| 1 | /* $NetBSD: subr_psref.c,v 1.6 2016/11/09 09:00:46 ozaki-r Exp $ */ |
| 2 | |
| 3 | /*- |
| 4 | * Copyright (c) 2016 The NetBSD Foundation, Inc. |
| 5 | * All rights reserved. |
| 6 | * |
| 7 | * This code is derived from software contributed to The NetBSD Foundation |
| 8 | * by Taylor R. Campbell. |
| 9 | * |
| 10 | * Redistribution and use in source and binary forms, with or without |
| 11 | * modification, are permitted provided that the following conditions |
| 12 | * are met: |
| 13 | * 1. Redistributions of source code must retain the above copyright |
| 14 | * notice, this list of conditions and the following disclaimer. |
| 15 | * 2. Redistributions in binary form must reproduce the above copyright |
| 16 | * notice, this list of conditions and the following disclaimer in the |
| 17 | * documentation and/or other materials provided with the distribution. |
| 18 | * |
| 19 | * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS |
| 20 | * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED |
| 21 | * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR |
| 22 | * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS |
| 23 | * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR |
| 24 | * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF |
| 25 | * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS |
| 26 | * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN |
| 27 | * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) |
| 28 | * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE |
| 29 | * POSSIBILITY OF SUCH DAMAGE. |
| 30 | */ |
| 31 | |
| 32 | /* |
| 33 | * Passive references |
| 34 | * |
| 35 | * Passive references are references to objects that guarantee the |
| 36 | * object will not be destroyed until the reference is released. |
| 37 | * |
| 38 | * Passive references require no interprocessor synchronization to |
| 39 | * acquire or release. However, destroying the target of passive |
| 40 | * references requires expensive interprocessor synchronization -- |
| 41 | * xcalls to determine on which CPUs the object is still in use. |
| 42 | * |
| 43 | * Passive references may be held only on a single CPU and by a |
| 44 | * single LWP. They require the caller to allocate a little stack |
| 45 | * space, a struct psref object. Sleeping while a passive |
| 46 | * reference is held is allowed, provided that the owner's LWP is |
| 47 | * bound to a CPU -- e.g., the owner is a softint or a bound |
| 48 | * kthread. However, sleeping should be kept to a short duration, |
| 49 | * e.g. sleeping on an adaptive lock. |
| 50 | * |
| 51 | * Passive references serve as an intermediate stage between |
| 52 | * reference counting and passive serialization (pserialize(9)): |
| 53 | * |
| 54 | * - If you need references to transfer from CPU to CPU or LWP to |
| 55 | * LWP, or if you need long-term references, you must use |
| 56 | * reference counting, e.g. with atomic operations or locks, |
| 57 | * which incurs interprocessor synchronization for every use -- |
| 58 | * cheaper than an xcall, but not scalable. |
| 59 | * |
| 60 | * - If all users *guarantee* that they will not sleep, then it is |
| 61 | * not necessary to use passive references: you may as well just |
| 62 | * use the even cheaper pserialize(9), because you have |
| 63 | * satisfied the requirements of a pserialize read section. |
| 64 | */ |
| 65 | |
| 66 | #include <sys/cdefs.h> |
| 67 | __KERNEL_RCSID(0, "$NetBSD: subr_psref.c,v 1.6 2016/11/09 09:00:46 ozaki-r Exp $" ); |
| 68 | |
| 69 | #include <sys/types.h> |
| 70 | #include <sys/condvar.h> |
| 71 | #include <sys/cpu.h> |
| 72 | #include <sys/intr.h> |
| 73 | #include <sys/kmem.h> |
| 74 | #include <sys/lwp.h> |
| 75 | #include <sys/mutex.h> |
| 76 | #include <sys/percpu.h> |
| 77 | #include <sys/psref.h> |
| 78 | #include <sys/queue.h> |
| 79 | #include <sys/xcall.h> |
| 80 | |
| 81 | LIST_HEAD(psref_head, psref); |
| 82 | |
| 83 | static bool _psref_held(const struct psref_target *, struct psref_class *, |
| 84 | bool); |
| 85 | |
| 86 | /* |
| 87 | * struct psref_class |
| 88 | * |
| 89 | * Private global state for a class of passive reference targets. |
| 90 | * Opaque to callers. |
| 91 | */ |
| 92 | struct psref_class { |
| 93 | kmutex_t prc_lock; |
| 94 | kcondvar_t prc_cv; |
| 95 | struct percpu *prc_percpu; /* struct psref_cpu */ |
| 96 | ipl_cookie_t prc_iplcookie; |
| 97 | }; |
| 98 | |
| 99 | /* |
| 100 | * struct psref_cpu |
| 101 | * |
| 102 | * Private per-CPU state for a class of passive reference targets. |
| 103 | * Not exposed by the API. |
| 104 | */ |
| 105 | struct psref_cpu { |
| 106 | struct psref_head pcpu_head; |
| 107 | }; |
| 108 | |
| 109 | /* |
| 110 | * psref_class_create(name, ipl) |
| 111 | * |
| 112 | * Create a new passive reference class, with the given wchan name |
| 113 | * and ipl. |
| 114 | */ |
| 115 | struct psref_class * |
| 116 | psref_class_create(const char *name, int ipl) |
| 117 | { |
| 118 | struct psref_class *class; |
| 119 | |
| 120 | ASSERT_SLEEPABLE(); |
| 121 | |
| 122 | class = kmem_alloc(sizeof(*class), KM_SLEEP); |
| 123 | if (class == NULL) |
| 124 | goto fail0; |
| 125 | |
| 126 | class->prc_percpu = percpu_alloc(sizeof(struct psref_cpu)); |
| 127 | if (class->prc_percpu == NULL) |
| 128 | goto fail1; |
| 129 | |
| 130 | mutex_init(&class->prc_lock, MUTEX_DEFAULT, ipl); |
| 131 | cv_init(&class->prc_cv, name); |
| 132 | class->prc_iplcookie = makeiplcookie(ipl); |
| 133 | |
| 134 | return class; |
| 135 | |
| 136 | fail1: kmem_free(class, sizeof(*class)); |
| 137 | fail0: return NULL; |
| 138 | } |
| 139 | |
| 140 | #ifdef DIAGNOSTIC |
| 141 | static void |
| 142 | psref_cpu_drained_p(void *p, void *cookie, struct cpu_info *ci __unused) |
| 143 | { |
| 144 | const struct psref_cpu *pcpu = p; |
| 145 | bool *retp = cookie; |
| 146 | |
| 147 | if (!LIST_EMPTY(&pcpu->pcpu_head)) |
| 148 | *retp = false; |
| 149 | } |
| 150 | |
| 151 | static bool |
| 152 | psref_class_drained_p(const struct psref_class *prc) |
| 153 | { |
| 154 | bool ret = true; |
| 155 | |
| 156 | percpu_foreach(prc->prc_percpu, &psref_cpu_drained_p, &ret); |
| 157 | |
| 158 | return ret; |
| 159 | } |
| 160 | #endif /* DIAGNOSTIC */ |
| 161 | |
| 162 | /* |
| 163 | * psref_class_destroy(class) |
| 164 | * |
| 165 | * Destroy a passive reference class and free memory associated |
| 166 | * with it. All targets in this class must have been drained and |
| 167 | * destroyed already. |
| 168 | */ |
| 169 | void |
| 170 | psref_class_destroy(struct psref_class *class) |
| 171 | { |
| 172 | |
| 173 | KASSERT(psref_class_drained_p(class)); |
| 174 | |
| 175 | cv_destroy(&class->prc_cv); |
| 176 | mutex_destroy(&class->prc_lock); |
| 177 | percpu_free(class->prc_percpu, sizeof(struct psref_cpu)); |
| 178 | kmem_free(class, sizeof(*class)); |
| 179 | } |
| 180 | |
| 181 | /* |
| 182 | * psref_target_init(target, class) |
| 183 | * |
| 184 | * Initialize a passive reference target in the specified class. |
| 185 | * The caller is responsible for issuing a membar_producer after |
| 186 | * psref_target_init and before exposing a pointer to the target |
| 187 | * to other CPUs. |
| 188 | */ |
| 189 | void |
| 190 | psref_target_init(struct psref_target *target, |
| 191 | struct psref_class *class) |
| 192 | { |
| 193 | |
| 194 | target->prt_class = class; |
| 195 | target->prt_draining = false; |
| 196 | } |
| 197 | |
| 198 | #ifdef DEBUG |
| 199 | static void |
| 200 | psref_check_duplication(struct psref_cpu *pcpu, struct psref *psref, |
| 201 | const struct psref_target *target) |
| 202 | { |
| 203 | bool found = false; |
| 204 | struct psref *_psref; |
| 205 | |
| 206 | LIST_FOREACH(_psref, &pcpu->pcpu_head, psref_entry) { |
| 207 | if (_psref == psref && |
| 208 | _psref->psref_target == target) { |
| 209 | found = true; |
| 210 | break; |
| 211 | } |
| 212 | } |
| 213 | if (found) { |
| 214 | panic("trying to acquire a target twice with the same psref: " |
| 215 | "psref=%p target=%p" , psref, target); |
| 216 | } |
| 217 | } |
| 218 | #endif /* DEBUG */ |
| 219 | |
| 220 | /* |
| 221 | * psref_acquire(psref, target, class) |
| 222 | * |
| 223 | * Acquire a passive reference to the specified target, which must |
| 224 | * be in the specified class. |
| 225 | * |
| 226 | * The caller must guarantee that the target will not be destroyed |
| 227 | * before psref_acquire returns. |
| 228 | * |
| 229 | * The caller must additionally guarantee that it will not switch |
| 230 | * CPUs before releasing the passive reference, either by |
| 231 | * disabling kpreemption and avoiding sleeps, or by being in a |
| 232 | * softint or in an LWP bound to a CPU. |
| 233 | */ |
| 234 | void |
| 235 | psref_acquire(struct psref *psref, const struct psref_target *target, |
| 236 | struct psref_class *class) |
| 237 | { |
| 238 | struct psref_cpu *pcpu; |
| 239 | int s; |
| 240 | |
| 241 | KASSERTMSG((kpreempt_disabled() || cpu_softintr_p() || |
| 242 | ISSET(curlwp->l_pflag, LP_BOUND)), |
| 243 | "passive references are CPU-local," |
| 244 | " but preemption is enabled and the caller is not" |
| 245 | " in a softint or CPU-bound LWP" ); |
| 246 | KASSERTMSG((target->prt_class == class), |
| 247 | "mismatched psref target class: %p (ref) != %p (expected)" , |
| 248 | target->prt_class, class); |
| 249 | KASSERTMSG(!target->prt_draining, "psref target already destroyed: %p" , |
| 250 | target); |
| 251 | |
| 252 | /* Block interrupts and acquire the current CPU's reference list. */ |
| 253 | s = splraiseipl(class->prc_iplcookie); |
| 254 | pcpu = percpu_getref(class->prc_percpu); |
| 255 | |
| 256 | #ifdef DEBUG |
| 257 | /* Sanity-check if the target is already acquired with the same psref. */ |
| 258 | psref_check_duplication(pcpu, psref, target); |
| 259 | #endif |
| 260 | |
| 261 | /* Record our reference. */ |
| 262 | LIST_INSERT_HEAD(&pcpu->pcpu_head, psref, psref_entry); |
| 263 | psref->psref_target = target; |
| 264 | psref->psref_lwp = curlwp; |
| 265 | psref->psref_cpu = curcpu(); |
| 266 | |
| 267 | /* Release the CPU list and restore interrupts. */ |
| 268 | percpu_putref(class->prc_percpu); |
| 269 | splx(s); |
| 270 | } |
| 271 | |
| 272 | /* |
| 273 | * psref_release(psref, target, class) |
| 274 | * |
| 275 | * Release a passive reference to the specified target, which must |
| 276 | * be in the specified class. |
| 277 | * |
| 278 | * The caller must not have switched CPUs or LWPs since acquiring |
| 279 | * the passive reference. |
| 280 | */ |
| 281 | void |
| 282 | psref_release(struct psref *psref, const struct psref_target *target, |
| 283 | struct psref_class *class) |
| 284 | { |
| 285 | int s; |
| 286 | |
| 287 | KASSERTMSG((kpreempt_disabled() || cpu_softintr_p() || |
| 288 | ISSET(curlwp->l_pflag, LP_BOUND)), |
| 289 | "passive references are CPU-local," |
| 290 | " but preemption is enabled and the caller is not" |
| 291 | " in a softint or CPU-bound LWP" ); |
| 292 | KASSERTMSG((target->prt_class == class), |
| 293 | "mismatched psref target class: %p (ref) != %p (expected)" , |
| 294 | target->prt_class, class); |
| 295 | |
| 296 | /* Make sure the psref looks sensible. */ |
| 297 | KASSERTMSG((psref->psref_target == target), |
| 298 | "passive reference target mismatch: %p (ref) != %p (expected)" , |
| 299 | psref->psref_target, target); |
| 300 | KASSERTMSG((psref->psref_lwp == curlwp), |
| 301 | "passive reference transferred from lwp %p to lwp %p" , |
| 302 | psref->psref_lwp, curlwp); |
| 303 | KASSERTMSG((psref->psref_cpu == curcpu()), |
| 304 | "passive reference transferred from CPU %u to CPU %u" , |
| 305 | cpu_index(psref->psref_cpu), cpu_index(curcpu())); |
| 306 | |
| 307 | /* |
| 308 | * Block interrupts and remove the psref from the current CPU's |
| 309 | * list. No need to percpu_getref or get the head of the list, |
| 310 | * and the caller guarantees that we are bound to a CPU anyway |
| 311 | * (as does blocking interrupts). |
| 312 | */ |
| 313 | s = splraiseipl(class->prc_iplcookie); |
| 314 | LIST_REMOVE(psref, psref_entry); |
| 315 | splx(s); |
| 316 | |
| 317 | /* If someone is waiting for users to drain, notify 'em. */ |
| 318 | if (__predict_false(target->prt_draining)) |
| 319 | cv_broadcast(&class->prc_cv); |
| 320 | } |
| 321 | |
| 322 | /* |
| 323 | * psref_copy(pto, pfrom, class) |
| 324 | * |
| 325 | * Copy a passive reference from pfrom, which must be in the |
| 326 | * specified class, to pto. Both pfrom and pto must later be |
| 327 | * released with psref_release. |
| 328 | * |
| 329 | * The caller must not have switched CPUs or LWPs since acquiring |
| 330 | * pfrom, and must not switch CPUs or LWPs before releasing both |
| 331 | * pfrom and pto. |
| 332 | */ |
| 333 | void |
| 334 | psref_copy(struct psref *pto, const struct psref *pfrom, |
| 335 | struct psref_class *class) |
| 336 | { |
| 337 | struct psref_cpu *pcpu; |
| 338 | int s; |
| 339 | |
| 340 | KASSERTMSG((kpreempt_disabled() || cpu_softintr_p() || |
| 341 | ISSET(curlwp->l_pflag, LP_BOUND)), |
| 342 | "passive references are CPU-local," |
| 343 | " but preemption is enabled and the caller is not" |
| 344 | " in a softint or CPU-bound LWP" ); |
| 345 | KASSERTMSG((pto != pfrom), |
| 346 | "can't copy passive reference to itself: %p" , |
| 347 | pto); |
| 348 | |
| 349 | /* Make sure the pfrom reference looks sensible. */ |
| 350 | KASSERTMSG((pfrom->psref_lwp == curlwp), |
| 351 | "passive reference transferred from lwp %p to lwp %p" , |
| 352 | pfrom->psref_lwp, curlwp); |
| 353 | KASSERTMSG((pfrom->psref_cpu == curcpu()), |
| 354 | "passive reference transferred from CPU %u to CPU %u" , |
| 355 | cpu_index(pfrom->psref_cpu), cpu_index(curcpu())); |
| 356 | KASSERTMSG((pfrom->psref_target->prt_class == class), |
| 357 | "mismatched psref target class: %p (ref) != %p (expected)" , |
| 358 | pfrom->psref_target->prt_class, class); |
| 359 | |
| 360 | /* Block interrupts and acquire the current CPU's reference list. */ |
| 361 | s = splraiseipl(class->prc_iplcookie); |
| 362 | pcpu = percpu_getref(class->prc_percpu); |
| 363 | |
| 364 | /* Record the new reference. */ |
| 365 | LIST_INSERT_HEAD(&pcpu->pcpu_head, pto, psref_entry); |
| 366 | pto->psref_target = pfrom->psref_target; |
| 367 | pto->psref_lwp = curlwp; |
| 368 | pto->psref_cpu = curcpu(); |
| 369 | |
| 370 | /* Release the CPU list and restore interrupts. */ |
| 371 | percpu_putref(class->prc_percpu); |
| 372 | splx(s); |
| 373 | } |
| 374 | |
| 375 | /* |
| 376 | * struct psreffed |
| 377 | * |
| 378 | * Global state for draining a psref target. |
| 379 | */ |
| 380 | struct psreffed { |
| 381 | struct psref_class *class; |
| 382 | struct psref_target *target; |
| 383 | bool ret; |
| 384 | }; |
| 385 | |
| 386 | static void |
| 387 | psreffed_p_xc(void *cookie0, void *cookie1 __unused) |
| 388 | { |
| 389 | struct psreffed *P = cookie0; |
| 390 | |
| 391 | /* |
| 392 | * If we hold a psref to the target, then answer true. |
| 393 | * |
| 394 | * This is the only dynamic decision that may be made with |
| 395 | * psref_held. |
| 396 | * |
| 397 | * No need to lock anything here: every write transitions from |
| 398 | * false to true, so there can be no conflicting writes. No |
| 399 | * need for a memory barrier here because P->ret is read only |
| 400 | * after xc_wait, which has already issued any necessary memory |
| 401 | * barriers. |
| 402 | */ |
| 403 | if (_psref_held(P->target, P->class, true)) |
| 404 | P->ret = true; |
| 405 | } |
| 406 | |
| 407 | static bool |
| 408 | psreffed_p(struct psref_target *target, struct psref_class *class) |
| 409 | { |
| 410 | struct psreffed P = { |
| 411 | .class = class, |
| 412 | .target = target, |
| 413 | .ret = false, |
| 414 | }; |
| 415 | |
| 416 | /* Ask all CPUs to say whether they hold a psref to the target. */ |
| 417 | xc_wait(xc_broadcast(0, &psreffed_p_xc, &P, NULL)); |
| 418 | |
| 419 | return P.ret; |
| 420 | } |
| 421 | |
| 422 | /* |
| 423 | * psref_target_destroy(target, class) |
| 424 | * |
| 425 | * Destroy a passive reference target. Waits for all existing |
| 426 | * references to drain. Caller must guarantee no new references |
| 427 | * will be acquired once it calls psref_target_destroy, e.g. by |
| 428 | * removing the target from a global list first. May sleep. |
| 429 | */ |
| 430 | void |
| 431 | psref_target_destroy(struct psref_target *target, struct psref_class *class) |
| 432 | { |
| 433 | |
| 434 | ASSERT_SLEEPABLE(); |
| 435 | |
| 436 | KASSERTMSG((target->prt_class == class), |
| 437 | "mismatched psref target class: %p (ref) != %p (expected)" , |
| 438 | target->prt_class, class); |
| 439 | |
| 440 | /* Request psref_release to notify us when done. */ |
| 441 | KASSERTMSG(!target->prt_draining, "psref target already destroyed: %p" , |
| 442 | target); |
| 443 | target->prt_draining = true; |
| 444 | |
| 445 | /* Wait until there are no more references on any CPU. */ |
| 446 | while (psreffed_p(target, class)) { |
| 447 | /* |
| 448 | * This enter/wait/exit business looks wrong, but it is |
| 449 | * both necessary, because psreffed_p performs a |
| 450 | * low-priority xcall and hence cannot run while a |
| 451 | * mutex is locked, and OK, because the wait is timed |
| 452 | * -- explicit wakeups are only an optimization. |
| 453 | */ |
| 454 | mutex_enter(&class->prc_lock); |
| 455 | (void)cv_timedwait(&class->prc_cv, &class->prc_lock, 1); |
| 456 | mutex_exit(&class->prc_lock); |
| 457 | } |
| 458 | |
| 459 | /* No more references. Cause subsequent psref_acquire to kassert. */ |
| 460 | target->prt_class = NULL; |
| 461 | } |
| 462 | |
| 463 | static bool |
| 464 | _psref_held(const struct psref_target *target, struct psref_class *class, |
| 465 | bool lwp_mismatch_ok) |
| 466 | { |
| 467 | const struct psref_cpu *pcpu; |
| 468 | const struct psref *psref; |
| 469 | int s; |
| 470 | bool held = false; |
| 471 | |
| 472 | KASSERTMSG((kpreempt_disabled() || cpu_softintr_p() || |
| 473 | ISSET(curlwp->l_pflag, LP_BOUND)), |
| 474 | "passive references are CPU-local," |
| 475 | " but preemption is enabled and the caller is not" |
| 476 | " in a softint or CPU-bound LWP" ); |
| 477 | KASSERTMSG((target->prt_class == class), |
| 478 | "mismatched psref target class: %p (ref) != %p (expected)" , |
| 479 | target->prt_class, class); |
| 480 | |
| 481 | /* Block interrupts and acquire the current CPU's reference list. */ |
| 482 | s = splraiseipl(class->prc_iplcookie); |
| 483 | pcpu = percpu_getref(class->prc_percpu); |
| 484 | |
| 485 | /* Search through all the references on this CPU. */ |
| 486 | LIST_FOREACH(psref, &pcpu->pcpu_head, psref_entry) { |
| 487 | /* Sanity-check the reference's CPU. */ |
| 488 | KASSERTMSG((psref->psref_cpu == curcpu()), |
| 489 | "passive reference transferred from CPU %u to CPU %u" , |
| 490 | cpu_index(psref->psref_cpu), cpu_index(curcpu())); |
| 491 | |
| 492 | /* If it doesn't match, skip it and move on. */ |
| 493 | if (psref->psref_target != target) |
| 494 | continue; |
| 495 | |
| 496 | /* |
| 497 | * Sanity-check the reference's LWP if we are asserting |
| 498 | * via psref_held that this LWP holds it, but not if we |
| 499 | * are testing in psref_target_destroy whether any LWP |
| 500 | * still holds it. |
| 501 | */ |
| 502 | KASSERTMSG((lwp_mismatch_ok || psref->psref_lwp == curlwp), |
| 503 | "passive reference transferred from lwp %p to lwp %p" , |
| 504 | psref->psref_lwp, curlwp); |
| 505 | |
| 506 | /* Stop here and report that we found it. */ |
| 507 | held = true; |
| 508 | break; |
| 509 | } |
| 510 | |
| 511 | /* Release the CPU list and restore interrupts. */ |
| 512 | percpu_putref(class->prc_percpu); |
| 513 | splx(s); |
| 514 | |
| 515 | return held; |
| 516 | } |
| 517 | |
| 518 | /* |
| 519 | * psref_held(target, class) |
| 520 | * |
| 521 | * True if the current CPU holds a passive reference to target, |
| 522 | * false otherwise. May be used only inside assertions. |
| 523 | */ |
| 524 | bool |
| 525 | psref_held(const struct psref_target *target, struct psref_class *class) |
| 526 | { |
| 527 | |
| 528 | return _psref_held(target, class, false); |
| 529 | } |
| 530 | |