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


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

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    1 /*-
    2  * Copyright (C) 2001 Jason Evans <jasone@freebsd.org>.  All rights reserved.
    3  *
    4  * Redistribution and use in source and binary forms, with or without
    5  * modification, are permitted provided that the following conditions
    6  * are met:
    7  * 1. Redistributions of source code must retain the above copyright
    8  *    notice(s), this list of conditions and the following disclaimer as
    9  *    the first lines of this file unmodified other than the possible 
   10  *    addition of one or more copyright notices.
   11  * 2. Redistributions in binary form must reproduce the above copyright
   12  *    notice(s), 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 COPYRIGHT HOLDER(S) ``AS IS'' AND ANY
   16  * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
   17  * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
   18  * DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT HOLDER(S) BE LIABLE FOR ANY
   19  * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
   20  * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
   21  * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
   22  * 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 SUCH
   25  * DAMAGE.
   26  */
   27 
   28 /*
   29  * Shared/exclusive locks.  This implementation assures deterministic lock
   30  * granting behavior, so that slocks and xlocks are interleaved.
   31  *
   32  * Priority propagation will not generally raise the priority of lock holders,
   33  * so should not be relied upon in combination with sx locks.
   34  */
   35 
   36 #include <sys/cdefs.h>
   37 __FBSDID("$FreeBSD: releng/6.1/sys/kern/kern_sx.c 158179 2006-04-30 16:44:43Z cvs2svn $");
   38 
   39 #include "opt_ddb.h"
   40 
   41 #include <sys/param.h>
   42 #include <sys/systm.h>
   43 #include <sys/ktr.h>
   44 #include <sys/linker_set.h>
   45 #include <sys/condvar.h>
   46 #include <sys/lock.h>
   47 #include <sys/mutex.h>
   48 #include <sys/proc.h>
   49 #include <sys/sx.h>
   50 
   51 #include <ddb/ddb.h>
   52 
   53 #ifdef DDB
   54 static void     db_show_sx(struct lock_object *lock);
   55 #endif
   56 
   57 struct lock_class lock_class_sx = {
   58         "sx",
   59         LC_SLEEPLOCK | LC_SLEEPABLE | LC_RECURSABLE | LC_UPGRADABLE,
   60 #ifdef DDB
   61         db_show_sx
   62 #endif
   63 };
   64 
   65 #ifndef INVARIANTS
   66 #define _sx_assert(sx, what, file, line)
   67 #endif
   68 
   69 void
   70 sx_sysinit(void *arg)
   71 {
   72         struct sx_args *sargs = arg;
   73 
   74         sx_init(sargs->sa_sx, sargs->sa_desc);
   75 }
   76 
   77 void
   78 sx_init(struct sx *sx, const char *description)
   79 {
   80         struct lock_object *lock;
   81 
   82         lock = &sx->sx_object;
   83         KASSERT((lock->lo_flags & LO_INITIALIZED) == 0,
   84             ("sx lock %s %p already initialized", description, sx));
   85         bzero(sx, sizeof(*sx));
   86         lock->lo_class = &lock_class_sx;
   87         lock->lo_type = lock->lo_name = description;
   88         lock->lo_flags = LO_WITNESS | LO_RECURSABLE | LO_SLEEPABLE |
   89             LO_UPGRADABLE;
   90         sx->sx_lock = mtx_pool_find(mtxpool_lockbuilder, sx);
   91         sx->sx_cnt = 0;
   92         cv_init(&sx->sx_shrd_cv, description);
   93         sx->sx_shrd_wcnt = 0;
   94         cv_init(&sx->sx_excl_cv, description);
   95         sx->sx_excl_wcnt = 0;
   96         sx->sx_xholder = NULL;
   97 
   98         LOCK_LOG_INIT(lock, 0);
   99 
  100         WITNESS_INIT(lock);
  101 }
  102 
  103 void
  104 sx_destroy(struct sx *sx)
  105 {
  106 
  107         LOCK_LOG_DESTROY(&sx->sx_object, 0);
  108 
  109         KASSERT((sx->sx_cnt == 0 && sx->sx_shrd_wcnt == 0 && sx->sx_excl_wcnt ==
  110             0), ("%s (%s): holders or waiters\n", __func__,
  111             sx->sx_object.lo_name));
  112 
  113         sx->sx_lock = NULL;
  114         cv_destroy(&sx->sx_shrd_cv);
  115         cv_destroy(&sx->sx_excl_cv);
  116 
  117         WITNESS_DESTROY(&sx->sx_object);
  118 }
  119 
  120 void
  121 _sx_slock(struct sx *sx, const char *file, int line)
  122 {
  123 
  124         mtx_lock(sx->sx_lock);
  125         KASSERT(sx->sx_xholder != curthread,
  126             ("%s (%s): slock while xlock is held @ %s:%d\n", __func__,
  127             sx->sx_object.lo_name, file, line));
  128         WITNESS_CHECKORDER(&sx->sx_object, LOP_NEWORDER, file, line);
  129 
  130         /*
  131          * Loop in case we lose the race for lock acquisition.
  132          */
  133         while (sx->sx_cnt < 0) {
  134                 sx->sx_shrd_wcnt++;
  135                 cv_wait(&sx->sx_shrd_cv, sx->sx_lock);
  136                 sx->sx_shrd_wcnt--;
  137         }
  138 
  139         /* Acquire a shared lock. */
  140         sx->sx_cnt++;
  141 
  142         LOCK_LOG_LOCK("SLOCK", &sx->sx_object, 0, 0, file, line);
  143         WITNESS_LOCK(&sx->sx_object, 0, file, line);
  144 
  145         mtx_unlock(sx->sx_lock);
  146 }
  147 
  148 int
  149 _sx_try_slock(struct sx *sx, const char *file, int line)
  150 {
  151 
  152         mtx_lock(sx->sx_lock);
  153         if (sx->sx_cnt >= 0) {
  154                 sx->sx_cnt++;
  155                 LOCK_LOG_TRY("SLOCK", &sx->sx_object, 0, 1, file, line);
  156                 WITNESS_LOCK(&sx->sx_object, LOP_TRYLOCK, file, line);
  157                 mtx_unlock(sx->sx_lock);
  158                 return (1);
  159         } else {
  160                 LOCK_LOG_TRY("SLOCK", &sx->sx_object, 0, 0, file, line);
  161                 mtx_unlock(sx->sx_lock);
  162                 return (0);
  163         }
  164 }
  165 
  166 void
  167 _sx_xlock(struct sx *sx, const char *file, int line)
  168 {
  169 
  170         mtx_lock(sx->sx_lock);
  171 
  172         /*
  173          * With sx locks, we're absolutely not permitted to recurse on
  174          * xlocks, as it is fatal (deadlock). Normally, recursion is handled
  175          * by WITNESS, but as it is not semantically correct to hold the
  176          * xlock while in here, we consider it API abuse and put it under
  177          * INVARIANTS.
  178          */
  179         KASSERT(sx->sx_xholder != curthread,
  180             ("%s (%s): xlock already held @ %s:%d", __func__,
  181             sx->sx_object.lo_name, file, line));
  182         WITNESS_CHECKORDER(&sx->sx_object, LOP_NEWORDER | LOP_EXCLUSIVE, file,
  183             line);
  184 
  185         /* Loop in case we lose the race for lock acquisition. */
  186         while (sx->sx_cnt != 0) {
  187                 sx->sx_excl_wcnt++;
  188                 cv_wait(&sx->sx_excl_cv, sx->sx_lock);
  189                 sx->sx_excl_wcnt--;
  190         }
  191 
  192         MPASS(sx->sx_cnt == 0);
  193 
  194         /* Acquire an exclusive lock. */
  195         sx->sx_cnt--;
  196         sx->sx_xholder = curthread;
  197 
  198         LOCK_LOG_LOCK("XLOCK", &sx->sx_object, 0, 0, file, line);
  199         WITNESS_LOCK(&sx->sx_object, LOP_EXCLUSIVE, file, line);
  200 
  201         mtx_unlock(sx->sx_lock);
  202 }
  203 
  204 int
  205 _sx_try_xlock(struct sx *sx, const char *file, int line)
  206 {
  207 
  208         mtx_lock(sx->sx_lock);
  209         if (sx->sx_cnt == 0) {
  210                 sx->sx_cnt--;
  211                 sx->sx_xholder = curthread;
  212                 LOCK_LOG_TRY("XLOCK", &sx->sx_object, 0, 1, file, line);
  213                 WITNESS_LOCK(&sx->sx_object, LOP_EXCLUSIVE | LOP_TRYLOCK, file,
  214                     line);
  215                 mtx_unlock(sx->sx_lock);
  216                 return (1);
  217         } else {
  218                 LOCK_LOG_TRY("XLOCK", &sx->sx_object, 0, 0, file, line);
  219                 mtx_unlock(sx->sx_lock);
  220                 return (0);
  221         }
  222 }
  223 
  224 void
  225 _sx_sunlock(struct sx *sx, const char *file, int line)
  226 {
  227 
  228         _sx_assert(sx, SX_SLOCKED, file, line);
  229         mtx_lock(sx->sx_lock);
  230 
  231         WITNESS_UNLOCK(&sx->sx_object, 0, file, line);
  232 
  233         /* Release. */
  234         sx->sx_cnt--;
  235 
  236         /*
  237          * If we just released the last shared lock, wake any waiters up, giving
  238          * exclusive lockers precedence.  In order to make sure that exclusive
  239          * lockers won't be blocked forever, don't wake shared lock waiters if
  240          * there are exclusive lock waiters.
  241          */
  242         if (sx->sx_excl_wcnt > 0) {
  243                 if (sx->sx_cnt == 0)
  244                         cv_signal(&sx->sx_excl_cv);
  245         } else if (sx->sx_shrd_wcnt > 0)
  246                 cv_broadcast(&sx->sx_shrd_cv);
  247 
  248         LOCK_LOG_LOCK("SUNLOCK", &sx->sx_object, 0, 0, file, line);
  249 
  250         mtx_unlock(sx->sx_lock);
  251 }
  252 
  253 void
  254 _sx_xunlock(struct sx *sx, const char *file, int line)
  255 {
  256 
  257         _sx_assert(sx, SX_XLOCKED, file, line);
  258         mtx_lock(sx->sx_lock);
  259         MPASS(sx->sx_cnt == -1);
  260 
  261         WITNESS_UNLOCK(&sx->sx_object, LOP_EXCLUSIVE, file, line);
  262 
  263         /* Release. */
  264         sx->sx_cnt++;
  265         sx->sx_xholder = NULL;
  266 
  267         /*
  268          * Wake up waiters if there are any.  Give precedence to slock waiters.
  269          */
  270         if (sx->sx_shrd_wcnt > 0)
  271                 cv_broadcast(&sx->sx_shrd_cv);
  272         else if (sx->sx_excl_wcnt > 0)
  273                 cv_signal(&sx->sx_excl_cv);
  274 
  275         LOCK_LOG_LOCK("XUNLOCK", &sx->sx_object, 0, 0, file, line);
  276 
  277         mtx_unlock(sx->sx_lock);
  278 }
  279 
  280 int
  281 _sx_try_upgrade(struct sx *sx, const char *file, int line)
  282 {
  283 
  284         _sx_assert(sx, SX_SLOCKED, file, line);
  285         mtx_lock(sx->sx_lock);
  286 
  287         if (sx->sx_cnt == 1) {
  288                 sx->sx_cnt = -1;
  289                 sx->sx_xholder = curthread;
  290 
  291                 LOCK_LOG_TRY("XUPGRADE", &sx->sx_object, 0, 1, file, line);
  292                 WITNESS_UPGRADE(&sx->sx_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
  293                     file, line);
  294 
  295                 mtx_unlock(sx->sx_lock);
  296                 return (1);
  297         } else {
  298                 LOCK_LOG_TRY("XUPGRADE", &sx->sx_object, 0, 0, file, line);
  299                 mtx_unlock(sx->sx_lock);
  300                 return (0);
  301         }
  302 }
  303 
  304 void
  305 _sx_downgrade(struct sx *sx, const char *file, int line)
  306 {
  307 
  308         _sx_assert(sx, SX_XLOCKED, file, line);
  309         mtx_lock(sx->sx_lock);
  310         MPASS(sx->sx_cnt == -1);
  311 
  312         WITNESS_DOWNGRADE(&sx->sx_object, 0, file, line);
  313 
  314         sx->sx_cnt = 1;
  315         sx->sx_xholder = NULL;
  316         if (sx->sx_shrd_wcnt > 0)
  317                 cv_broadcast(&sx->sx_shrd_cv);
  318 
  319         LOCK_LOG_LOCK("XDOWNGRADE", &sx->sx_object, 0, 0, file, line);
  320 
  321         mtx_unlock(sx->sx_lock);
  322 }
  323 
  324 #ifdef INVARIANT_SUPPORT
  325 #ifndef INVARIANTS
  326 #undef  _sx_assert
  327 #endif
  328 
  329 /*
  330  * In the non-WITNESS case, sx_assert() can only detect that at least
  331  * *some* thread owns an slock, but it cannot guarantee that *this*
  332  * thread owns an slock.
  333  */
  334 void
  335 _sx_assert(struct sx *sx, int what, const char *file, int line)
  336 {
  337 
  338         if (panicstr != NULL)
  339                 return;
  340         switch (what) {
  341         case SX_LOCKED:
  342         case SX_SLOCKED:
  343 #ifdef WITNESS
  344                 witness_assert(&sx->sx_object, what, file, line);
  345 #else
  346                 mtx_lock(sx->sx_lock);
  347                 if (sx->sx_cnt <= 0 &&
  348                     (what == SX_SLOCKED || sx->sx_xholder != curthread))
  349                         panic("Lock %s not %slocked @ %s:%d\n",
  350                             sx->sx_object.lo_name, (what == SX_SLOCKED) ?
  351                             "share " : "", file, line);
  352                 mtx_unlock(sx->sx_lock);
  353 #endif
  354                 break;
  355         case SX_XLOCKED:
  356                 mtx_lock(sx->sx_lock);
  357                 if (sx->sx_xholder != curthread)
  358                         panic("Lock %s not exclusively locked @ %s:%d\n",
  359                             sx->sx_object.lo_name, file, line);
  360                 mtx_unlock(sx->sx_lock);
  361                 break;
  362         case SX_UNLOCKED:
  363 #ifdef WITNESS
  364                 witness_assert(&sx->sx_object, what, file, line);
  365 #else
  366                 /*
  367                  * We are able to check only exclusive lock here,
  368                  * we cannot assert that *this* thread owns slock.
  369                  */
  370                 mtx_lock(sx->sx_lock);
  371                 if (sx->sx_xholder == curthread)
  372                         panic("Lock %s exclusively locked @ %s:%d\n",
  373                             sx->sx_object.lo_name, file, line);
  374                 mtx_unlock(sx->sx_lock);
  375 #endif
  376                 break;
  377         default:
  378                 panic("Unknown sx lock assertion: %d @ %s:%d", what, file,
  379                     line);
  380         }
  381 }
  382 #endif  /* INVARIANT_SUPPORT */
  383 
  384 #ifdef DDB
  385 void
  386 db_show_sx(struct lock_object *lock)
  387 {
  388         struct thread *td;
  389         struct sx *sx;
  390 
  391         sx = (struct sx *)lock;
  392 
  393         db_printf(" state: ");
  394         if (sx->sx_cnt < 0) {
  395                 td = sx->sx_xholder;
  396                 db_printf("XLOCK: %p (tid %d, pid %d, \"%s\")\n", td,
  397                     td->td_tid, td->td_proc->p_pid, td->td_proc->p_comm);
  398         } else if (sx->sx_cnt > 0)
  399                 db_printf("SLOCK: %d locks\n", sx->sx_cnt);
  400         else
  401                 db_printf("UNLOCKED\n");
  402         db_printf(" waiters: %d shared, %d exclusive\n", sx->sx_shrd_wcnt,
  403             sx->sx_excl_wcnt);
  404 }
  405 #endif

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