kern_rwlock.c

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    1 /*-
    2  * Copyright (c) 2006 John Baldwin <jhb@FreeBSD.org>
    3  * All rights reserved.
    4  *
    5  * Redistribution and use in source and binary forms, with or without
    6  * modification, are permitted provided that the following conditions
    7  * are met:
    8  * 1. Redistributions of source code must retain the above copyright
    9  *    notice, this list of conditions and the following disclaimer.
   10  * 2. Redistributions in binary form must reproduce the above copyright
   11  *    notice, this list of conditions and the following disclaimer in the
   12  *    documentation and/or other materials provided with the distribution.
   13  * 3. Neither the name of the author nor the names of any co-contributors
   14  *    may be used to endorse or promote products derived from this software
   15  *    without specific prior written permission.
   16  *
   17  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
   18  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   19  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   20  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
   21  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   22  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   23  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   24  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   25  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   26  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   27  * SUCH DAMAGE.
   28  */
   29 
   30 /*
   31  * Machine independent bits of reader/writer lock implementation.
   32  */
   33 
   34 #include <sys/cdefs.h>
   35 __FBSDID("$FreeBSD: releng/9.2/sys/kern/kern_rwlock.c 252172 2013-06-24 19:18:01Z jhb $");
   36 
   37 #include "opt_ddb.h"
   38 #include "opt_hwpmc_hooks.h"
   39 #include "opt_kdtrace.h"
   40 #include "opt_no_adaptive_rwlocks.h"
   41 
   42 #include <sys/param.h>
   43 #include <sys/ktr.h>
   44 #include <sys/kernel.h>
   45 #include <sys/lock.h>
   46 #include <sys/mutex.h>
   47 #include <sys/proc.h>
   48 #include <sys/rwlock.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 #ifdef ADAPTIVE_RWLOCKS
   65 #define ROWNER_RETRIES  10
   66 #define ROWNER_LOOPS    10000
   67 #endif
   68 
   69 #ifdef DDB
   70 #include <ddb/ddb.h>
   71 
   72 static void     db_show_rwlock(struct lock_object *lock);
   73 #endif
   74 static void     assert_rw(struct lock_object *lock, int what);
   75 static void     lock_rw(struct lock_object *lock, int how);
   76 #ifdef KDTRACE_HOOKS
   77 static int      owner_rw(struct lock_object *lock, struct thread **owner);
   78 #endif
   79 static int      unlock_rw(struct lock_object *lock);
   80 
   81 struct lock_class lock_class_rw = {
   82         .lc_name = "rw",
   83         .lc_flags = LC_SLEEPLOCK | LC_RECURSABLE | LC_UPGRADABLE,
   84         .lc_assert = assert_rw,
   85 #ifdef DDB
   86         .lc_ddb_show = db_show_rwlock,
   87 #endif
   88         .lc_lock = lock_rw,
   89         .lc_unlock = unlock_rw,
   90 #ifdef KDTRACE_HOOKS
   91         .lc_owner = owner_rw,
   92 #endif
   93 };
   94 
   95 /*
   96  * Return a pointer to the owning thread if the lock is write-locked or
   97  * NULL if the lock is unlocked or read-locked.
   98  */
   99 #define rw_wowner(rw)                                                   \
  100         ((rw)->rw_lock & RW_LOCK_READ ? NULL :                          \
  101             (struct thread *)RW_OWNER((rw)->rw_lock))
  102 
  103 /*
  104  * Returns if a write owner is recursed.  Write ownership is not assured
  105  * here and should be previously checked.
  106  */
  107 #define rw_recursed(rw)         ((rw)->rw_recurse != 0)
  108 
  109 /*
  110  * Return true if curthread helds the lock.
  111  */
  112 #define rw_wlocked(rw)          (rw_wowner((rw)) == curthread)
  113 
  114 /*
  115  * Return a pointer to the owning thread for this lock who should receive
  116  * any priority lent by threads that block on this lock.  Currently this
  117  * is identical to rw_wowner().
  118  */
  119 #define rw_owner(rw)            rw_wowner(rw)
  120 
  121 #ifndef INVARIANTS
  122 #define _rw_assert(rw, what, file, line)
  123 #endif
  124 
  125 void
  126 assert_rw(struct lock_object *lock, int what)
  127 {
  128 
  129         rw_assert((struct rwlock *)lock, what);
  130 }
  131 
  132 void
  133 lock_rw(struct lock_object *lock, int how)
  134 {
  135         struct rwlock *rw;
  136 
  137         rw = (struct rwlock *)lock;
  138         if (how)
  139                 rw_wlock(rw);
  140         else
  141                 rw_rlock(rw);
  142 }
  143 
  144 int
  145 unlock_rw(struct lock_object *lock)
  146 {
  147         struct rwlock *rw;
  148 
  149         rw = (struct rwlock *)lock;
  150         rw_assert(rw, RA_LOCKED | LA_NOTRECURSED);
  151         if (rw->rw_lock & RW_LOCK_READ) {
  152                 rw_runlock(rw);
  153                 return (0);
  154         } else {
  155                 rw_wunlock(rw);
  156                 return (1);
  157         }
  158 }
  159 
  160 #ifdef KDTRACE_HOOKS
  161 int
  162 owner_rw(struct lock_object *lock, struct thread **owner)
  163 {
  164         struct rwlock *rw = (struct rwlock *)lock;
  165         uintptr_t x = rw->rw_lock;
  166 
  167         *owner = rw_wowner(rw);
  168         return ((x & RW_LOCK_READ) != 0 ?  (RW_READERS(x) != 0) :
  169             (*owner != NULL));
  170 }
  171 #endif
  172 
  173 void
  174 rw_init_flags(struct rwlock *rw, const char *name, int opts)
  175 {
  176         int flags;
  177 
  178         MPASS((opts & ~(RW_DUPOK | RW_NOPROFILE | RW_NOWITNESS | RW_QUIET |
  179             RW_RECURSE)) == 0);
  180         ASSERT_ATOMIC_LOAD_PTR(rw->rw_lock,
  181             ("%s: rw_lock not aligned for %s: %p", __func__, name,
  182             &rw->rw_lock));
  183 
  184         flags = LO_UPGRADABLE;
  185         if (opts & RW_DUPOK)
  186                 flags |= LO_DUPOK;
  187         if (opts & RW_NOPROFILE)
  188                 flags |= LO_NOPROFILE;
  189         if (!(opts & RW_NOWITNESS))
  190                 flags |= LO_WITNESS;
  191         if (opts & RW_RECURSE)
  192                 flags |= LO_RECURSABLE;
  193         if (opts & RW_QUIET)
  194                 flags |= LO_QUIET;
  195 
  196         rw->rw_lock = RW_UNLOCKED;
  197         rw->rw_recurse = 0;
  198         lock_init(&rw->lock_object, &lock_class_rw, name, NULL, flags);
  199 }
  200 
  201 void
  202 rw_destroy(struct rwlock *rw)
  203 {
  204 
  205         KASSERT(rw->rw_lock == RW_UNLOCKED, ("rw lock %p not unlocked", rw));
  206         KASSERT(rw->rw_recurse == 0, ("rw lock %p still recursed", rw));
  207         rw->rw_lock = RW_DESTROYED;
  208         lock_destroy(&rw->lock_object);
  209 }
  210 
  211 void
  212 rw_sysinit(void *arg)
  213 {
  214         struct rw_args *args = arg;
  215 
  216         rw_init(args->ra_rw, args->ra_desc);
  217 }
  218 
  219 void
  220 rw_sysinit_flags(void *arg)
  221 {
  222         struct rw_args_flags *args = arg;
  223 
  224         rw_init_flags(args->ra_rw, args->ra_desc, args->ra_flags);
  225 }
  226 
  227 int
  228 rw_wowned(struct rwlock *rw)
  229 {
  230 
  231         return (rw_wowner(rw) == curthread);
  232 }
  233 
  234 void
  235 _rw_wlock(struct rwlock *rw, const char *file, int line)
  236 {
  237 
  238         if (SCHEDULER_STOPPED())
  239                 return;
  240         KASSERT(rw->rw_lock != RW_DESTROYED,
  241             ("rw_wlock() of destroyed rwlock @ %s:%d", file, line));
  242         WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER | LOP_EXCLUSIVE, file,
  243             line, NULL);
  244         __rw_wlock(rw, curthread, file, line);
  245         LOCK_LOG_LOCK("WLOCK", &rw->lock_object, 0, rw->rw_recurse, file, line);
  246         WITNESS_LOCK(&rw->lock_object, LOP_EXCLUSIVE, file, line);
  247         curthread->td_locks++;
  248 }
  249 
  250 int
  251 _rw_try_wlock(struct rwlock *rw, const char *file, int line)
  252 {
  253         int rval;
  254 
  255         if (SCHEDULER_STOPPED())
  256                 return (1);
  257 
  258         KASSERT(rw->rw_lock != RW_DESTROYED,
  259             ("rw_try_wlock() of destroyed rwlock @ %s:%d", file, line));
  260 
  261         if (rw_wlocked(rw) &&
  262             (rw->lock_object.lo_flags & LO_RECURSABLE) != 0) {
  263                 rw->rw_recurse++;
  264                 rval = 1;
  265         } else
  266                 rval = atomic_cmpset_acq_ptr(&rw->rw_lock, RW_UNLOCKED,
  267                     (uintptr_t)curthread);
  268 
  269         LOCK_LOG_TRY("WLOCK", &rw->lock_object, 0, rval, file, line);
  270         if (rval) {
  271                 WITNESS_LOCK(&rw->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
  272                     file, line);
  273                 curthread->td_locks++;
  274         }
  275         return (rval);
  276 }
  277 
  278 void
  279 _rw_wunlock(struct rwlock *rw, const char *file, int line)
  280 {
  281 
  282         if (SCHEDULER_STOPPED())
  283                 return;
  284         KASSERT(rw->rw_lock != RW_DESTROYED,
  285             ("rw_wunlock() of destroyed rwlock @ %s:%d", file, line));
  286         _rw_assert(rw, RA_WLOCKED, file, line);
  287         curthread->td_locks--;
  288         WITNESS_UNLOCK(&rw->lock_object, LOP_EXCLUSIVE, file, line);
  289         LOCK_LOG_LOCK("WUNLOCK", &rw->lock_object, 0, rw->rw_recurse, file,
  290             line);
  291         if (!rw_recursed(rw))
  292                 LOCKSTAT_PROFILE_RELEASE_LOCK(LS_RW_WUNLOCK_RELEASE, rw);
  293         __rw_wunlock(rw, curthread, file, line);
  294 }
  295 /*
  296  * Determines whether a new reader can acquire a lock.  Succeeds if the
  297  * reader already owns a read lock and the lock is locked for read to
  298  * prevent deadlock from reader recursion.  Also succeeds if the lock
  299  * is unlocked and has no writer waiters or spinners.  Failing otherwise
  300  * prioritizes writers before readers.
  301  */
  302 #define RW_CAN_READ(_rw)                                                \
  303     ((curthread->td_rw_rlocks && (_rw) & RW_LOCK_READ) || ((_rw) &      \
  304     (RW_LOCK_READ | RW_LOCK_WRITE_WAITERS | RW_LOCK_WRITE_SPINNER)) ==  \
  305     RW_LOCK_READ)
  306 
  307 void
  308 _rw_rlock(struct rwlock *rw, const char *file, int line)
  309 {
  310         struct turnstile *ts;
  311 #ifdef ADAPTIVE_RWLOCKS
  312         volatile struct thread *owner;
  313         int spintries = 0;
  314         int i;
  315 #endif
  316 #ifdef LOCK_PROFILING
  317         uint64_t waittime = 0;
  318         int contested = 0;
  319 #endif
  320         uintptr_t v;
  321 #ifdef KDTRACE_HOOKS
  322         uint64_t spin_cnt = 0;
  323         uint64_t sleep_cnt = 0;
  324         int64_t sleep_time = 0;
  325 #endif
  326 
  327         if (SCHEDULER_STOPPED())
  328                 return;
  329 
  330         KASSERT(rw->rw_lock != RW_DESTROYED,
  331             ("rw_rlock() of destroyed rwlock @ %s:%d", file, line));
  332         KASSERT(rw_wowner(rw) != curthread,
  333             ("rw_rlock: wlock already held for %s @ %s:%d",
  334             rw->lock_object.lo_name, file, line));
  335         WITNESS_CHECKORDER(&rw->lock_object, LOP_NEWORDER, file, line, NULL);
  336 
  337         for (;;) {
  338 #ifdef KDTRACE_HOOKS
  339                 spin_cnt++;
  340 #endif
  341                 /*
  342                  * Handle the easy case.  If no other thread has a write
  343                  * lock, then try to bump up the count of read locks.  Note
  344                  * that we have to preserve the current state of the
  345                  * RW_LOCK_WRITE_WAITERS flag.  If we fail to acquire a
  346                  * read lock, then rw_lock must have changed, so restart
  347                  * the loop.  Note that this handles the case of a
  348                  * completely unlocked rwlock since such a lock is encoded
  349                  * as a read lock with no waiters.
  350                  */
  351                 v = rw->rw_lock;
  352                 if (RW_CAN_READ(v)) {
  353                         /*
  354                          * The RW_LOCK_READ_WAITERS flag should only be set
  355                          * if the lock has been unlocked and write waiters
  356                          * were present.
  357                          */
  358                         if (atomic_cmpset_acq_ptr(&rw->rw_lock, v,
  359                             v + RW_ONE_READER)) {
  360                                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
  361                                         CTR4(KTR_LOCK,
  362                                             "%s: %p succeed %p -> %p", __func__,
  363                                             rw, (void *)v,
  364                                             (void *)(v + RW_ONE_READER));
  365                                 break;
  366                         }
  367                         continue;
  368                 }
  369 #ifdef HWPMC_HOOKS
  370                 PMC_SOFT_CALL( , , lock, failed);
  371 #endif
  372                 lock_profile_obtain_lock_failed(&rw->lock_object,
  373                     &contested, &waittime);
  374 
  375 #ifdef ADAPTIVE_RWLOCKS
  376                 /*
  377                  * If the owner is running on another CPU, spin until
  378                  * the owner stops running or the state of the lock
  379                  * changes.
  380                  */
  381                 if ((v & RW_LOCK_READ) == 0) {
  382                         owner = (struct thread *)RW_OWNER(v);
  383                         if (TD_IS_RUNNING(owner)) {
  384                                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
  385                                         CTR3(KTR_LOCK,
  386                                             "%s: spinning on %p held by %p",
  387                                             __func__, rw, owner);
  388                                 while ((struct thread*)RW_OWNER(rw->rw_lock) ==
  389                                     owner && TD_IS_RUNNING(owner)) {
  390                                         cpu_spinwait();
  391 #ifdef KDTRACE_HOOKS
  392                                         spin_cnt++;
  393 #endif
  394                                 }
  395                                 continue;
  396                         }
  397                 } else if (spintries < ROWNER_RETRIES) {
  398                         spintries++;
  399                         for (i = 0; i < ROWNER_LOOPS; i++) {
  400                                 v = rw->rw_lock;
  401                                 if ((v & RW_LOCK_READ) == 0 || RW_CAN_READ(v))
  402                                         break;
  403                                 cpu_spinwait();
  404                         }
  405                         if (i != ROWNER_LOOPS)
  406                                 continue;
  407                 }
  408 #endif
  409 
  410                 /*
  411                  * Okay, now it's the hard case.  Some other thread already
  412                  * has a write lock or there are write waiters present,
  413                  * acquire the turnstile lock so we can begin the process
  414                  * of blocking.
  415                  */
  416                 ts = turnstile_trywait(&rw->lock_object);
  417 
  418                 /*
  419                  * The lock might have been released while we spun, so
  420                  * recheck its state and restart the loop if needed.
  421                  */
  422                 v = rw->rw_lock;
  423                 if (RW_CAN_READ(v)) {
  424                         turnstile_cancel(ts);
  425                         continue;
  426                 }
  427 
  428 #ifdef ADAPTIVE_RWLOCKS
  429                 /*
  430                  * The current lock owner might have started executing
  431                  * on another CPU (or the lock could have changed
  432                  * owners) while we were waiting on the turnstile
  433                  * chain lock.  If so, drop the turnstile lock and try
  434                  * again.
  435                  */
  436                 if ((v & RW_LOCK_READ) == 0) {
  437                         owner = (struct thread *)RW_OWNER(v);
  438                         if (TD_IS_RUNNING(owner)) {
  439                                 turnstile_cancel(ts);
  440                                 continue;
  441                         }
  442                 }
  443 #endif
  444 
  445                 /*
  446                  * The lock is held in write mode or it already has waiters.
  447                  */
  448                 MPASS(!RW_CAN_READ(v));
  449 
  450                 /*
  451                  * If the RW_LOCK_READ_WAITERS flag is already set, then
  452                  * we can go ahead and block.  If it is not set then try
  453                  * to set it.  If we fail to set it drop the turnstile
  454                  * lock and restart the loop.
  455                  */
  456                 if (!(v & RW_LOCK_READ_WAITERS)) {
  457                         if (!atomic_cmpset_ptr(&rw->rw_lock, v,
  458                             v | RW_LOCK_READ_WAITERS)) {
  459                                 turnstile_cancel(ts);
  460                                 continue;
  461                         }
  462                         if (LOCK_LOG_TEST(&rw->lock_object, 0))
  463                                 CTR2(KTR_LOCK, "%s: %p set read waiters flag",
  464                                     __func__, rw);
  465                 }
  466 
  467                 /*
  468                  * We were unable to acquire the lock and the read waiters
  469                  * flag is set, so we must block on the turnstile.
  470                  */
  471                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
  472                         CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__,
  473                             rw);
  474 #ifdef KDTRACE_HOOKS
  475                 sleep_time -= lockstat_nsecs();
  476 #endif
  477                 turnstile_wait(ts, rw_owner(rw), TS_SHARED_QUEUE);
  478 #ifdef KDTRACE_HOOKS
  479                 sleep_time += lockstat_nsecs();
  480                 sleep_cnt++;
  481 #endif
  482                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
  483                         CTR2(KTR_LOCK, "%s: %p resuming from turnstile",
  484                             __func__, rw);
  485         }
  486 
  487         /*
  488          * TODO: acquire "owner of record" here.  Here be turnstile dragons
  489          * however.  turnstiles don't like owners changing between calls to
  490          * turnstile_wait() currently.
  491          */
  492         LOCKSTAT_PROFILE_OBTAIN_LOCK_SUCCESS(LS_RW_RLOCK_ACQUIRE, rw, contested,
  493             waittime, file, line);
  494         LOCK_LOG_LOCK("RLOCK", &rw->lock_object, 0, 0, file, line);
  495         WITNESS_LOCK(&rw->lock_object, 0, file, line);
  496         curthread->td_locks++;
  497         curthread->td_rw_rlocks++;
  498 #ifdef KDTRACE_HOOKS
  499         if (sleep_time)
  500                 LOCKSTAT_RECORD1(LS_RW_RLOCK_BLOCK, rw, sleep_time);
  501 
  502         /*
  503          * Record only the loops spinning and not sleeping. 
  504          */
  505         if (spin_cnt > sleep_cnt)
  506                 LOCKSTAT_RECORD1(LS_RW_RLOCK_SPIN, rw, (spin_cnt - sleep_cnt));
  507 #endif
  508 }
  509 
  510 int
  511 _rw_try_rlock(struct rwlock *rw, const char *file, int line)
  512 {
  513         uintptr_t x;
  514 
  515         if (SCHEDULER_STOPPED())
  516                 return (1);
  517 
  518         for (;;) {
  519                 x = rw->rw_lock;
  520                 KASSERT(rw->rw_lock != RW_DESTROYED,
  521                     ("rw_try_rlock() of destroyed rwlock @ %s:%d", file, line));
  522                 if (!(x & RW_LOCK_READ))
  523                         break;
  524                 if (atomic_cmpset_acq_ptr(&rw->rw_lock, x, x + RW_ONE_READER)) {
  525                         LOCK_LOG_TRY("RLOCK", &rw->lock_object, 0, 1, file,
  526                             line);
  527                         WITNESS_LOCK(&rw->lock_object, LOP_TRYLOCK, file, line);
  528                         curthread->td_locks++;
  529                         curthread->td_rw_rlocks++;
  530                         return (1);
  531                 }
  532         }
  533 
  534         LOCK_LOG_TRY("RLOCK", &rw->lock_object, 0, 0, file, line);
  535         return (0);
  536 }
  537 
  538 void
  539 _rw_runlock(struct rwlock *rw, const char *file, int line)
  540 {
  541         struct turnstile *ts;
  542         uintptr_t x, v, queue;
  543 
  544         if (SCHEDULER_STOPPED())
  545                 return;
  546 
  547         KASSERT(rw->rw_lock != RW_DESTROYED,
  548             ("rw_runlock() of destroyed rwlock @ %s:%d", file, line));
  549         _rw_assert(rw, RA_RLOCKED, file, line);
  550         curthread->td_locks--;
  551         curthread->td_rw_rlocks--;
  552         WITNESS_UNLOCK(&rw->lock_object, 0, file, line);
  553         LOCK_LOG_LOCK("RUNLOCK", &rw->lock_object, 0, 0, file, line);
  554 
  555         /* TODO: drop "owner of record" here. */
  556 
  557         for (;;) {
  558                 /*
  559                  * See if there is more than one read lock held.  If so,
  560                  * just drop one and return.
  561                  */
  562                 x = rw->rw_lock;
  563                 if (RW_READERS(x) > 1) {
  564                         if (atomic_cmpset_rel_ptr(&rw->rw_lock, x,
  565                             x - RW_ONE_READER)) {
  566                                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
  567                                         CTR4(KTR_LOCK,
  568                                             "%s: %p succeeded %p -> %p",
  569                                             __func__, rw, (void *)x,
  570                                             (void *)(x - RW_ONE_READER));
  571                                 break;
  572                         }
  573                         continue;
  574                 }
  575                 /*
  576                  * If there aren't any waiters for a write lock, then try
  577                  * to drop it quickly.
  578                  */
  579                 if (!(x & RW_LOCK_WAITERS)) {
  580                         MPASS((x & ~RW_LOCK_WRITE_SPINNER) ==
  581                             RW_READERS_LOCK(1));
  582                         if (atomic_cmpset_rel_ptr(&rw->rw_lock, x,
  583                             RW_UNLOCKED)) {
  584                                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
  585                                         CTR2(KTR_LOCK, "%s: %p last succeeded",
  586                                             __func__, rw);
  587                                 break;
  588                         }
  589                         continue;
  590                 }
  591                 /*
  592                  * Ok, we know we have waiters and we think we are the
  593                  * last reader, so grab the turnstile lock.
  594                  */
  595                 turnstile_chain_lock(&rw->lock_object);
  596                 v = rw->rw_lock & (RW_LOCK_WAITERS | RW_LOCK_WRITE_SPINNER);
  597                 MPASS(v & RW_LOCK_WAITERS);
  598 
  599                 /*
  600                  * Try to drop our lock leaving the lock in a unlocked
  601                  * state.
  602                  *
  603                  * If you wanted to do explicit lock handoff you'd have to
  604                  * do it here.  You'd also want to use turnstile_signal()
  605                  * and you'd have to handle the race where a higher
  606                  * priority thread blocks on the write lock before the
  607                  * thread you wakeup actually runs and have the new thread
  608                  * "steal" the lock.  For now it's a lot simpler to just
  609                  * wakeup all of the waiters.
  610                  *
  611                  * As above, if we fail, then another thread might have
  612                  * acquired a read lock, so drop the turnstile lock and
  613                  * restart.
  614                  */
  615                 x = RW_UNLOCKED;
  616                 if (v & RW_LOCK_WRITE_WAITERS) {
  617                         queue = TS_EXCLUSIVE_QUEUE;
  618                         x |= (v & RW_LOCK_READ_WAITERS);
  619                 } else
  620                         queue = TS_SHARED_QUEUE;
  621                 if (!atomic_cmpset_rel_ptr(&rw->rw_lock, RW_READERS_LOCK(1) | v,
  622                     x)) {
  623                         turnstile_chain_unlock(&rw->lock_object);
  624                         continue;
  625                 }
  626                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
  627                         CTR2(KTR_LOCK, "%s: %p last succeeded with waiters",
  628                             __func__, rw);
  629 
  630                 /*
  631                  * Ok.  The lock is released and all that's left is to
  632                  * wake up the waiters.  Note that the lock might not be
  633                  * free anymore, but in that case the writers will just
  634                  * block again if they run before the new lock holder(s)
  635                  * release the lock.
  636                  */
  637                 ts = turnstile_lookup(&rw->lock_object);
  638                 MPASS(ts != NULL);
  639                 turnstile_broadcast(ts, queue);
  640                 turnstile_unpend(ts, TS_SHARED_LOCK);
  641                 turnstile_chain_unlock(&rw->lock_object);
  642                 break;
  643         }
  644         LOCKSTAT_PROFILE_RELEASE_LOCK(LS_RW_RUNLOCK_RELEASE, rw);
  645 }
  646 
  647 /*
  648  * This function is called when we are unable to obtain a write lock on the
  649  * first try.  This means that at least one other thread holds either a
  650  * read or write lock.
  651  */
  652 void
  653 _rw_wlock_hard(struct rwlock *rw, uintptr_t tid, const char *file, int line)
  654 {
  655         struct turnstile *ts;
  656 #ifdef ADAPTIVE_RWLOCKS
  657         volatile struct thread *owner;
  658         int spintries = 0;
  659         int i;
  660 #endif
  661         uintptr_t v, x;
  662 #ifdef LOCK_PROFILING
  663         uint64_t waittime = 0;
  664         int contested = 0;
  665 #endif
  666 #ifdef KDTRACE_HOOKS
  667         uint64_t spin_cnt = 0;
  668         uint64_t sleep_cnt = 0;
  669         int64_t sleep_time = 0;
  670 #endif
  671 
  672         if (SCHEDULER_STOPPED())
  673                 return;
  674 
  675         if (rw_wlocked(rw)) {
  676                 KASSERT(rw->lock_object.lo_flags & LO_RECURSABLE,
  677                     ("%s: recursing but non-recursive rw %s @ %s:%d\n",
  678                     __func__, rw->lock_object.lo_name, file, line));
  679                 rw->rw_recurse++;
  680                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
  681                         CTR2(KTR_LOCK, "%s: %p recursing", __func__, rw);
  682                 return;
  683         }
  684 
  685         if (LOCK_LOG_TEST(&rw->lock_object, 0))
  686                 CTR5(KTR_LOCK, "%s: %s contested (lock=%p) at %s:%d", __func__,
  687                     rw->lock_object.lo_name, (void *)rw->rw_lock, file, line);
  688 
  689         while (!_rw_write_lock(rw, tid)) {
  690 #ifdef KDTRACE_HOOKS
  691                 spin_cnt++;
  692 #endif
  693 #ifdef HWPMC_HOOKS
  694                 PMC_SOFT_CALL( , , lock, failed);
  695 #endif
  696                 lock_profile_obtain_lock_failed(&rw->lock_object,
  697                     &contested, &waittime);
  698 #ifdef ADAPTIVE_RWLOCKS
  699                 /*
  700                  * If the lock is write locked and the owner is
  701                  * running on another CPU, spin until the owner stops
  702                  * running or the state of the lock changes.
  703                  */
  704                 v = rw->rw_lock;
  705                 owner = (struct thread *)RW_OWNER(v);
  706                 if (!(v & RW_LOCK_READ) && TD_IS_RUNNING(owner)) {
  707                         if (LOCK_LOG_TEST(&rw->lock_object, 0))
  708                                 CTR3(KTR_LOCK, "%s: spinning on %p held by %p",
  709                                     __func__, rw, owner);
  710                         while ((struct thread*)RW_OWNER(rw->rw_lock) == owner &&
  711                             TD_IS_RUNNING(owner)) {
  712                                 cpu_spinwait();
  713 #ifdef KDTRACE_HOOKS
  714                                 spin_cnt++;
  715 #endif
  716                         }
  717                         continue;
  718                 }
  719                 if ((v & RW_LOCK_READ) && RW_READERS(v) &&
  720                     spintries < ROWNER_RETRIES) {
  721                         if (!(v & RW_LOCK_WRITE_SPINNER)) {
  722                                 if (!atomic_cmpset_ptr(&rw->rw_lock, v,
  723                                     v | RW_LOCK_WRITE_SPINNER)) {
  724                                         continue;
  725                                 }
  726                         }
  727                         spintries++;
  728                         for (i = 0; i < ROWNER_LOOPS; i++) {
  729                                 if ((rw->rw_lock & RW_LOCK_WRITE_SPINNER) == 0)
  730                                         break;
  731                                 cpu_spinwait();
  732                         }
  733 #ifdef KDTRACE_HOOKS
  734                         spin_cnt += ROWNER_LOOPS - i;
  735 #endif
  736                         if (i != ROWNER_LOOPS)
  737                                 continue;
  738                 }
  739 #endif
  740                 ts = turnstile_trywait(&rw->lock_object);
  741                 v = rw->rw_lock;
  742 
  743 #ifdef ADAPTIVE_RWLOCKS
  744                 /*
  745                  * The current lock owner might have started executing
  746                  * on another CPU (or the lock could have changed
  747                  * owners) while we were waiting on the turnstile
  748                  * chain lock.  If so, drop the turnstile lock and try
  749                  * again.
  750                  */
  751                 if (!(v & RW_LOCK_READ)) {
  752                         owner = (struct thread *)RW_OWNER(v);
  753                         if (TD_IS_RUNNING(owner)) {
  754                                 turnstile_cancel(ts);
  755                                 continue;
  756                         }
  757                 }
  758 #endif
  759                 /*
  760                  * Check for the waiters flags about this rwlock.
  761                  * If the lock was released, without maintain any pending
  762                  * waiters queue, simply try to acquire it.
  763                  * If a pending waiters queue is present, claim the lock
  764                  * ownership and maintain the pending queue.
  765                  */
  766                 x = v & (RW_LOCK_WAITERS | RW_LOCK_WRITE_SPINNER);
  767                 if ((v & ~x) == RW_UNLOCKED) {
  768                         x &= ~RW_LOCK_WRITE_SPINNER;
  769                         if (atomic_cmpset_acq_ptr(&rw->rw_lock, v, tid | x)) {
  770                                 if (x)
  771                                         turnstile_claim(ts);
  772                                 else
  773                                         turnstile_cancel(ts);
  774                                 break;
  775                         }
  776                         turnstile_cancel(ts);
  777                         continue;
  778                 }
  779                 /*
  780                  * If the RW_LOCK_WRITE_WAITERS flag isn't set, then try to
  781                  * set it.  If we fail to set it, then loop back and try
  782                  * again.
  783                  */
  784                 if (!(v & RW_LOCK_WRITE_WAITERS)) {
  785                         if (!atomic_cmpset_ptr(&rw->rw_lock, v,
  786                             v | RW_LOCK_WRITE_WAITERS)) {
  787                                 turnstile_cancel(ts);
  788                                 continue;
  789                         }
  790                         if (LOCK_LOG_TEST(&rw->lock_object, 0))
  791                                 CTR2(KTR_LOCK, "%s: %p set write waiters flag",
  792                                     __func__, rw);
  793                 }
  794                 /*
  795                  * We were unable to acquire the lock and the write waiters
  796                  * flag is set, so we must block on the turnstile.
  797                  */
  798                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
  799                         CTR2(KTR_LOCK, "%s: %p blocking on turnstile", __func__,
  800                             rw);
  801 #ifdef KDTRACE_HOOKS
  802                 sleep_time -= lockstat_nsecs();
  803 #endif
  804                 turnstile_wait(ts, rw_owner(rw), TS_EXCLUSIVE_QUEUE);
  805 #ifdef KDTRACE_HOOKS
  806                 sleep_time += lockstat_nsecs();
  807                 sleep_cnt++;
  808 #endif
  809                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
  810                         CTR2(KTR_LOCK, "%s: %p resuming from turnstile",
  811                             __func__, rw);
  812 #ifdef ADAPTIVE_RWLOCKS
  813                 spintries = 0;
  814 #endif
  815         }
  816         LOCKSTAT_PROFILE_OBTAIN_LOCK_SUCCESS(LS_RW_WLOCK_ACQUIRE, rw, contested,
  817             waittime, file, line);
  818 #ifdef KDTRACE_HOOKS
  819         if (sleep_time)
  820                 LOCKSTAT_RECORD1(LS_RW_WLOCK_BLOCK, rw, sleep_time);
  821 
  822         /*
  823          * Record only the loops spinning and not sleeping.
  824          */ 
  825         if (spin_cnt > sleep_cnt)
  826                 LOCKSTAT_RECORD1(LS_RW_WLOCK_SPIN, rw, (spin_cnt - sleep_cnt));
  827 #endif
  828 }
  829 
  830 /*
  831  * This function is called if the first try at releasing a write lock failed.
  832  * This means that one of the 2 waiter bits must be set indicating that at
  833  * least one thread is waiting on this lock.
  834  */
  835 void
  836 _rw_wunlock_hard(struct rwlock *rw, uintptr_t tid, const char *file, int line)
  837 {
  838         struct turnstile *ts;
  839         uintptr_t v;
  840         int queue;
  841 
  842         if (SCHEDULER_STOPPED())
  843                 return;
  844 
  845         if (rw_wlocked(rw) && rw_recursed(rw)) {
  846                 rw->rw_recurse--;
  847                 if (LOCK_LOG_TEST(&rw->lock_object, 0))
  848                         CTR2(KTR_LOCK, "%s: %p unrecursing", __func__, rw);
  849                 return;
  850         }
  851 
  852         KASSERT(rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS),
  853             ("%s: neither of the waiter flags are set", __func__));
  854 
  855         if (LOCK_LOG_TEST(&rw->lock_object, 0))
  856                 CTR2(KTR_LOCK, "%s: %p contested", __func__, rw);
  857 
  858         turnstile_chain_lock(&rw->lock_object);
  859         ts = turnstile_lookup(&rw->lock_object);
  860         MPASS(ts != NULL);
  861 
  862         /*
  863          * Use the same algo as sx locks for now.  Prefer waking up shared
  864          * waiters if we have any over writers.  This is probably not ideal.
  865          *
  866          * 'v' is the value we are going to write back to rw_lock.  If we
  867          * have waiters on both queues, we need to preserve the state of
  868          * the waiter flag for the queue we don't wake up.  For now this is
  869          * hardcoded for the algorithm mentioned above.
  870          *
  871          * In the case of both readers and writers waiting we wakeup the
  872          * readers but leave the RW_LOCK_WRITE_WAITERS flag set.  If a
  873          * new writer comes in before a reader it will claim the lock up
  874          * above.  There is probably a potential priority inversion in
  875          * there that could be worked around either by waking both queues
  876          * of waiters or doing some complicated lock handoff gymnastics.
  877          */
  878         v = RW_UNLOCKED;
  879         if (rw->rw_lock & RW_LOCK_WRITE_WAITERS) {
  880                 queue = TS_EXCLUSIVE_QUEUE;
  881                 v |= (rw->rw_lock & RW_LOCK_READ_WAITERS);
  882         } else
  883                 queue = TS_SHARED_QUEUE;
  884 
  885         /* Wake up all waiters for the specific queue. */
  886         if (LOCK_LOG_TEST(&rw->lock_object, 0))
  887                 CTR3(KTR_LOCK, "%s: %p waking up %s waiters", __func__, rw,
  888                     queue == TS_SHARED_QUEUE ? "read" : "write");
  889         turnstile_broadcast(ts, queue);
  890         atomic_store_rel_ptr(&rw->rw_lock, v);
  891         turnstile_unpend(ts, TS_EXCLUSIVE_LOCK);
  892         turnstile_chain_unlock(&rw->lock_object);
  893 }
  894 
  895 /*
  896  * Attempt to do a non-blocking upgrade from a read lock to a write
  897  * lock.  This will only succeed if this thread holds a single read
  898  * lock.  Returns true if the upgrade succeeded and false otherwise.
  899  */
  900 int
  901 _rw_try_upgrade(struct rwlock *rw, const char *file, int line)
  902 {
  903         uintptr_t v, x, tid;
  904         struct turnstile *ts;
  905         int success;
  906 
  907         if (SCHEDULER_STOPPED())
  908                 return (1);
  909 
  910         KASSERT(rw->rw_lock != RW_DESTROYED,
  911             ("rw_try_upgrade() of destroyed rwlock @ %s:%d", file, line));
  912         _rw_assert(rw, RA_RLOCKED, file, line);
  913 
  914         /*
  915          * Attempt to switch from one reader to a writer.  If there
  916          * are any write waiters, then we will have to lock the
  917          * turnstile first to prevent races with another writer
  918          * calling turnstile_wait() before we have claimed this
  919          * turnstile.  So, do the simple case of no waiters first.
  920          */
  921         tid = (uintptr_t)curthread;
  922         success = 0;
  923         for (;;) {
  924                 v = rw->rw_lock;
  925                 if (RW_READERS(v) > 1)
  926                         break;
  927                 if (!(v & RW_LOCK_WAITERS)) {
  928                         success = atomic_cmpset_ptr(&rw->rw_lock, v, tid);
  929                         if (!success)
  930                                 continue;
  931                         break;
  932                 }
  933 
  934                 /*
  935                  * Ok, we think we have waiters, so lock the turnstile.
  936                  */
  937                 ts = turnstile_trywait(&rw->lock_object);
  938                 v = rw->rw_lock;
  939                 if (RW_READERS(v) > 1) {
  940                         turnstile_cancel(ts);
  941                         break;
  942                 }
  943                 /*
  944                  * Try to switch from one reader to a writer again.  This time
  945                  * we honor the current state of the waiters flags.
  946                  * If we obtain the lock with the flags set, then claim
  947                  * ownership of the turnstile.
  948                  */
  949                 x = rw->rw_lock & RW_LOCK_WAITERS;
  950                 success = atomic_cmpset_ptr(&rw->rw_lock, v, tid | x);
  951                 if (success) {
  952                         if (x)
  953                                 turnstile_claim(ts);
  954                         else
  955                                 turnstile_cancel(ts);
  956                         break;
  957                 }
  958                 turnstile_cancel(ts);
  959         }
  960         LOCK_LOG_TRY("WUPGRADE", &rw->lock_object, 0, success, file, line);
  961         if (success) {
  962                 curthread->td_rw_rlocks--;
  963                 WITNESS_UPGRADE(&rw->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
  964                     file, line);
  965                 LOCKSTAT_RECORD0(LS_RW_TRYUPGRADE_UPGRADE, rw);
  966         }
  967         return (success);
  968 }
  969 
  970 /*
  971  * Downgrade a write lock into a single read lock.
  972  */
  973 void
  974 _rw_downgrade(struct rwlock *rw, const char *file, int line)
  975 {
  976         struct turnstile *ts;
  977         uintptr_t tid, v;
  978         int rwait, wwait;
  979 
  980         if (SCHEDULER_STOPPED())
  981                 return;
  982 
  983         KASSERT(rw->rw_lock != RW_DESTROYED,
  984             ("rw_downgrade() of destroyed rwlock @ %s:%d", file, line));
  985         _rw_assert(rw, RA_WLOCKED | RA_NOTRECURSED, file, line);
  986 #ifndef INVARIANTS
  987         if (rw_recursed(rw))
  988                 panic("downgrade of a recursed lock");
  989 #endif
  990 
  991         WITNESS_DOWNGRADE(&rw->lock_object, 0, file, line);
  992 
  993         /*
  994          * Convert from a writer to a single reader.  First we handle
  995          * the easy case with no waiters.  If there are any waiters, we
  996          * lock the turnstile and "disown" the lock.
  997          */
  998         tid = (uintptr_t)curthread;
  999         if (atomic_cmpset_rel_ptr(&rw->rw_lock, tid, RW_READERS_LOCK(1)))
 1000                 goto out;
 1001 
 1002         /*
 1003          * Ok, we think we have waiters, so lock the turnstile so we can
 1004          * read the waiter flags without any races.
 1005          */
 1006         turnstile_chain_lock(&rw->lock_object);
 1007         v = rw->rw_lock & RW_LOCK_WAITERS;
 1008         rwait = v & RW_LOCK_READ_WAITERS;
 1009         wwait = v & RW_LOCK_WRITE_WAITERS;
 1010         MPASS(rwait | wwait);
 1011 
 1012         /*
 1013          * Downgrade from a write lock while preserving waiters flag
 1014          * and give up ownership of the turnstile.
 1015          */
 1016         ts = turnstile_lookup(&rw->lock_object);
 1017         MPASS(ts != NULL);
 1018         if (!wwait)
 1019                 v &= ~RW_LOCK_READ_WAITERS;
 1020         atomic_store_rel_ptr(&rw->rw_lock, RW_READERS_LOCK(1) | v);
 1021         /*
 1022          * Wake other readers if there are no writers pending.  Otherwise they
 1023          * won't be able to acquire the lock anyway.
 1024          */
 1025         if (rwait && !wwait) {
 1026                 turnstile_broadcast(ts, TS_SHARED_QUEUE);
 1027                 turnstile_unpend(ts, TS_EXCLUSIVE_LOCK);
 1028         } else
 1029                 turnstile_disown(ts);
 1030         turnstile_chain_unlock(&rw->lock_object);
 1031 out:
 1032         curthread->td_rw_rlocks++;
 1033         LOCK_LOG_LOCK("WDOWNGRADE", &rw->lock_object, 0, 0, file, line);
 1034         LOCKSTAT_RECORD0(LS_RW_DOWNGRADE_DOWNGRADE, rw);
 1035 }
 1036 
 1037 #ifdef INVARIANT_SUPPORT
 1038 #ifndef INVARIANTS
 1039 #undef _rw_assert
 1040 #endif
 1041 
 1042 /*
 1043  * In the non-WITNESS case, rw_assert() can only detect that at least
 1044  * *some* thread owns an rlock, but it cannot guarantee that *this*
 1045  * thread owns an rlock.
 1046  */
 1047 void
 1048 _rw_assert(struct rwlock *rw, int what, const char *file, int line)
 1049 {
 1050 
 1051         if (panicstr != NULL)
 1052                 return;
 1053         switch (what) {
 1054         case RA_LOCKED:
 1055         case RA_LOCKED | RA_RECURSED:
 1056         case RA_LOCKED | RA_NOTRECURSED:
 1057         case RA_RLOCKED:
 1058         case RA_RLOCKED | RA_RECURSED:
 1059         case RA_RLOCKED | RA_NOTRECURSED:
 1060 #ifdef WITNESS
 1061                 witness_assert(&rw->lock_object, what, file, line);
 1062 #else
 1063                 /*
 1064                  * If some other thread has a write lock or we have one
 1065                  * and are asserting a read lock, fail.  Also, if no one
 1066                  * has a lock at all, fail.
 1067                  */
 1068                 if (rw->rw_lock == RW_UNLOCKED ||
 1069                     (!(rw->rw_lock & RW_LOCK_READ) && (what & RA_RLOCKED ||
 1070                     rw_wowner(rw) != curthread)))
 1071                         panic("Lock %s not %slocked @ %s:%d\n",
 1072                             rw->lock_object.lo_name, (what & RA_RLOCKED) ?
 1073                             "read " : "", file, line);
 1074 
 1075                 if (!(rw->rw_lock & RW_LOCK_READ) && !(what & RA_RLOCKED)) {
 1076                         if (rw_recursed(rw)) {
 1077                                 if (what & RA_NOTRECURSED)
 1078                                         panic("Lock %s recursed @ %s:%d\n",
 1079                                             rw->lock_object.lo_name, file,
 1080                                             line);
 1081                         } else if (what & RA_RECURSED)
 1082                                 panic("Lock %s not recursed @ %s:%d\n",
 1083                                     rw->lock_object.lo_name, file, line);
 1084                 }
 1085 #endif
 1086                 break;
 1087         case RA_WLOCKED:
 1088         case RA_WLOCKED | RA_RECURSED:
 1089         case RA_WLOCKED | RA_NOTRECURSED:
 1090                 if (rw_wowner(rw) != curthread)
 1091                         panic("Lock %s not exclusively locked @ %s:%d\n",
 1092                             rw->lock_object.lo_name, file, line);
 1093                 if (rw_recursed(rw)) {
 1094                         if (what & RA_NOTRECURSED)
 1095                                 panic("Lock %s recursed @ %s:%d\n",
 1096                                     rw->lock_object.lo_name, file, line);
 1097                 } else if (what & RA_RECURSED)
 1098                         panic("Lock %s not recursed @ %s:%d\n",
 1099                             rw->lock_object.lo_name, file, line);
 1100                 break;
 1101         case RA_UNLOCKED:
 1102 #ifdef WITNESS
 1103                 witness_assert(&rw->lock_object, what, file, line);
 1104 #else
 1105                 /*
 1106                  * If we hold a write lock fail.  We can't reliably check
 1107                  * to see if we hold a read lock or not.
 1108                  */
 1109                 if (rw_wowner(rw) == curthread)
 1110                         panic("Lock %s exclusively locked @ %s:%d\n",
 1111                             rw->lock_object.lo_name, file, line);
 1112 #endif
 1113                 break;
 1114         default:
 1115                 panic("Unknown rw lock assertion: %d @ %s:%d", what, file,
 1116                     line);
 1117         }
 1118 }
 1119 #endif /* INVARIANT_SUPPORT */
 1120 
 1121 #ifdef DDB
 1122 void
 1123 db_show_rwlock(struct lock_object *lock)
 1124 {
 1125         struct rwlock *rw;
 1126         struct thread *td;
 1127 
 1128         rw = (struct rwlock *)lock;
 1129 
 1130         db_printf(" state: ");
 1131         if (rw->rw_lock == RW_UNLOCKED)
 1132                 db_printf("UNLOCKED\n");
 1133         else if (rw->rw_lock == RW_DESTROYED) {
 1134                 db_printf("DESTROYED\n");
 1135                 return;
 1136         } else if (rw->rw_lock & RW_LOCK_READ)
 1137                 db_printf("RLOCK: %ju locks\n",
 1138                     (uintmax_t)(RW_READERS(rw->rw_lock)));
 1139         else {
 1140                 td = rw_wowner(rw);
 1141                 db_printf("WLOCK: %p (tid %d, pid %d, \"%s\")\n", td,
 1142                     td->td_tid, td->td_proc->p_pid, td->td_name);
 1143                 if (rw_recursed(rw))
 1144                         db_printf(" recursed: %u\n", rw->rw_recurse);
 1145         }
 1146         db_printf(" waiters: ");
 1147         switch (rw->rw_lock & (RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS)) {
 1148         case RW_LOCK_READ_WAITERS:
 1149                 db_printf("readers\n");
 1150                 break;
 1151         case RW_LOCK_WRITE_WAITERS:
 1152                 db_printf("writers\n");
 1153                 break;
 1154         case RW_LOCK_READ_WAITERS | RW_LOCK_WRITE_WAITERS:
 1155                 db_printf("readers and writers\n");
 1156                 break;
 1157         default:
 1158                 db_printf("none\n");
 1159                 break;
 1160         }
 1161 }
 1162 
 1163 #endif
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FreeBSD/Linux Kernel Cross Reference sys/kern/kern_rwlock.c

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