The Design and Implementation of the FreeBSD Operating System, Second Edition
Now available: The Design and Implementation of the FreeBSD Operating System (Second Edition)


[ source navigation ] [ diff markup ] [ identifier search ] [ freetext search ] [ file search ] [ list types ] [ track identifier ]

FreeBSD/Linux Kernel Cross Reference
sys/kern/kern_rwlock.c

Version: -  FREEBSD  -  FREEBSD-13-STABLE  -  FREEBSD-13-0  -  FREEBSD-12-STABLE  -  FREEBSD-12-0  -  FREEBSD-11-STABLE  -  FREEBSD-11-0  -  FREEBSD-10-STABLE  -  FREEBSD-10-0  -  FREEBSD-9-STABLE  -  FREEBSD-9-0  -  FREEBSD-8-STABLE  -  FREEBSD-8-0  -  FREEBSD-7-STABLE  -  FREEBSD-7-0  -  FREEBSD-6-STABLE  -  FREEBSD-6-0  -  FREEBSD-5-STABLE  -  FREEBSD-5-0  -  FREEBSD-4-STABLE  -  FREEBSD-3-STABLE  -  FREEBSD22  -  l41  -  OPENBSD  -  linux-2.6  -  MK84  -  PLAN9  -  xnu-8792 
SearchContext: -  none  -  3  -  10 

    1 /*-
    2  * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
    3  *
    4  * Copyright (c) 2006 John Baldwin <jhb@FreeBSD.org>
    5  *
    6  * Redistribution and use in source and binary forms, with or without
    7  * modification, are permitted provided that the following conditions
    8  * are met:
    9  * 1. Redistributions of source code must retain the above copyright
   10  *    notice, this list of conditions and the following disclaimer.
   11  * 2. Redistributions in binary form must reproduce the above copyright
   12  *    notice, this list of conditions and the following disclaimer in the
   13  *    documentation and/or other materials provided with the distribution.
   14  *
   15  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
   16  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   17  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   18  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
   19  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   20  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   21  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   22  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   23  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   24  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   25  * SUCH DAMAGE.
   26  */
   27 
   28 /*
   29  * Machine independent bits of reader/writer lock implementation.
   30  */
   31 
   32 #include <sys/cdefs.h>
   33 __FBSDID("$FreeBSD$");
   34 
   35 #include "opt_ddb.h"
   36 #include "opt_hwpmc_hooks.h"
   37 #include "opt_no_adaptive_rwlocks.h"
   38 
   39 #include <sys/param.h>
   40 #include <sys/kdb.h>
   41 #include <sys/ktr.h>
   42 #include <sys/kernel.h>
   43 #include <sys/lock.h>
   44 #include <sys/mutex.h>
   45 #include <sys/proc.h>
   46 #include <sys/rwlock.h>
   47 #include <sys/sched.h>
   48 #include <sys/smp.h>
   49 #include <sys/sysctl.h>
   50 #include <sys/systm.h>
   51 #include <sys/turnstile.h>
   52 
   53 #include <machine/cpu.h>
   54 
   55 #if defined(SMP) && !defined(NO_ADAPTIVE_RWLOCKS)
   56 #define ADAPTIVE_RWLOCKS
   57 #endif
   58 
   59 #ifdef HWPMC_HOOKS
   60 #include <sys/pmckern.h>
   61 PMC_SOFT_DECLARE( , , lock, failed);
   62 #endif
   63 
   64 /*
   65  * Return the rwlock address when the lock cookie address is provided.
   66  * This functionality assumes that struct rwlock* have a member named rw_lock.
   67  */
   68 #define rwlock2rw(c)    (__containerof(c, struct rwlock, rw_lock))
   69 
   70 #ifdef DDB
   71 #include <ddb/ddb.h>
   72 
   73 static void     db_show_rwlock(const struct lock_object *lock);
   74 #endif
   75 static void     assert_rw(const struct lock_object *lock, int what);
   76 static void     lock_rw(struct lock_object *lock, uintptr_t how);
   77 #ifdef KDTRACE_HOOKS
   78 static int      owner_rw(const struct lock_object *lock, struct thread **owner);
   79 #endif
   80 static uintptr_t unlock_rw(struct lock_object *lock);
   81 
   82 struct lock_class lock_class_rw = {
   83         .lc_name = "rw",
   84         .lc_flags = LC_SLEEPLOCK | LC_RECURSABLE | LC_UPGRADABLE,
   85         .lc_assert = assert_rw,
   86 #ifdef DDB
   87         .lc_ddb_show = db_show_rwlock,
   88 #endif
   89         .lc_lock = lock_rw,
   90         .lc_unlock = unlock_rw,
   91 #ifdef KDTRACE_HOOKS
   92         .lc_owner = owner_rw,
   93 #endif
   94 };
   95 
   96 #ifdef ADAPTIVE_RWLOCKS
   97 #ifdef RWLOCK_CUSTOM_BACKOFF
   98 static u_short __read_frequently rowner_retries;
   99 static u_short __read_frequently rowner_loops;
  100 static SYSCTL_NODE(_debug, OID_AUTO, rwlock,
  101     CTLFLAG_RD | CTLFLAG_MPSAFE, NULL,
  102     "rwlock debugging");
  103 SYSCTL_U16(_debug_rwlock, OID_AUTO, retry, CTLFLAG_RW, &rowner_retries, 0, "");
  104 SYSCTL_U16(_debug_rwlock, OID_AUTO, loops, CTLFLAG_RW, &rowner_loops, 0, "");
  105 
  106 static struct lock_delay_config __read_frequently rw_delay;
  107 
  108 SYSCTL_U16(_debug_rwlock, OID_AUTO, delay_base, CTLFLAG_RW, &rw_delay.base,
  109     0, "");
  110 SYSCTL_U16(_debug_rwlock, OID_AUTO, delay_max, CTLFLAG_RW, &rw_delay.max,
  111     0, "");
  112 
  113 static void
  114 rw_lock_delay_init(void *arg __unused)
  115 {
  116 
  117         lock_delay_default_init(&rw_delay);
  118         rowner_retries = 10;
  119         rowner_loops = max(10000, rw_delay.max);
  120 }
  121 LOCK_DELAY_SYSINIT(rw_lock_delay_init);
  122 #else
  123 #define rw_delay        locks_delay
  124 #define rowner_retries  locks_delay_retries
  125 #define rowner_loops    locks_delay_loops
  126 #endif
  127 #endif
  128 
  129 /*
  130  * Return a pointer to the owning thread if the lock is write-locked or
  131  * NULL if the lock is unlocked or read-locked.
  132  */
  133 
  134 #define lv_rw_wowner(v)                                                 \
  135         ((v) & RW_LOCK_READ ? NULL :                                    \
  136          (struct thread *)RW_OWNER((v)))
  137 
  138 #define rw_wowner(rw)   lv_rw_wowner(RW_READ_VALUE(rw))
  139 
  140 /*
  141  * Returns if a write owner is recursed.  Write ownership is not assured
  142  * here and should be previously checked.
  143  */
  144 #define rw_recursed(rw)         ((rw)->rw_recurse != 0)
  145 
  146 /*
  147  * Return true if curthread helds the lock.
  148  */
  149 #define rw_wlocked(rw)          (rw_wowner((rw)) == curthread)
  150 
  151 /*
  152  * Return a pointer to the owning thread for this lock who should receive
  153  * any priority lent by threads that block on this lock.  Currently this
  154  * is identical to rw_wowner().
  155  */
  156 #define rw_owner(rw)            rw_wowner(rw)
  157 
  158 #ifndef INVARIANTS
  159 #define __rw_assert(c, what, file, line)
  160 #endif
  161 
  162 void
  163 assert_rw(const struct lock_object *lock, int what)
  164 {
  165 
  166         rw_assert((const struct rwlock *)lock, what);
  167 }
  168 
  169 void
  170 lock_rw(struct lock_object *lock, uintptr_t how)
  171 {
  172         struct rwlock *rw;
  173 
  174         rw = (struct rwlock *)lock;
  175         if (how)
  176                 rw_rlock(rw);
  177         else
  178                 rw_wlock(rw);
  179 }
  180 
  181 uintptr_t
  182 unlock_rw(struct lock_object *lock)
  183 {
  184         struct rwlock *rw;
  185 
  186         rw = (struct rwlock *)lock;
  187         rw_assert(rw, RA_LOCKED | LA_NOTRECURSED);
  188         if (rw->rw_lock & RW_LOCK_READ) {
  189                 rw_runlock(rw);
  190                 return (1);
  191         } else {
  192                 rw_wunlock(rw);
  193                 return (0);
  194         }
  195 }
  196 
  197 #ifdef KDTRACE_HOOKS
  198 int
  199 owner_rw(const struct lock_object *lock, struct thread **owner)
  200 {
  201         const struct rwlock *rw = (const struct rwlock *)lock;
  202         uintptr_t x = rw->rw_lock;
  203 
  204         *owner = rw_wowner(rw);
  205         return ((x & RW_LOCK_READ) != 0 ?  (RW_READERS(x) != 0) :
  206             (*owner != NULL));
  207 }
  208 #endif
  209 
  210 void
  211 _rw_init_flags(volatile uintptr_t *c, const char *name, int opts)
  212 {
  213         struct rwlock *rw;
  214         int flags;
  215 
  216         rw = rwlock2rw(c);
  217 
  218         MPASS((opts & ~(RW_DUPOK | RW_NOPROFILE | RW_NOWITNESS | RW_QUIET |
  219             RW_RECURSE | RW_NEW)) == 0);
  220         ASSERT_ATOMIC_LOAD_PTR(rw->rw_lock,
  221             ("%s: rw_lock not aligned for %s: %p", __func__, name,
  222             &rw->rw_lock));
  223 
  224         flags = LO_UPGRADABLE;
  225         if (opts & RW_DUPOK)
  226                 flags |= LO_DUPOK;
  227         if (opts & RW_NOPROFILE)
  228                 flags |= LO_NOPROFILE;
  229         if (!(opts & RW_NOWITNESS))
  230                 flags |= LO_WITNESS;
  231         if (opts & RW_RECURSE)
  232                 flags |= LO_RECURSABLE;
  233         if (opts & RW_QUIET)
  234                 flags |= LO_QUIET;
  235         if (opts & RW_NEW)
  236                 flags |= LO_NEW;
  237 
  238         lock_init(&rw->lock_object, &lock_class_rw, name, NULL, flags);
  239         rw->rw_lock = RW_UNLOCKED;
  240         rw->rw_recurse = 0;
  241 }
  242 
  243 void
  244 _rw_destroy(volatile uintptr_t *c)
  245 {
  246         struct rwlock *rw;
  247 
  248         rw = rwlock2rw(c);
  249 
  250         KASSERT(rw->rw_lock == RW_UNLOCKED, ("rw lock %p not unlocked", rw));
  251         KASSERT(rw->rw_recurse == 0, ("rw lock %p still recursed", rw));
  252         rw->rw_lock = RW_DESTROYED;
  253         lock_destroy(&rw->lock_object);
  254 }
  255 
  256 void
  257 rw_sysinit(void *arg)
  258 {
  259         struct rw_args *args;
  260 
  261         args = arg;
  262         rw_init_flags((struct rwlock *)args->ra_rw, args->ra_desc,
  263             args->ra_flags);
  264 }
  265 
  266 int
  267 _rw_wowned(const volatile uintptr_t *c)
  268 {
  269 
  270         return (rw_wowner(rwlock2rw(c)) == curthread);
  271 }
  272 
  273 void
  274 _rw_wlock_cookie(volatile uintptr_t *c, const char *file, int line)
  275 {
  276         struct rwlock *rw;
  277         uintptr_t tid, v;
  278 
  279         rw = rwlock2rw(c);
  280 
  281         KASSERT(kdb_active != 0 || SCHEDULER_STOPPED() ||
  282             !TD_IS_IDLETHREAD(curthread),
  283             ("rw_wlock() by idle thread %p on rwlock %s @ %s:%d",
  284             curthread, rw->lock_object.lo_name, file, line));
  285         KASSERT(rw->rw_lock != RW_DESTROYED,
  286             ("rw_wlock() of destroyed rwlock @ %s:%d", file, line));
  287         WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER | LOP_EXCLUSIVE, file,
  288             line, NULL);
  289         tid = (uintptr_t)curthread;
  290         v = RW_UNLOCKED;
  291         if (!_rw_write_lock_fetch(rw, &v, tid))
  292                 _rw_wlock_hard(rw, v, file, line);
  293         else
  294                 LOCKSTAT_PROFILE_OBTAIN_RWLOCK_SUCCESS(rw__acquire, rw,
  295                     0, 0, file, line, LOCKSTAT_WRITER);
  296 
  297         LOCK_LOG_LOCK("WLOCK", &rw->lock_object, 0, rw->rw_recurse, file, line);
  298         WITNESS_LOCK(&rw->lock_object, LOP_EXCLUSIVE, file, line);
  299         TD_LOCKS_INC(curthread);
  300 }
  301 
  302 int
  303 __rw_try_wlock_int(struct rwlock *rw LOCK_FILE_LINE_ARG_DEF)
  304 {
  305         struct thread *td;
  306         uintptr_t tid, v;
  307         int rval;
  308         bool recursed;
  309 
  310         td = curthread;
  311         tid = (uintptr_t)td;
  312         if (SCHEDULER_STOPPED_TD(td))
  313                 return (1);
  314 
  315         KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(td),
  316             ("rw_try_wlock() by idle thread %p on rwlock %s @ %s:%d",
  317             curthread, rw->lock_object.lo_name, file, line));
  318         KASSERT(rw->rw_lock != RW_DESTROYED,
  319             ("rw_try_wlock() of destroyed rwlock @ %s:%d", file, line));
  320 
  321         rval = 1;
  322         recursed = false;
  323         v = RW_UNLOCKED;
  324         for (;;) {
  325                 if (atomic_fcmpset_acq_ptr(&rw->rw_lock, &v, tid))
  326                         break;
  327                 if (v == RW_UNLOCKED)
  328                         continue;
  329                 if (v == tid && (rw->lock_object.lo_flags & LO_RECURSABLE)) {
  330                         rw->rw_recurse++;
  331                         atomic_set_ptr(&rw->rw_lock, RW_LOCK_WRITER_RECURSED);
  332                         break;
  333                 }
  334                 rval = 0;
  335                 break;
  336         }
  337 
  338         LOCK_LOG_TRY("WLOCK", &rw->lock_object, 0, rval, file, line);
  339         if (rval) {
  340                 WITNESS_LOCK(&rw->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
  341                     file, line);
  342                 if (!recursed)
  343                         LOCKSTAT_PROFILE_OBTAIN_RWLOCK_SUCCESS(rw__acquire,
  344                             rw, 0, 0, file, line, LOCKSTAT_WRITER);
  345                 TD_LOCKS_INC(curthread);
  346         }
  347         return (rval);
  348 }
  349 
  350 int
  351 __rw_try_wlock(volatile uintptr_t *c, const char *file, int line)
  352 {
  353         struct rwlock *rw;
  354 
  355         rw = rwlock2rw(c);
  356         return (__rw_try_wlock_int(rw LOCK_FILE_LINE_ARG));
  357 }
  358 
  359 void
  360 _rw_wunlock_cookie(volatile uintptr_t *c, const char *file, int line)
  361 {
  362         struct rwlock *rw;
  363 
  364         rw = rwlock2rw(c);
  365 
  366         KASSERT(rw->rw_lock != RW_DESTROYED,
  367             ("rw_wunlock() of destroyed rwlock @ %s:%d", file, line));
  368         __rw_assert(c, RA_WLOCKED, file, line);
  369         WITNESS_UNLOCK(&rw->lock_object, LOP_EXCLUSIVE, file, line);
  370         LOCK_LOG_LOCK("WUNLOCK", &rw->lock_object, 0, rw->rw_recurse, file,
  371             line);
  372 
  373 #ifdef LOCK_PROFILING
  374         _rw_wunlock_hard(rw, (uintptr_t)curthread, file, line);
  375 #else
  376         __rw_wunlock(rw, curthread, file, line);
  377 #endif
  378 
  379         TD_LOCKS_DEC(curthread);
  380 }
  381 
  382 /*
  383  * Determines whether a new reader can acquire a lock.  Succeeds if the
  384  * reader already owns a read lock and the lock is locked for read to
  385  * prevent deadlock from reader recursion.  Also succeeds if the lock
  386  * is unlocked and has no writer waiters or spinners.  Failing otherwise
  387  * prioritizes writers before readers.
  388  */
  389 static bool __always_inline
  390 __rw_can_read(struct thread *td, uintptr_t v, bool fp)
  391 {
  392 
  393         if ((v & (RW_LOCK_READ | RW_LOCK_WRITE_WAITERS | RW_LOCK_WRITE_SPINNER))
  394             == RW_LOCK_READ)
  395                 return (true);
  396         if (!fp && td->td_rw_rlocks && (v & RW_LOCK_READ))
  397                 return (true);
  398         return (false);
  399 }
  400 
  401 static bool __always_inline
  402 __rw_rlock_try(struct rwlock *rw, struct thread *td, uintptr_t *vp, bool fp
  403     LOCK_FILE_LINE_ARG_DEF)
  404 {
  405 
  406         /*
  407          * Handle the easy case.  If no other thread has a write
  408          * lock, then try to bump up the count of read locks.  Note
  409          * that we have to preserve the current state of the
  410          * RW_LOCK_WRITE_WAITERS flag.  If we fail to acquire a
  411          * read lock, then rw_lock must have changed, so restart
  412          * the loop.  Note that this handles the case of a
  413          * completely unlocked rwlock since such a lock is encoded
  414          * as a read lock with no waiters.
  415          */
  416         while (__rw_can_read(td, *vp, fp)) {
  417                 if (atomic_fcmpset_acq_ptr(&rw->rw_lock, vp,
  418                         *vp + RW_ONE_READER)) {
  419                         if (LOCK_LOG_TEST(&rw->lock_object, 0))
  420                                 CTR4(KTR_LOCK,
  421                                     "%s: %p succeed %p -> %p", __func__,
  422                                     rw, (void *)*vp,
  423                                     (void *)(*vp + RW_ONE_READER));
  424                         td->td_rw_rlocks++;
  425                         return (true);
  426                 }
  427         }
  428         return (false);
  429 }
  430 
  431 static void __noinline
  432 __rw_rlock_hard(struct rwlock *rw, struct thread *td, uintptr_t v
  433     LOCK_FILE_LINE_ARG_DEF)
  434 {
  435         struct turnstile *ts;
  436         struct thread *owner;
  437 #ifdef ADAPTIVE_RWLOCKS
  438         int spintries = 0;
  439         int i, n;
  440 #endif
  441 #ifdef LOCK_PROFILING
  442         uint64_t waittime = 0;
  443         int contested = 0;
  444 #endif
  445 #if defined(ADAPTIVE_RWLOCKS) || defined(KDTRACE_HOOKS)
  446         struct lock_delay_arg lda;
  447 #endif
  448 #ifdef KDTRACE_HOOKS
  449         u_int sleep_cnt = 0;
  450         int64_t sleep_time = 0;
  451         int64_t all_time = 0;
  452 #endif
  453 #if defined(KDTRACE_HOOKS) || defined(LOCK_PROFILING)
  454         uintptr_t state = 0;
  455         int doing_lockprof = 0;
  456 #endif
  457 
  458 #ifdef KDTRACE_HOOKS
  459         if (LOCKSTAT_PROFILE_ENABLED(rw__acquire)) {
  460                 if (__rw_rlock_try(rw, td, &v, false LOCK_FILE_LINE_ARG))
  461                         goto out_lockstat;
  462                 doing_lockprof = 1;
  463                 all_time -= lockstat_nsecs(&rw->lock_object);
  464                 state = v;
  465         }
  466 #endif
  467 #ifdef LOCK_PROFILING
  468         doing_lockprof = 1;
  469         state = v;
  470 #endif
  471 
  472         if (SCHEDULER_STOPPED())
  473                 return;
  474 
  475 #if defined(ADAPTIVE_RWLOCKS)
  476         lock_delay_arg_init(&lda, &rw_delay);
  477 #elif defined(KDTRACE_HOOKS)
  478         lock_delay_arg_init_noadapt(&lda);
  479 #endif
  480 
  481 #ifdef HWPMC_HOOKS
  482         PMC_SOFT_CALL( , , lock, failed);
  483 #endif
  484         lock_profile_obtain_lock_failed(&rw->lock_object,
  485             &contested, &waittime);
  486 
  487         for (;;) {
  488                 if (__rw_rlock_try(rw, td, &v, false LOCK_FILE_LINE_ARG))
  489                         break;
  490 #ifdef KDTRACE_HOOKS
  491                 lda.spin_cnt++;
  492 #endif
  493 
  494 #ifdef ADAPTIVE_RWLOCKS
  495                 /*
  496                  * If the owner is running on another CPU, spin until
  497                  * the owner stops running or the state of the lock
  498                  * changes.
  499                  */
  500                 if ((v & RW_LOCK_READ) == 0) {
  501                         owner = (struct thread *)RW_OWNER(v);
  502                         if (TD_IS_RUNNING(owner)) {
  503                                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
  504                                         CTR3(KTR_LOCK,
  505                                             "%s: spinning on %p held by %p",
  506                                             __func__, rw, owner);
  507                                 KTR_STATE1(KTR_SCHED, "thread",
  508                                     sched_tdname(curthread), "spinning",
  509                                     "lockname:\"%s\"", rw->lock_object.lo_name);
  510                                 do {
  511                                         lock_delay(&lda);
  512                                         v = RW_READ_VALUE(rw);
  513                                         owner = lv_rw_wowner(v);
  514                                 } while (owner != NULL && TD_IS_RUNNING(owner));
  515                                 KTR_STATE0(KTR_SCHED, "thread",
  516                                     sched_tdname(curthread), "running");
  517                                 continue;
  518                         }
  519                 } else {
  520                         if ((v & RW_LOCK_WRITE_SPINNER) && RW_READERS(v) == 0) {
  521                                 MPASS(!__rw_can_read(td, v, false));
  522                                 lock_delay_spin(2);
  523                                 v = RW_READ_VALUE(rw);
  524                                 continue;
  525                         }
  526                         if (spintries < rowner_retries) {
  527                                 spintries++;
  528                                 KTR_STATE1(KTR_SCHED, "thread", sched_tdname(curthread),
  529                                     "spinning", "lockname:\"%s\"",
  530                                     rw->lock_object.lo_name);
  531                                 n = RW_READERS(v);
  532                                 for (i = 0; i < rowner_loops; i += n) {
  533                                         lock_delay_spin(n);
  534                                         v = RW_READ_VALUE(rw);
  535                                         if (!(v & RW_LOCK_READ))
  536                                                 break;
  537                                         n = RW_READERS(v);
  538                                         if (n == 0)
  539                                                 break;
  540                                         if (__rw_can_read(td, v, false))
  541                                                 break;
  542                                 }
  543 #ifdef KDTRACE_HOOKS
  544                                 lda.spin_cnt += rowner_loops - i;
  545 #endif
  546                                 KTR_STATE0(KTR_SCHED, "thread", sched_tdname(curthread),
  547                                     "running");
  548                                 if (i < rowner_loops)
  549                                         continue;
  550                         }
  551                 }
  552 #endif
  553 
  554                 /*
  555                  * Okay, now it's the hard case.  Some other thread already
  556                  * has a write lock or there are write waiters present,
  557                  * acquire the turnstile lock so we can begin the process
  558                  * of blocking.
  559                  */
  560                 ts = turnstile_trywait(&rw->lock_object);
  561 
  562                 /*
  563                  * The lock might have been released while we spun, so
  564                  * recheck its state and restart the loop if needed.
  565                  */
  566                 v = RW_READ_VALUE(rw);
  567 retry_ts:
  568                 if (((v & RW_LOCK_WRITE_SPINNER) && RW_READERS(v) == 0) ||
  569                     __rw_can_read(td, v, false)) {
  570                         turnstile_cancel(ts);
  571                         continue;
  572                 }
  573 
  574                 owner = lv_rw_wowner(v);
  575 
  576 #ifdef ADAPTIVE_RWLOCKS
  577                 /*
  578                  * The current lock owner might have started executing
  579                  * on another CPU (or the lock could have changed
  580                  * owners) while we were waiting on the turnstile
  581                  * chain lock.  If so, drop the turnstile lock and try
  582                  * again.
  583                  */
  584                 if (owner != NULL) {
  585                         if (TD_IS_RUNNING(owner)) {
  586                                 turnstile_cancel(ts);
  587                                 continue;
  588                         }
  589                 }
  590 #endif
  591 
  592                 /*
  593                  * The lock is held in write mode or it already has waiters.
  594                  */
  595                 MPASS(!__rw_can_read(td, v, false));
  596 
  597                 /*
  598                  * If the RW_LOCK_READ_WAITERS flag is already set, then
  599                  * we can go ahead and block.  If it is not set then try
  600                  * to set it.  If we fail to set it drop the turnstile
  601                  * lock and restart the loop.
  602                  */
  603                 if (!(v & RW_LOCK_READ_WAITERS)) {
  604                         if (!atomic_fcmpset_ptr(&rw->rw_lock, &v,
  605                             v | RW_LOCK_READ_WAITERS))
  606                                 goto retry_ts;
  607                         if (LOCK_LOG_TEST(&rw->lock_object, 0))
  608                                 CTR2(KTR_LOCK, "%s: %p set read waiters flag",
  609                                     __func__, rw);
  610                 }
  611 
  612                 /*
  613                  * We were unable to acquire the lock and the read waiters
  614                  * flag is set, so we must block on the turnstile.
  615                  */
  616                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
  617                         CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__,
  618                             rw);
  619 #ifdef KDTRACE_HOOKS
  620                 sleep_time -= lockstat_nsecs(&rw->lock_object);
  621 #endif
  622                 MPASS(owner == rw_owner(rw));
  623                 turnstile_wait(ts, owner, TS_SHARED_QUEUE);
  624 #ifdef KDTRACE_HOOKS
  625                 sleep_time += lockstat_nsecs(&rw->lock_object);
  626                 sleep_cnt++;
  627 #endif
  628                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
  629                         CTR2(KTR_LOCK, "%s: %p resuming from turnstile",
  630                             __func__, rw);
  631                 v = RW_READ_VALUE(rw);
  632         }
  633 #if defined(KDTRACE_HOOKS) || defined(LOCK_PROFILING)
  634         if (__predict_true(!doing_lockprof))
  635                 return;
  636 #endif
  637 #ifdef KDTRACE_HOOKS
  638         all_time += lockstat_nsecs(&rw->lock_object);
  639         if (sleep_time)
  640                 LOCKSTAT_RECORD4(rw__block, rw, sleep_time,
  641                     LOCKSTAT_READER, (state & RW_LOCK_READ) == 0,
  642                     (state & RW_LOCK_READ) == 0 ? 0 : RW_READERS(state));
  643 
  644         /* Record only the loops spinning and not sleeping. */
  645         if (lda.spin_cnt > sleep_cnt)
  646                 LOCKSTAT_RECORD4(rw__spin, rw, all_time - sleep_time,
  647                     LOCKSTAT_READER, (state & RW_LOCK_READ) == 0,
  648                     (state & RW_LOCK_READ) == 0 ? 0 : RW_READERS(state));
  649 out_lockstat:
  650 #endif
  651         /*
  652          * TODO: acquire "owner of record" here.  Here be turnstile dragons
  653          * however.  turnstiles don't like owners changing between calls to
  654          * turnstile_wait() currently.
  655          */
  656         LOCKSTAT_PROFILE_OBTAIN_RWLOCK_SUCCESS(rw__acquire, rw, contested,
  657             waittime, file, line, LOCKSTAT_READER);
  658 }
  659 
  660 void
  661 __rw_rlock_int(struct rwlock *rw LOCK_FILE_LINE_ARG_DEF)
  662 {
  663         struct thread *td;
  664         uintptr_t v;
  665 
  666         td = curthread;
  667 
  668         KASSERT(kdb_active != 0 || SCHEDULER_STOPPED_TD(td) ||
  669             !TD_IS_IDLETHREAD(td),
  670             ("rw_rlock() by idle thread %p on rwlock %s @ %s:%d",
  671             td, rw->lock_object.lo_name, file, line));
  672         KASSERT(rw->rw_lock != RW_DESTROYED,
  673             ("rw_rlock() of destroyed rwlock @ %s:%d", file, line));
  674         KASSERT(rw_wowner(rw) != td,
  675             ("rw_rlock: wlock already held for %s @ %s:%d",
  676             rw->lock_object.lo_name, file, line));
  677         WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER, file, line, NULL);
  678 
  679         v = RW_READ_VALUE(rw);
  680         if (__predict_false(LOCKSTAT_PROFILE_ENABLED(rw__acquire) ||
  681             !__rw_rlock_try(rw, td, &v, true LOCK_FILE_LINE_ARG)))
  682                 __rw_rlock_hard(rw, td, v LOCK_FILE_LINE_ARG);
  683         else
  684                 lock_profile_obtain_lock_success(&rw->lock_object, 0, 0,
  685                     file, line);
  686 
  687         LOCK_LOG_LOCK("RLOCK", &rw->lock_object, 0, 0, file, line);
  688         WITNESS_LOCK(&rw->lock_object, 0, file, line);
  689         TD_LOCKS_INC(curthread);
  690 }
  691 
  692 void
  693 __rw_rlock(volatile uintptr_t *c, const char *file, int line)
  694 {
  695         struct rwlock *rw;
  696 
  697         rw = rwlock2rw(c);
  698         __rw_rlock_int(rw LOCK_FILE_LINE_ARG);
  699 }
  700 
  701 int
  702 __rw_try_rlock_int(struct rwlock *rw LOCK_FILE_LINE_ARG_DEF)
  703 {
  704         uintptr_t x;
  705 
  706         if (SCHEDULER_STOPPED())
  707                 return (1);
  708 
  709         KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread),
  710             ("rw_try_rlock() by idle thread %p on rwlock %s @ %s:%d",
  711             curthread, rw->lock_object.lo_name, file, line));
  712 
  713         x = rw->rw_lock;
  714         for (;;) {
  715                 KASSERT(rw->rw_lock != RW_DESTROYED,
  716                     ("rw_try_rlock() of destroyed rwlock @ %s:%d", file, line));
  717                 if (!(x & RW_LOCK_READ))
  718                         break;
  719                 if (atomic_fcmpset_acq_ptr(&rw->rw_lock, &x, x + RW_ONE_READER)) {
  720                         LOCK_LOG_TRY("RLOCK", &rw->lock_object, 0, 1, file,
  721                             line);
  722                         WITNESS_LOCK(&rw->lock_object, LOP_TRYLOCK, file, line);
  723                         LOCKSTAT_PROFILE_OBTAIN_RWLOCK_SUCCESS(rw__acquire,
  724                             rw, 0, 0, file, line, LOCKSTAT_READER);
  725                         TD_LOCKS_INC(curthread);
  726                         curthread->td_rw_rlocks++;
  727                         return (1);
  728                 }
  729         }
  730 
  731         LOCK_LOG_TRY("RLOCK", &rw->lock_object, 0, 0, file, line);
  732         return (0);
  733 }
  734 
  735 int
  736 __rw_try_rlock(volatile uintptr_t *c, const char *file, int line)
  737 {
  738         struct rwlock *rw;
  739 
  740         rw = rwlock2rw(c);
  741         return (__rw_try_rlock_int(rw LOCK_FILE_LINE_ARG));
  742 }
  743 
  744 static bool __always_inline
  745 __rw_runlock_try(struct rwlock *rw, struct thread *td, uintptr_t *vp)
  746 {
  747 
  748         for (;;) {
  749                 if (RW_READERS(*vp) > 1 || !(*vp & RW_LOCK_WAITERS)) {
  750                         if (atomic_fcmpset_rel_ptr(&rw->rw_lock, vp,
  751                             *vp - RW_ONE_READER)) {
  752                                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
  753                                         CTR4(KTR_LOCK,
  754                                             "%s: %p succeeded %p -> %p",
  755                                             __func__, rw, (void *)*vp,
  756                                             (void *)(*vp - RW_ONE_READER));
  757                                 td->td_rw_rlocks--;
  758                                 return (true);
  759                         }
  760                         continue;
  761                 }
  762                 break;
  763         }
  764         return (false);
  765 }
  766 
  767 static void __noinline
  768 __rw_runlock_hard(struct rwlock *rw, struct thread *td, uintptr_t v
  769     LOCK_FILE_LINE_ARG_DEF)
  770 {
  771         struct turnstile *ts;
  772         uintptr_t setv, queue;
  773 
  774         if (SCHEDULER_STOPPED())
  775                 return;
  776 
  777         if (__rw_runlock_try(rw, td, &v))
  778                 goto out_lockstat;
  779 
  780         /*
  781          * Ok, we know we have waiters and we think we are the
  782          * last reader, so grab the turnstile lock.
  783          */
  784         turnstile_chain_lock(&rw->lock_object);
  785         v = RW_READ_VALUE(rw);
  786         for (;;) {
  787                 if (__rw_runlock_try(rw, td, &v))
  788                         break;
  789 
  790                 MPASS(v & RW_LOCK_WAITERS);
  791 
  792                 /*
  793                  * Try to drop our lock leaving the lock in a unlocked
  794                  * state.
  795                  *
  796                  * If you wanted to do explicit lock handoff you'd have to
  797                  * do it here.  You'd also want to use turnstile_signal()
  798                  * and you'd have to handle the race where a higher
  799                  * priority thread blocks on the write lock before the
  800                  * thread you wakeup actually runs and have the new thread
  801                  * "steal" the lock.  For now it's a lot simpler to just
  802                  * wakeup all of the waiters.
  803                  *
  804                  * As above, if we fail, then another thread might have
  805                  * acquired a read lock, so drop the turnstile lock and
  806                  * restart.
  807                  */
  808                 setv = RW_UNLOCKED;
  809                 queue = TS_SHARED_QUEUE;
  810                 if (v & RW_LOCK_WRITE_WAITERS) {
  811                         queue = TS_EXCLUSIVE_QUEUE;
  812                         setv |= (v & RW_LOCK_READ_WAITERS);
  813                 }
  814                 setv |= (v & RW_LOCK_WRITE_SPINNER);
  815                 if (!atomic_fcmpset_rel_ptr(&rw->rw_lock, &v, setv))
  816                         continue;
  817                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
  818                         CTR2(KTR_LOCK, "%s: %p last succeeded with waiters",
  819                             __func__, rw);
  820 
  821                 /*
  822                  * Ok.  The lock is released and all that's left is to
  823                  * wake up the waiters.  Note that the lock might not be
  824                  * free anymore, but in that case the writers will just
  825                  * block again if they run before the new lock holder(s)
  826                  * release the lock.
  827                  */
  828                 ts = turnstile_lookup(&rw->lock_object);
  829                 MPASS(ts != NULL);
  830                 turnstile_broadcast(ts, queue);
  831                 turnstile_unpend(ts);
  832                 td->td_rw_rlocks--;
  833                 break;
  834         }
  835         turnstile_chain_unlock(&rw->lock_object);
  836 out_lockstat:
  837         LOCKSTAT_PROFILE_RELEASE_RWLOCK(rw__release, rw, LOCKSTAT_READER);
  838 }
  839 
  840 void
  841 _rw_runlock_cookie_int(struct rwlock *rw LOCK_FILE_LINE_ARG_DEF)
  842 {
  843         struct thread *td;
  844         uintptr_t v;
  845 
  846         KASSERT(rw->rw_lock != RW_DESTROYED,
  847             ("rw_runlock() of destroyed rwlock @ %s:%d", file, line));
  848         __rw_assert(&rw->rw_lock, RA_RLOCKED, file, line);
  849         WITNESS_UNLOCK(&rw->lock_object, 0, file, line);
  850         LOCK_LOG_LOCK("RUNLOCK", &rw->lock_object, 0, 0, file, line);
  851 
  852         td = curthread;
  853         v = RW_READ_VALUE(rw);
  854 
  855         if (__predict_false(LOCKSTAT_PROFILE_ENABLED(rw__release) ||
  856             !__rw_runlock_try(rw, td, &v)))
  857                 __rw_runlock_hard(rw, td, v LOCK_FILE_LINE_ARG);
  858         else
  859                 lock_profile_release_lock(&rw->lock_object);
  860 
  861         TD_LOCKS_DEC(curthread);
  862 }
  863 
  864 void
  865 _rw_runlock_cookie(volatile uintptr_t *c, const char *file, int line)
  866 {
  867         struct rwlock *rw;
  868 
  869         rw = rwlock2rw(c);
  870         _rw_runlock_cookie_int(rw LOCK_FILE_LINE_ARG);
  871 }
  872 
  873 #ifdef ADAPTIVE_RWLOCKS
  874 static inline void
  875 rw_drop_critical(uintptr_t v, bool *in_critical, int *extra_work)
  876 {
  877 
  878         if (v & RW_LOCK_WRITE_SPINNER)
  879                 return;
  880         if (*in_critical) {
  881                 critical_exit();
  882                 *in_critical = false;
  883                 (*extra_work)--;
  884         }
  885 }
  886 #else
  887 #define rw_drop_critical(v, in_critical, extra_work) do { } while (0)
  888 #endif
  889 
  890 /*
  891  * This function is called when we are unable to obtain a write lock on the
  892  * first try.  This means that at least one other thread holds either a
  893  * read or write lock.
  894  */
  895 void
  896 __rw_wlock_hard(volatile uintptr_t *c, uintptr_t v LOCK_FILE_LINE_ARG_DEF)
  897 {
  898         uintptr_t tid;
  899         struct rwlock *rw;
  900         struct turnstile *ts;
  901         struct thread *owner;
  902 #ifdef ADAPTIVE_RWLOCKS
  903         int spintries = 0;
  904         int i, n;
  905         enum { READERS, WRITER } sleep_reason = READERS;
  906         bool in_critical = false;
  907 #endif
  908         uintptr_t setv;
  909 #ifdef LOCK_PROFILING
  910         uint64_t waittime = 0;
  911         int contested = 0;
  912 #endif
  913 #if defined(ADAPTIVE_RWLOCKS) || defined(KDTRACE_HOOKS)
  914         struct lock_delay_arg lda;
  915 #endif
  916 #ifdef KDTRACE_HOOKS
  917         u_int sleep_cnt = 0;
  918         int64_t sleep_time = 0;
  919         int64_t all_time = 0;
  920 #endif
  921 #if defined(KDTRACE_HOOKS) || defined(LOCK_PROFILING)
  922         uintptr_t state = 0;
  923         int doing_lockprof = 0;
  924 #endif
  925         int extra_work = 0;
  926 
  927         tid = (uintptr_t)curthread;
  928         rw = rwlock2rw(c);
  929 
  930 #ifdef KDTRACE_HOOKS
  931         if (LOCKSTAT_PROFILE_ENABLED(rw__acquire)) {
  932                 while (v == RW_UNLOCKED) {
  933                         if (_rw_write_lock_fetch(rw, &v, tid))
  934                                 goto out_lockstat;
  935                 }
  936                 extra_work = 1;
  937                 doing_lockprof = 1;
  938                 all_time -= lockstat_nsecs(&rw->lock_object);
  939                 state = v;
  940         }
  941 #endif
  942 #ifdef LOCK_PROFILING
  943         extra_work = 1;
  944         doing_lockprof = 1;
  945         state = v;
  946 #endif
  947 
  948         if (SCHEDULER_STOPPED())
  949                 return;
  950 
  951         if (__predict_false(v == RW_UNLOCKED))
  952                 v = RW_READ_VALUE(rw);
  953 
  954         if (__predict_false(lv_rw_wowner(v) == (struct thread *)tid)) {
  955                 KASSERT(rw->lock_object.lo_flags & LO_RECURSABLE,
  956                     ("%s: recursing but non-recursive rw %s @ %s:%d\n",
  957                     __func__, rw->lock_object.lo_name, file, line));
  958                 rw->rw_recurse++;
  959                 atomic_set_ptr(&rw->rw_lock, RW_LOCK_WRITER_RECURSED);
  960                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
  961                         CTR2(KTR_LOCK, "%s: %p recursing", __func__, rw);
  962                 return;
  963         }
  964 
  965         if (LOCK_LOG_TEST(&rw->lock_object, 0))
  966                 CTR5(KTR_LOCK, "%s: %s contested (lock=%p) at %s:%d", __func__,
  967                     rw->lock_object.lo_name, (void *)rw->rw_lock, file, line);
  968 
  969 #if defined(ADAPTIVE_RWLOCKS)
  970         lock_delay_arg_init(&lda, &rw_delay);
  971 #elif defined(KDTRACE_HOOKS)
  972         lock_delay_arg_init_noadapt(&lda);
  973 #endif
  974 
  975 #ifdef HWPMC_HOOKS
  976         PMC_SOFT_CALL( , , lock, failed);
  977 #endif
  978         lock_profile_obtain_lock_failed(&rw->lock_object,
  979             &contested, &waittime);
  980 
  981         for (;;) {
  982                 if (v == RW_UNLOCKED) {
  983                         if (_rw_write_lock_fetch(rw, &v, tid))
  984                                 break;
  985                         continue;
  986                 }
  987 #ifdef KDTRACE_HOOKS
  988                 lda.spin_cnt++;
  989 #endif
  990 
  991 #ifdef ADAPTIVE_RWLOCKS
  992                 if (v == (RW_LOCK_READ | RW_LOCK_WRITE_SPINNER)) {
  993                         if (atomic_fcmpset_acq_ptr(&rw->rw_lock, &v, tid))
  994                                 break;
  995                         continue;
  996                 }
  997 
  998                 /*
  999                  * If the lock is write locked and the owner is
 1000                  * running on another CPU, spin until the owner stops
 1001                  * running or the state of the lock changes.
 1002                  */
 1003                 if (!(v & RW_LOCK_READ)) {
 1004                         rw_drop_critical(v, &in_critical, &extra_work);
 1005                         sleep_reason = WRITER;
 1006                         owner = lv_rw_wowner(v);
 1007                         if (!TD_IS_RUNNING(owner))
 1008                                 goto ts;
 1009                         if (LOCK_LOG_TEST(&rw->lock_object, 0))
 1010                                 CTR3(KTR_LOCK, "%s: spinning on %p held by %p",
 1011                                     __func__, rw, owner);
 1012                         KTR_STATE1(KTR_SCHED, "thread", sched_tdname(curthread),
 1013                             "spinning", "lockname:\"%s\"",
 1014                             rw->lock_object.lo_name);
 1015                         do {
 1016                                 lock_delay(&lda);
 1017                                 v = RW_READ_VALUE(rw);
 1018                                 owner = lv_rw_wowner(v);
 1019                         } while (owner != NULL && TD_IS_RUNNING(owner));
 1020                         KTR_STATE0(KTR_SCHED, "thread", sched_tdname(curthread),
 1021                             "running");
 1022                         continue;
 1023                 } else if (RW_READERS(v) > 0) {
 1024                         sleep_reason = READERS;
 1025                         if (spintries == rowner_retries)
 1026                                 goto ts;
 1027                         if (!(v & RW_LOCK_WRITE_SPINNER)) {
 1028                                 if (!in_critical) {
 1029                                         critical_enter();
 1030                                         in_critical = true;
 1031                                         extra_work++;
 1032                                 }
 1033                                 if (!atomic_fcmpset_ptr(&rw->rw_lock, &v,
 1034                                     v | RW_LOCK_WRITE_SPINNER)) {
 1035                                         critical_exit();
 1036                                         in_critical = false;
 1037                                         extra_work--;
 1038                                         continue;
 1039                                 }
 1040                         }
 1041                         spintries++;
 1042                         KTR_STATE1(KTR_SCHED, "thread", sched_tdname(curthread),
 1043                             "spinning", "lockname:\"%s\"",
 1044                             rw->lock_object.lo_name);
 1045                         n = RW_READERS(v);
 1046                         for (i = 0; i < rowner_loops; i += n) {
 1047                                 lock_delay_spin(n);
 1048                                 v = RW_READ_VALUE(rw);
 1049                                 if (!(v & RW_LOCK_WRITE_SPINNER))
 1050                                         break;
 1051                                 if (!(v & RW_LOCK_READ))
 1052                                         break;
 1053                                 n = RW_READERS(v);
 1054                                 if (n == 0)
 1055                                         break;
 1056                         }
 1057 #ifdef KDTRACE_HOOKS
 1058                         lda.spin_cnt += i;
 1059 #endif
 1060                         KTR_STATE0(KTR_SCHED, "thread", sched_tdname(curthread),
 1061                             "running");
 1062                         if (i < rowner_loops)
 1063                                 continue;
 1064                 }
 1065 ts:
 1066 #endif
 1067                 ts = turnstile_trywait(&rw->lock_object);
 1068                 v = RW_READ_VALUE(rw);
 1069 retry_ts:
 1070                 owner = lv_rw_wowner(v);
 1071 
 1072 #ifdef ADAPTIVE_RWLOCKS
 1073                 /*
 1074                  * The current lock owner might have started executing
 1075                  * on another CPU (or the lock could have changed
 1076                  * owners) while we were waiting on the turnstile
 1077                  * chain lock.  If so, drop the turnstile lock and try
 1078                  * again.
 1079                  */
 1080                 if (owner != NULL) {
 1081                         if (TD_IS_RUNNING(owner)) {
 1082                                 turnstile_cancel(ts);
 1083                                 rw_drop_critical(v, &in_critical, &extra_work);
 1084                                 continue;
 1085                         }
 1086                 } else if (RW_READERS(v) > 0 && sleep_reason == WRITER) {
 1087                         turnstile_cancel(ts);
 1088                         rw_drop_critical(v, &in_critical, &extra_work);
 1089                         continue;
 1090                 }
 1091 #endif
 1092                 /*
 1093                  * Check for the waiters flags about this rwlock.
 1094                  * If the lock was released, without maintain any pending
 1095                  * waiters queue, simply try to acquire it.
 1096                  * If a pending waiters queue is present, claim the lock
 1097                  * ownership and maintain the pending queue.
 1098                  */
 1099                 setv = v & (RW_LOCK_WAITERS | RW_LOCK_WRITE_SPINNER);
 1100                 if ((v & ~setv) == RW_UNLOCKED) {
 1101                         setv &= ~RW_LOCK_WRITE_SPINNER;
 1102                         if (atomic_fcmpset_acq_ptr(&rw->rw_lock, &v, tid | setv)) {
 1103                                 if (setv)
 1104                                         turnstile_claim(ts);
 1105                                 else
 1106                                         turnstile_cancel(ts);
 1107                                 break;
 1108                         }
 1109                         goto retry_ts;
 1110                 }
 1111 
 1112 #ifdef ADAPTIVE_RWLOCKS
 1113                 if (in_critical) {
 1114                         if ((v & RW_LOCK_WRITE_SPINNER) ||
 1115                             !((v & RW_LOCK_WRITE_WAITERS))) {
 1116                                 setv = v & ~RW_LOCK_WRITE_SPINNER;
 1117                                 setv |= RW_LOCK_WRITE_WAITERS;
 1118                                 if (!atomic_fcmpset_ptr(&rw->rw_lock, &v, setv))
 1119                                         goto retry_ts;
 1120                         }
 1121                         critical_exit();
 1122                         in_critical = false;
 1123                         extra_work--;
 1124                 } else {
 1125 #endif
 1126                         /*
 1127                          * If the RW_LOCK_WRITE_WAITERS flag isn't set, then try to
 1128                          * set it.  If we fail to set it, then loop back and try
 1129                          * again.
 1130                          */
 1131                         if (!(v & RW_LOCK_WRITE_WAITERS)) {
 1132                                 if (!atomic_fcmpset_ptr(&rw->rw_lock, &v,
 1133                                     v | RW_LOCK_WRITE_WAITERS))
 1134                                         goto retry_ts;
 1135                                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 1136                                         CTR2(KTR_LOCK, "%s: %p set write waiters flag",
 1137                                             __func__, rw);
 1138                         }
 1139 #ifdef ADAPTIVE_RWLOCKS
 1140                 }
 1141 #endif
 1142                 /*
 1143                  * We were unable to acquire the lock and the write waiters
 1144                  * flag is set, so we must block on the turnstile.
 1145                  */
 1146                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 1147                         CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__,
 1148                             rw);
 1149 #ifdef KDTRACE_HOOKS
 1150                 sleep_time -= lockstat_nsecs(&rw->lock_object);
 1151 #endif
 1152                 MPASS(owner == rw_owner(rw));
 1153                 turnstile_wait(ts, owner, TS_EXCLUSIVE_QUEUE);
 1154 #ifdef KDTRACE_HOOKS
 1155                 sleep_time += lockstat_nsecs(&rw->lock_object);
 1156                 sleep_cnt++;
 1157 #endif
 1158                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 1159                         CTR2(KTR_LOCK, "%s: %p resuming from turnstile",
 1160                             __func__, rw);
 1161 #ifdef ADAPTIVE_RWLOCKS
 1162                 spintries = 0;
 1163 #endif
 1164                 v = RW_READ_VALUE(rw);
 1165         }
 1166         if (__predict_true(!extra_work))
 1167                 return;
 1168 #ifdef ADAPTIVE_RWLOCKS
 1169         if (in_critical)
 1170                 critical_exit();
 1171 #endif
 1172 #if defined(KDTRACE_HOOKS) || defined(LOCK_PROFILING)
 1173         if (__predict_true(!doing_lockprof))
 1174                 return;
 1175 #endif
 1176 #ifdef KDTRACE_HOOKS
 1177         all_time += lockstat_nsecs(&rw->lock_object);
 1178         if (sleep_time)
 1179                 LOCKSTAT_RECORD4(rw__block, rw, sleep_time,
 1180                     LOCKSTAT_WRITER, (state & RW_LOCK_READ) == 0,
 1181                     (state & RW_LOCK_READ) == 0 ? 0 : RW_READERS(state));
 1182 
 1183         /* Record only the loops spinning and not sleeping. */
 1184         if (lda.spin_cnt > sleep_cnt)
 1185                 LOCKSTAT_RECORD4(rw__spin, rw, all_time - sleep_time,
 1186                     LOCKSTAT_WRITER, (state & RW_LOCK_READ) == 0,
 1187                     (state & RW_LOCK_READ) == 0 ? 0 : RW_READERS(state));
 1188 out_lockstat:
 1189 #endif
 1190         LOCKSTAT_PROFILE_OBTAIN_RWLOCK_SUCCESS(rw__acquire, rw, contested,
 1191             waittime, file, line, LOCKSTAT_WRITER);
 1192 }
 1193 
 1194 /*
 1195  * This function is called if lockstat is active or the first try at releasing
 1196  * a write lock failed.  The latter means that the lock is recursed or one of
 1197  * the 2 waiter bits must be set indicating that at least one thread is waiting
 1198  * on this lock.
 1199  */
 1200 void
 1201 __rw_wunlock_hard(volatile uintptr_t *c, uintptr_t v LOCK_FILE_LINE_ARG_DEF)
 1202 {
 1203         struct rwlock *rw;
 1204         struct turnstile *ts;
 1205         uintptr_t tid, setv;
 1206         int queue;
 1207 
 1208         tid = (uintptr_t)curthread;
 1209         if (SCHEDULER_STOPPED())
 1210                 return;
 1211 
 1212         rw = rwlock2rw(c);
 1213         if (__predict_false(v == tid))
 1214                 v = RW_READ_VALUE(rw);
 1215 
 1216         if (v & RW_LOCK_WRITER_RECURSED) {
 1217                 if (--(rw->rw_recurse) == 0)
 1218                         atomic_clear_ptr(&rw->rw_lock, RW_LOCK_WRITER_RECURSED);
 1219                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
 1220                         CTR2(KTR_LOCK, "%s: %p unrecursing", __func__, rw);
 1221                 return;
 1222         }
 1223 
 1224         LOCKSTAT_PROFILE_RELEASE_RWLOCK(rw__release, rw, LOCKSTAT_WRITER);
 1225         if (v == tid && _rw_write_unlock(rw, tid))
 1226                 return;
 1227 
 1228         KASSERT(rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS),
 1229             ("%s: neither of the waiter flags are set", __func__));
 1230 
 1231         if (LOCK_LOG_TEST(&rw->lock_object, 0))
 1232                 CTR2(KTR_LOCK, "%s: %p contested", __func__, rw);
 1233 
 1234         turnstile_chain_lock(&rw->lock_object);
 1235 
 1236         /*
 1237          * Use the same algo as sx locks for now.  Prefer waking up shared
 1238          * waiters if we have any over writers.  This is probably not ideal.
 1239          *
 1240          * 'v' is the value we are going to write back to rw_lock.  If we
 1241          * have waiters on both queues, we need to preserve the state of
 1242          * the waiter flag for the queue we don't wake up.  For now this is
 1243          * hardcoded for the algorithm mentioned above.
 1244          *
 1245          * In the case of both readers and writers waiting we wakeup the
 1246          * readers but leave the RW_LOCK_WRITE_WAITERS flag set.  If a
 1247          * new writer comes in before a reader it will claim the lock up
 1248          * above.  There is probably a potential priority inversion in
 1249          * there that could be worked around either by waking both queues
 1250          * of waiters or doing some complicated lock handoff gymnastics.
 1251          */
 1252         setv = RW_UNLOCKED;
 1253         v = RW_READ_VALUE(rw);
 1254         queue = TS_SHARED_QUEUE;
 1255         if (v & RW_LOCK_WRITE_WAITERS) {
 1256                 queue = TS_EXCLUSIVE_QUEUE;
 1257                 setv |= (v & RW_LOCK_READ_WAITERS);
 1258         }
 1259         atomic_store_rel_ptr(&rw->rw_lock, setv);
 1260 
 1261         /* Wake up all waiters for the specific queue. */
 1262         if (LOCK_LOG_TEST(&rw->lock_object, 0))
 1263                 CTR3(KTR_LOCK, "%s: %p waking up %s waiters", __func__, rw,
 1264                     queue == TS_SHARED_QUEUE ? "read" : "write");
 1265 
 1266         ts = turnstile_lookup(&rw->lock_object);
 1267         MPASS(ts != NULL);
 1268         turnstile_broadcast(ts, queue);
 1269         turnstile_unpend(ts);
 1270         turnstile_chain_unlock(&rw->lock_object);
 1271 }
 1272 
 1273 /*
 1274  * Attempt to do a non-blocking upgrade from a read lock to a write
 1275  * lock.  This will only succeed if this thread holds a single read
 1276  * lock.  Returns true if the upgrade succeeded and false otherwise.
 1277  */
 1278 int
 1279 __rw_try_upgrade_int(struct rwlock *rw LOCK_FILE_LINE_ARG_DEF)
 1280 {
 1281         uintptr_t v, setv, tid;
 1282         struct turnstile *ts;
 1283         int success;
 1284 
 1285         if (SCHEDULER_STOPPED())
 1286                 return (1);
 1287 
 1288         KASSERT(rw->rw_lock != RW_DESTROYED,
 1289             ("rw_try_upgrade() of destroyed rwlock @ %s:%d", file, line));
 1290         __rw_assert(&rw->rw_lock, RA_RLOCKED, file, line);
 1291 
 1292         /*
 1293          * Attempt to switch from one reader to a writer.  If there
 1294          * are any write waiters, then we will have to lock the
 1295          * turnstile first to prevent races with another writer
 1296          * calling turnstile_wait() before we have claimed this
 1297          * turnstile.  So, do the simple case of no waiters first.
 1298          */
 1299         tid = (uintptr_t)curthread;
 1300         success = 0;
 1301         v = RW_READ_VALUE(rw);
 1302         for (;;) {
 1303                 if (RW_READERS(v) > 1)
 1304                         break;
 1305                 if (!(v & RW_LOCK_WAITERS)) {
 1306                         success = atomic_fcmpset_acq_ptr(&rw->rw_lock, &v, tid);
 1307                         if (!success)
 1308                                 continue;
 1309                         break;
 1310                 }
 1311 
 1312                 /*
 1313                  * Ok, we think we have waiters, so lock the turnstile.
 1314                  */
 1315                 ts = turnstile_trywait(&rw->lock_object);
 1316                 v = RW_READ_VALUE(rw);
 1317 retry_ts:
 1318                 if (RW_READERS(v) > 1) {
 1319                         turnstile_cancel(ts);
 1320                         break;
 1321                 }
 1322                 /*
 1323                  * Try to switch from one reader to a writer again.  This time
 1324                  * we honor the current state of the waiters flags.
 1325                  * If we obtain the lock with the flags set, then claim
 1326                  * ownership of the turnstile.
 1327                  */
 1328                 setv = tid | (v & RW_LOCK_WAITERS);
 1329                 success = atomic_fcmpset_ptr(&rw->rw_lock, &v, setv);
 1330                 if (success) {
 1331                         if (v & RW_LOCK_WAITERS)
 1332                                 turnstile_claim(ts);
 1333                         else
 1334                                 turnstile_cancel(ts);
 1335                         break;
 1336                 }
 1337                 goto retry_ts;
 1338         }
 1339         LOCK_LOG_TRY("WUPGRADE", &rw->lock_object, 0, success, file, line);
 1340         if (success) {
 1341                 curthread->td_rw_rlocks--;
 1342                 WITNESS_UPGRADE(&rw->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
 1343                     file, line);
 1344                 LOCKSTAT_RECORD0(rw__upgrade, rw);
 1345         }
 1346         return (success);
 1347 }
 1348 
 1349 int
 1350 __rw_try_upgrade(volatile uintptr_t *c, const char *file, int line)
 1351 {
 1352         struct rwlock *rw;
 1353 
 1354         rw = rwlock2rw(c);
 1355         return (__rw_try_upgrade_int(rw LOCK_FILE_LINE_ARG));
 1356 }
 1357 
 1358 /*
 1359  * Downgrade a write lock into a single read lock.
 1360  */
 1361 void
 1362 __rw_downgrade_int(struct rwlock *rw LOCK_FILE_LINE_ARG_DEF)
 1363 {
 1364         struct turnstile *ts;
 1365         uintptr_t tid, v;
 1366         int rwait, wwait;
 1367 
 1368         if (SCHEDULER_STOPPED())
 1369                 return;
 1370 
 1371         KASSERT(rw->rw_lock != RW_DESTROYED,
 1372             ("rw_downgrade() of destroyed rwlock @ %s:%d", file, line));
 1373         __rw_assert(&rw->rw_lock, RA_WLOCKED | RA_NOTRECURSED, file, line);
 1374 #ifndef INVARIANTS
 1375         if (rw_recursed(rw))
 1376                 panic("downgrade of a recursed lock");
 1377 #endif
 1378 
 1379         WITNESS_DOWNGRADE(&rw->lock_object, 0, file, line);
 1380 
 1381         /*
 1382          * Convert from a writer to a single reader.  First we handle
 1383          * the easy case with no waiters.  If there are any waiters, we
 1384          * lock the turnstile and "disown" the lock.
 1385          */
 1386         tid = (uintptr_t)curthread;
 1387         if (atomic_cmpset_rel_ptr(&rw->rw_lock, tid, RW_READERS_LOCK(1)))
 1388                 goto out;
 1389 
 1390         /*
 1391          * Ok, we think we have waiters, so lock the turnstile so we can
 1392          * read the waiter flags without any races.
 1393          */
 1394         turnstile_chain_lock(&rw->lock_object);
 1395         v = rw->rw_lock & RW_LOCK_WAITERS;
 1396         rwait = v & RW_LOCK_READ_WAITERS;
 1397         wwait = v & RW_LOCK_WRITE_WAITERS;
 1398         MPASS(rwait | wwait);
 1399 
 1400         /*
 1401          * Downgrade from a write lock while preserving waiters flag
 1402          * and give up ownership of the turnstile.
 1403          */
 1404         ts = turnstile_lookup(&rw->lock_object);
 1405         MPASS(ts != NULL);
 1406         if (!wwait)
 1407                 v &= ~RW_LOCK_READ_WAITERS;
 1408         atomic_store_rel_ptr(&rw->rw_lock, RW_READERS_LOCK(1) | v);
 1409         /*
 1410          * Wake other readers if there are no writers pending.  Otherwise they
 1411          * won't be able to acquire the lock anyway.
 1412          */
 1413         if (rwait && !wwait) {
 1414                 turnstile_broadcast(ts, TS_SHARED_QUEUE);
 1415                 turnstile_unpend(ts);
 1416         } else
 1417                 turnstile_disown(ts);
 1418         turnstile_chain_unlock(&rw->lock_object);
 1419 out:
 1420         curthread->td_rw_rlocks++;
 1421         LOCK_LOG_LOCK("WDOWNGRADE", &rw->lock_object, 0, 0, file, line);
 1422         LOCKSTAT_RECORD0(rw__downgrade, rw);
 1423 }
 1424 
 1425 void
 1426 __rw_downgrade(volatile uintptr_t *c, const char *file, int line)
 1427 {
 1428         struct rwlock *rw;
 1429 
 1430         rw = rwlock2rw(c);
 1431         __rw_downgrade_int(rw LOCK_FILE_LINE_ARG);
 1432 }
 1433 
 1434 #ifdef INVARIANT_SUPPORT
 1435 #ifndef INVARIANTS
 1436 #undef __rw_assert
 1437 #endif
 1438 
 1439 /*
 1440  * In the non-WITNESS case, rw_assert() can only detect that at least
 1441  * *some* thread owns an rlock, but it cannot guarantee that *this*
 1442  * thread owns an rlock.
 1443  */
 1444 void
 1445 __rw_assert(const volatile uintptr_t *c, int what, const char *file, int line)
 1446 {
 1447         const struct rwlock *rw;
 1448 
 1449         if (SCHEDULER_STOPPED())
 1450                 return;
 1451 
 1452         rw = rwlock2rw(c);
 1453 
 1454         switch (what) {
 1455         case RA_LOCKED:
 1456         case RA_LOCKED | RA_RECURSED:
 1457         case RA_LOCKED | RA_NOTRECURSED:
 1458         case RA_RLOCKED:
 1459         case RA_RLOCKED | RA_RECURSED:
 1460         case RA_RLOCKED | RA_NOTRECURSED:
 1461 #ifdef WITNESS
 1462                 witness_assert(&rw->lock_object, what, file, line);
 1463 #else
 1464                 /*
 1465                  * If some other thread has a write lock or we have one
 1466                  * and are asserting a read lock, fail.  Also, if no one
 1467                  * has a lock at all, fail.
 1468                  */
 1469                 if (rw->rw_lock == RW_UNLOCKED ||
 1470                     (!(rw->rw_lock & RW_LOCK_READ) && (what & RA_RLOCKED ||
 1471                     rw_wowner(rw) != curthread)))
 1472                         panic("Lock %s not %slocked @ %s:%d\n",
 1473                             rw->lock_object.lo_name, (what & RA_RLOCKED) ?
 1474                             "read " : "", file, line);
 1475 
 1476                 if (!(rw->rw_lock & RW_LOCK_READ) && !(what & RA_RLOCKED)) {
 1477                         if (rw_recursed(rw)) {
 1478                                 if (what & RA_NOTRECURSED)
 1479                                         panic("Lock %s recursed @ %s:%d\n",
 1480                                             rw->lock_object.lo_name, file,
 1481                                             line);
 1482                         } else if (what & RA_RECURSED)
 1483                                 panic("Lock %s not recursed @ %s:%d\n",
 1484                                     rw->lock_object.lo_name, file, line);
 1485                 }
 1486 #endif
 1487                 break;
 1488         case RA_WLOCKED:
 1489         case RA_WLOCKED | RA_RECURSED:
 1490         case RA_WLOCKED | RA_NOTRECURSED:
 1491                 if (rw_wowner(rw) != curthread)
 1492                         panic("Lock %s not exclusively locked @ %s:%d\n",
 1493                             rw->lock_object.lo_name, file, line);
 1494                 if (rw_recursed(rw)) {
 1495                         if (what & RA_NOTRECURSED)
 1496                                 panic("Lock %s recursed @ %s:%d\n",
 1497                                     rw->lock_object.lo_name, file, line);
 1498                 } else if (what & RA_RECURSED)
 1499                         panic("Lock %s not recursed @ %s:%d\n",
 1500                             rw->lock_object.lo_name, file, line);
 1501                 break;
 1502         case RA_UNLOCKED:
 1503 #ifdef WITNESS
 1504                 witness_assert(&rw->lock_object, what, file, line);
 1505 #else
 1506                 /*
 1507                  * If we hold a write lock fail.  We can't reliably check
 1508                  * to see if we hold a read lock or not.
 1509                  */
 1510                 if (rw_wowner(rw) == curthread)
 1511                         panic("Lock %s exclusively locked @ %s:%d\n",
 1512                             rw->lock_object.lo_name, file, line);
 1513 #endif
 1514                 break;
 1515         default:
 1516                 panic("Unknown rw lock assertion: %d @ %s:%d", what, file,
 1517                     line);
 1518         }
 1519 }
 1520 #endif /* INVARIANT_SUPPORT */
 1521 
 1522 #ifdef DDB
 1523 void
 1524 db_show_rwlock(const struct lock_object *lock)
 1525 {
 1526         const struct rwlock *rw;
 1527         struct thread *td;
 1528 
 1529         rw = (const struct rwlock *)lock;
 1530 
 1531         db_printf(" state: ");
 1532         if (rw->rw_lock == RW_UNLOCKED)
 1533                 db_printf("UNLOCKED\n");
 1534         else if (rw->rw_lock == RW_DESTROYED) {
 1535                 db_printf("DESTROYED\n");
 1536                 return;
 1537         } else if (rw->rw_lock & RW_LOCK_READ)
 1538                 db_printf("RLOCK: %ju locks\n",
 1539                     (uintmax_t)(RW_READERS(rw->rw_lock)));
 1540         else {
 1541                 td = rw_wowner(rw);
 1542                 db_printf("WLOCK: %p (tid %d, pid %d, \"%s\")\n", td,
 1543                     td->td_tid, td->td_proc->p_pid, td->td_name);
 1544                 if (rw_recursed(rw))
 1545                         db_printf(" recursed: %u\n", rw->rw_recurse);
 1546         }
 1547         db_printf(" waiters: ");
 1548         switch (rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS)) {
 1549         case RW_LOCK_READ_WAITERS:
 1550                 db_printf("readers\n");
 1551                 break;
 1552         case RW_LOCK_WRITE_WAITERS:
 1553                 db_printf("writers\n");
 1554                 break;
 1555         case RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS:
 1556                 db_printf("readers and writers\n");
 1557                 break;
 1558         default:
 1559                 db_printf("none\n");
 1560                 break;
 1561         }
 1562 }
 1563 
 1564 #endif

Cache object: fee9ef9c21d85afaa3e375394e8b2d9f


[ source navigation ] [ diff markup ] [ identifier search ] [ freetext search ] [ file search ] [ list types ] [ track identifier ]


This page is part of the FreeBSD/Linux Linux Kernel Cross-Reference, and was automatically generated using a modified version of the LXR engine.