCK 43 * is set and increment the count. 44 * 45 * Exclusive spinlock failure case: While maintaining the count, clear the 46 * SHARED_SPINLOCK flag unconditionally. Then use an atomic add to transfer 47 * the count from the low bits to the high bits of counta. Then loop until 48 * all low bits are 0. Once the low bits drop to 0 we can transfer the 49 * count back with an atomic_cmpset_int(), atomically, and return. 50 */ 51 #include <sys/param.h> 52 #include <sys/systm.h> 53 #include <sys/types.h> 54 #include <sys/kernel.h> 55 #include <sys/sysctl.h> 56 #ifdef INVARIANTS 57 #include <sys/proc.h> 58 #endif 59 #include <sys/priv.h> 60 #include <machine/atomic.h> 61 #include <machine/cpu.h> 62 #include <machine/cpufunc.h> 63 #include <machine/specialreg.h> 64 #include <machine/clock.h> 65 #include <sys/indefinite2.h> 66 #include <sys/spinlock.h> 67 #include <sys/spinlock2.h> 68 #include <sys/ktr.h> 69 70 #ifdef _KERNEL_VIRTUAL 71 #include <pthread.h> 72 #endif 73 74 struct spinlock pmap_spin = SPINLOCK_INITIALIZER(pmap_spin, "pmap_spin"); 75 76 /* 77 * Kernal Trace 78 */ 79 #if !defined(KTR_SPIN_CONTENTION) 80 #define KTR_SPIN_CONTENTION KTR_ALL 81 #endif 82 #define SPIN_STRING "spin=%p type=%c" 83 #define SPIN_ARG_SIZE (sizeof(void *) + sizeof(int)) 84 85 KTR_INFO_MASTER(spin); 86 #if 0 87 KTR_INFO(KTR_SPIN_CONTENTION, spin, beg, 0, SPIN_STRING, SPIN_ARG_SIZE); 88 KTR_INFO(KTR_SPIN_CONTENTION, spin, end, 1, SPIN_STRING, SPIN_ARG_SIZE); 89 #endif 90 91 #define logspin(name, spin, type) \ 92 KTR_LOG(spin_ ## name, spin, type) 93 94 #ifdef INVARIANTS 95 static int spin_lock_test_mode; 96 #endif 97 98 #ifdef DEBUG_LOCKS_LATENCY 99 100 static long spinlocks_add_latency; 101 SYSCTL_LONG(_debug, OID_AUTO, spinlocks_add_latency, CTLFLAG_RW, 102 &spinlocks_add_latency, 0, 103 "Add spinlock latency"); 104 105 #endif 106 107 /* 108 * We contested due to another exclusive lock holder. We lose. 109 * 110 * We have to unwind the attempt and may acquire the spinlock 111 * anyway while doing so. 112 */ 113 int 114 spin_trylock_contested(struct spinlock *spin) 115 { 116 globaldata_t gd = mycpu; 117 118 /* 119 * Handle degenerate case, else fail. 120 */ 121 if (atomic_cmpset_int(&spin->counta, SPINLOCK_SHARED|0, 1)) 122 return TRUE; 123 /*atomic_add_int(&spin->counta, -1);*/ 124 --gd->gd_spinlocks; 125 crit_exit_raw(gd->gd_curthread); 126 127 return (FALSE); 128 } 129 130 /* 131 * The spin_lock() inline was unable to acquire the lock and calls this 132 * function with spin->counta already incremented, passing (spin->counta - 1) 133 * to the function (the result of the inline's fetchadd). 134 * 135 * Note that we implement both exclusive and shared spinlocks, so we cannot 136 * use atomic_swap_int(). Instead, we try to use atomic_fetchadd_int() 137 * to put most of the burden on the cpu. Atomic_cmpset_int() (cmpxchg) 138 * can cause a lot of unnecessary looping in situations where it is just 139 * trying to increment the count. 140 * 141 * Similarly, we leave the SHARED flag intact and incur slightly more 142 * overhead when switching from shared to exclusive. This allows us to 143 * use atomic_fetchadd_int() for both spinlock types in the critical 144 * path. 145 * 146 * Backoff algorithms can create even worse starvation problems, particularly 147 * on multi-socket cpus, and don't really improve performance when a lot 148 * of cores are contending. However, if we are contested on an exclusive 149 * lock due to a large number of shared locks being present, we throw in 150 * extra cpu_pause()'s to account for the necessary time it will take other 151 * cores to contend among themselves and release their shared locks. 152 */ 153 void 154 _spin_lock_contested(struct spinlock *spin, const char *ident, int value) 155 { 156 indefinite_info_t info; 157 uint32_t ovalue; 158 159 /* 160 * WARNING! Caller has already incremented the lock. We must 161 * increment the count value (from the inline's fetch-add) 162 * to match. 163 * 164 * Handle the degenerate case where the spinlock is flagged SHARED 165 * with only our reference. We can convert it to EXCLUSIVE. 166 */ 167 ++value; 168 if (value == (SPINLOCK_SHARED | 1)) { 169 if (atomic_cmpset_int(&spin->counta, SPINLOCK_SHARED | 1, 1)) 170 return; 171 } 172 indefinite_init(&info, ident, 0, 'S'); 173 174 /* 175 * Transfer our exclusive request to the high bits and clear the 176 * SPINLOCK_SHARED bit if it was set. This makes the spinlock 177 * appear exclusive, preventing any NEW shared or exclusive 178 * spinlocks from being obtained while we wait for existing 179 * shared or exclusive holders to unlock. 180 * 181 * Don't tread on earlier exclusive waiters by stealing the lock 182 * away early if the low bits happen to now be 1. 183 * 184 * The shared unlock understands that this may occur. 185 */ 186 ovalue = atomic_fetchadd_int(&spin->counta, SPINLOCK_EXCLWAIT - 1); 187 ovalue += SPINLOCK_EXCLWAIT - 1; 188 if (value & SPINLOCK_SHARED) { 189 atomic_clear_int(&spin->counta, SPINLOCK_SHARED); 190 ovalue &= ~SPINLOCK_SHARED; 191 } 192 193 /* 194 * Spin until we can acquire a low-count of 1. 195 */ 196 for (;;) { 197 /* 198 * If the low bits are zero, try to acquire the exclusive lock 199 * by transfering our high bit reservation to the low bits. 200 * 201 * NOTE: Reading spin->counta prior to the swap is extremely 202 * important on multi-chip/many-core boxes. On 48-core 203 * this one change improves fully concurrent all-cores 204 * compiles by 100% or better. 205 * 206 * I can't emphasize enough how important the pre-read 207 * is in preventing hw cache bus armageddon on 208 * multi-chip systems. And on single-chip/multi-core 209 * systems it just doesn't hurt. 210 */ 211 cpu_ccfence(); 212 if ((ovalue & (SPINLOCK_EXCLWAIT - 1)) == 0) { 213 if (atomic_fcmpset_int(&spin->counta, &ovalue, 214 (ovalue - SPINLOCK_EXCLWAIT) | 1)) { 215 break; 216 } 217 continue; 218 } 219 220 /* 221 * Throw in extra cpu_pause()'s when we are waiting on 222 * multiple other shared lock holders to release (the 223 * indefinite_check() also throws one in). 224 * 225 * We know these are shared lock holders when the count 226 * is larger than 1, because an exclusive lock holder can 227 * only have one count. Do this optimization only when 228 * the number of shared lock holders is 3 or greater. 229 */ 230 ovalue &= SPINLOCK_EXCLWAIT - 1; 231 while (ovalue > 2) { 232 cpu_pause(); 233 cpu_pause(); 234 --ovalue; 235 } 236 237 if (indefinite_check(&info)) 238 break; 239 /* 240 * ovalue was wrong anyway, just reload 241 */ 242 ovalue = spin->counta; 243 } 244 indefinite_done(&info); 245 } 246 247 /* 248 * The spin_lock_shared() inline was unable to acquire the lock and calls 249 * this function with spin->counta already incremented. 250 * 251 * This is not in the critical path unless there is contention between 252 * shared and exclusive holders. 253 */ 254 void 255 _spin_lock_shared_contested(struct spinlock *spin, const char *ident) 256 { 257 indefinite_info_t info; 258 uint32_t ovalue; 259 260 indefinite_init(&info, ident, 0, 's'); 261 262 /* 263 * Undo the inline's increment. 264 */ 265 cpu_pause(); 266 ovalue = atomic_fetchadd_int(&spin->counta, -1); 267 ovalue += -1; 268 269 #ifdef DEBUG_LOCKS_LATENCY 270 long j; 271 for (j = spinlocks_add_latency; j > 0; --j) 272 cpu_ccfence(); 273 #endif 274 275 for (;;) { 276 /* 277 * Loop until we can acquire the shared spinlock. Note that 278 * the low bits can be zero while the high EXCLWAIT bits are 279 * non-zero. In this situation exclusive requesters have 280 * priority (otherwise shared users on multiple cpus can hog 281 * the spinlnock). 282 * 283 * NOTE: Reading spin->counta prior to the swap is extremely 284 * important on multi-chip/many-core boxes. On 48-core 285 * this one change improves fully concurrent all-cores 286 * compiles by 100% or better. 287 * 288 * I can't emphasize enough how important the pre-read 289 * is in preventing hw cache bus armageddon on 290 * multi-chip systems. And on single-chip/multi-core 291 * systems it just doesn't hurt. 292 */ 293 cpu_ccfence(); 294 if (ovalue == 0) { 295 if (atomic_fcmpset_int(&spin->counta, &ovalue, 296 SPINLOCK_SHARED | 1)) { 297 break; 298 } 299 continue; 300 } 301 if (ovalue & SPINLOCK_SHARED) { 302 /* 303 * Go for the increment, improving the exclusive 304 * to multiple-readers transition. 305 */ 306 ovalue = atomic_fetchadd_int(&spin->counta, 1); 307 /* ovalue += 1; NOT NEEDED */ 308 if (ovalue & SPINLOCK_SHARED) 309 break; 310 ovalue = atomic_fetchadd_int(&spin->counta, -1); 311 ovalue += -1; 312 continue; 313 } 314 if (indefinite_check(&info)) 315 break; 316 /* 317 * ovalue was wrong anyway, just reload 318 */ 319 ovalue = spin->counta; 320 } 321 indefinite_done(&info); 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 "Number of iterations to use for spinlock wait code test"); 338 339 static int 340 sysctl_spin_lock_test(SYSCTL_HANDLER_ARGS) 341 { 342 struct spinlock spin; 343 int error; 344 int value = 0; 345 int i; 346 347 if ((error = priv_check(curthread, PRIV_ROOT)) != 0) 348 return (error); 349 if ((error = SYSCTL_IN(req, &value, sizeof(value))) != 0) 350 return (error); 351 352 /* 353 * Indefinite wait test 354 */ 355 if (value == 1) { 356 spin_init(&spin, "sysctllock"); 357 spin_lock(&spin); /* force an indefinite wait */ 358 spin_lock_test_mode = 1; 359 spin_lock(&spin); 360 spin_unlock(&spin); /* Clean up the spinlock count */ 361 spin_unlock(&spin); 362 spin_lock_test_mode = 0; 363 } 364 365 /* 366 * Time best-case exclusive spinlocks 367 */ 368 if (value == 2) { 369 globaldata_t gd = mycpu; 370 371 spin_init(&spin, "sysctllocktest"); 372 for (i = spin_test_count; i > 0; --i) { 373 _spin_lock_quick(gd, &spin, "test"); 374 spin_unlock_quick(gd, &spin); 375 } 376 } 377 378 return (0); 379 } 380 381 SYSCTL_PROC(_debug, KERN_PROC_ALL, spin_lock_test, CTLFLAG_RW|CTLTYPE_INT, 382 0, 0, sysctl_spin_lock_test, "I", "Test spinlock wait code"); 383 384 #endif /* INVARIANTS */ 385