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/6.0/sys/kern/kern_sx.c 139804 2005-01-06 23:35:40Z imp $");
   38 
   39 #include <sys/param.h>
   40 #include <sys/systm.h>
   41 #include <sys/ktr.h>
   42 #include <sys/condvar.h>
   43 #include <sys/lock.h>
   44 #include <sys/mutex.h>
   45 #include <sys/sx.h>
   46 
   47 struct lock_class lock_class_sx = {
   48         "sx",
   49         LC_SLEEPLOCK | LC_SLEEPABLE | LC_RECURSABLE | LC_UPGRADABLE
   50 };
   51 
   52 #ifndef INVARIANTS
   53 #define _sx_assert(sx, what, file, line)
   54 #endif
   55 
   56 void
   57 sx_sysinit(void *arg)
   58 {
   59         struct sx_args *sargs = arg;
   60 
   61         sx_init(sargs->sa_sx, sargs->sa_desc);
   62 }
   63 
   64 void
   65 sx_init(struct sx *sx, const char *description)
   66 {
   67         struct lock_object *lock;
   68 
   69         lock = &sx->sx_object;
   70         KASSERT((lock->lo_flags & LO_INITIALIZED) == 0,
   71             ("sx lock %s %p already initialized", description, sx));
   72         bzero(sx, sizeof(*sx));
   73         lock->lo_class = &lock_class_sx;
   74         lock->lo_type = lock->lo_name = description;
   75         lock->lo_flags = LO_WITNESS | LO_RECURSABLE | LO_SLEEPABLE |
   76             LO_UPGRADABLE;
   77         sx->sx_lock = mtx_pool_find(mtxpool_lockbuilder, sx);
   78         sx->sx_cnt = 0;
   79         cv_init(&sx->sx_shrd_cv, description);
   80         sx->sx_shrd_wcnt = 0;
   81         cv_init(&sx->sx_excl_cv, description);
   82         sx->sx_excl_wcnt = 0;
   83         sx->sx_xholder = NULL;
   84 
   85         LOCK_LOG_INIT(lock, 0);
   86 
   87         WITNESS_INIT(lock);
   88 }
   89 
   90 void
   91 sx_destroy(struct sx *sx)
   92 {
   93 
   94         LOCK_LOG_DESTROY(&sx->sx_object, 0);
   95 
   96         KASSERT((sx->sx_cnt == 0 && sx->sx_shrd_wcnt == 0 && sx->sx_excl_wcnt ==
   97             0), ("%s (%s): holders or waiters\n", __func__,
   98             sx->sx_object.lo_name));
   99 
  100         sx->sx_lock = NULL;
  101         cv_destroy(&sx->sx_shrd_cv);
  102         cv_destroy(&sx->sx_excl_cv);
  103 
  104         WITNESS_DESTROY(&sx->sx_object);
  105 }
  106 
  107 void
  108 _sx_slock(struct sx *sx, const char *file, int line)
  109 {
  110 
  111         mtx_lock(sx->sx_lock);
  112         KASSERT(sx->sx_xholder != curthread,
  113             ("%s (%s): slock while xlock is held @ %s:%d\n", __func__,
  114             sx->sx_object.lo_name, file, line));
  115         WITNESS_CHECKORDER(&sx->sx_object, LOP_NEWORDER, file, line);
  116 
  117         /*
  118          * Loop in case we lose the race for lock acquisition.
  119          */
  120         while (sx->sx_cnt < 0) {
  121                 sx->sx_shrd_wcnt++;
  122                 cv_wait(&sx->sx_shrd_cv, sx->sx_lock);
  123                 sx->sx_shrd_wcnt--;
  124         }
  125 
  126         /* Acquire a shared lock. */
  127         sx->sx_cnt++;
  128 
  129         LOCK_LOG_LOCK("SLOCK", &sx->sx_object, 0, 0, file, line);
  130         WITNESS_LOCK(&sx->sx_object, 0, file, line);
  131 
  132         mtx_unlock(sx->sx_lock);
  133 }
  134 
  135 int
  136 _sx_try_slock(struct sx *sx, const char *file, int line)
  137 {
  138 
  139         mtx_lock(sx->sx_lock);
  140         if (sx->sx_cnt >= 0) {
  141                 sx->sx_cnt++;
  142                 LOCK_LOG_TRY("SLOCK", &sx->sx_object, 0, 1, file, line);
  143                 WITNESS_LOCK(&sx->sx_object, LOP_TRYLOCK, file, line);
  144                 mtx_unlock(sx->sx_lock);
  145                 return (1);
  146         } else {
  147                 LOCK_LOG_TRY("SLOCK", &sx->sx_object, 0, 0, file, line);
  148                 mtx_unlock(sx->sx_lock);
  149                 return (0);
  150         }
  151 }
  152 
  153 void
  154 _sx_xlock(struct sx *sx, const char *file, int line)
  155 {
  156 
  157         mtx_lock(sx->sx_lock);
  158 
  159         /*
  160          * With sx locks, we're absolutely not permitted to recurse on
  161          * xlocks, as it is fatal (deadlock). Normally, recursion is handled
  162          * by WITNESS, but as it is not semantically correct to hold the
  163          * xlock while in here, we consider it API abuse and put it under
  164          * INVARIANTS.
  165          */
  166         KASSERT(sx->sx_xholder != curthread,
  167             ("%s (%s): xlock already held @ %s:%d", __func__,
  168             sx->sx_object.lo_name, file, line));
  169         WITNESS_CHECKORDER(&sx->sx_object, LOP_NEWORDER | LOP_EXCLUSIVE, file,
  170             line);
  171 
  172         /* Loop in case we lose the race for lock acquisition. */
  173         while (sx->sx_cnt != 0) {
  174                 sx->sx_excl_wcnt++;
  175                 cv_wait(&sx->sx_excl_cv, sx->sx_lock);
  176                 sx->sx_excl_wcnt--;
  177         }
  178 
  179         MPASS(sx->sx_cnt == 0);
  180 
  181         /* Acquire an exclusive lock. */
  182         sx->sx_cnt--;
  183         sx->sx_xholder = curthread;
  184 
  185         LOCK_LOG_LOCK("XLOCK", &sx->sx_object, 0, 0, file, line);
  186         WITNESS_LOCK(&sx->sx_object, LOP_EXCLUSIVE, file, line);
  187 
  188         mtx_unlock(sx->sx_lock);
  189 }
  190 
  191 int
  192 _sx_try_xlock(struct sx *sx, const char *file, int line)
  193 {
  194 
  195         mtx_lock(sx->sx_lock);
  196         if (sx->sx_cnt == 0) {
  197                 sx->sx_cnt--;
  198                 sx->sx_xholder = curthread;
  199                 LOCK_LOG_TRY("XLOCK", &sx->sx_object, 0, 1, file, line);
  200                 WITNESS_LOCK(&sx->sx_object, LOP_EXCLUSIVE | LOP_TRYLOCK, file,
  201                     line);
  202                 mtx_unlock(sx->sx_lock);
  203                 return (1);
  204         } else {
  205                 LOCK_LOG_TRY("XLOCK", &sx->sx_object, 0, 0, file, line);
  206                 mtx_unlock(sx->sx_lock);
  207                 return (0);
  208         }
  209 }
  210 
  211 void
  212 _sx_sunlock(struct sx *sx, const char *file, int line)
  213 {
  214 
  215         _sx_assert(sx, SX_SLOCKED, file, line);
  216         mtx_lock(sx->sx_lock);
  217 
  218         WITNESS_UNLOCK(&sx->sx_object, 0, file, line);
  219 
  220         /* Release. */
  221         sx->sx_cnt--;
  222 
  223         /*
  224          * If we just released the last shared lock, wake any waiters up, giving
  225          * exclusive lockers precedence.  In order to make sure that exclusive
  226          * lockers won't be blocked forever, don't wake shared lock waiters if
  227          * there are exclusive lock waiters.
  228          */
  229         if (sx->sx_excl_wcnt > 0) {
  230                 if (sx->sx_cnt == 0)
  231                         cv_signal(&sx->sx_excl_cv);
  232         } else if (sx->sx_shrd_wcnt > 0)
  233                 cv_broadcast(&sx->sx_shrd_cv);
  234 
  235         LOCK_LOG_LOCK("SUNLOCK", &sx->sx_object, 0, 0, file, line);
  236 
  237         mtx_unlock(sx->sx_lock);
  238 }
  239 
  240 void
  241 _sx_xunlock(struct sx *sx, const char *file, int line)
  242 {
  243 
  244         _sx_assert(sx, SX_XLOCKED, file, line);
  245         mtx_lock(sx->sx_lock);
  246         MPASS(sx->sx_cnt == -1);
  247 
  248         WITNESS_UNLOCK(&sx->sx_object, LOP_EXCLUSIVE, file, line);
  249 
  250         /* Release. */
  251         sx->sx_cnt++;
  252         sx->sx_xholder = NULL;
  253 
  254         /*
  255          * Wake up waiters if there are any.  Give precedence to slock waiters.
  256          */
  257         if (sx->sx_shrd_wcnt > 0)
  258                 cv_broadcast(&sx->sx_shrd_cv);
  259         else if (sx->sx_excl_wcnt > 0)
  260                 cv_signal(&sx->sx_excl_cv);
  261 
  262         LOCK_LOG_LOCK("XUNLOCK", &sx->sx_object, 0, 0, file, line);
  263 
  264         mtx_unlock(sx->sx_lock);
  265 }
  266 
  267 int
  268 _sx_try_upgrade(struct sx *sx, const char *file, int line)
  269 {
  270 
  271         _sx_assert(sx, SX_SLOCKED, file, line);
  272         mtx_lock(sx->sx_lock);
  273 
  274         if (sx->sx_cnt == 1) {
  275                 sx->sx_cnt = -1;
  276                 sx->sx_xholder = curthread;
  277 
  278                 LOCK_LOG_TRY("XUPGRADE", &sx->sx_object, 0, 1, file, line);
  279                 WITNESS_UPGRADE(&sx->sx_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
  280                     file, line);
  281 
  282                 mtx_unlock(sx->sx_lock);
  283                 return (1);
  284         } else {
  285                 LOCK_LOG_TRY("XUPGRADE", &sx->sx_object, 0, 0, file, line);
  286                 mtx_unlock(sx->sx_lock);
  287                 return (0);
  288         }
  289 }
  290 
  291 void
  292 _sx_downgrade(struct sx *sx, const char *file, int line)
  293 {
  294 
  295         _sx_assert(sx, SX_XLOCKED, file, line);
  296         mtx_lock(sx->sx_lock);
  297         MPASS(sx->sx_cnt == -1);
  298 
  299         WITNESS_DOWNGRADE(&sx->sx_object, 0, file, line);
  300 
  301         sx->sx_cnt = 1;
  302         sx->sx_xholder = NULL;
  303         if (sx->sx_shrd_wcnt > 0)
  304                 cv_broadcast(&sx->sx_shrd_cv);
  305 
  306         LOCK_LOG_LOCK("XDOWNGRADE", &sx->sx_object, 0, 0, file, line);
  307 
  308         mtx_unlock(sx->sx_lock);
  309 }
  310 
  311 #ifdef INVARIANT_SUPPORT
  312 #ifndef INVARIANTS
  313 #undef  _sx_assert
  314 #endif
  315 
  316 /*
  317  * In the non-WITNESS case, sx_assert() can only detect that at least
  318  * *some* thread owns an slock, but it cannot guarantee that *this*
  319  * thread owns an slock.
  320  */
  321 void
  322 _sx_assert(struct sx *sx, int what, const char *file, int line)
  323 {
  324 
  325         if (panicstr != NULL)
  326                 return;
  327         switch (what) {
  328         case SX_LOCKED:
  329         case SX_SLOCKED:
  330 #ifdef WITNESS
  331                 witness_assert(&sx->sx_object, what, file, line);
  332 #else
  333                 mtx_lock(sx->sx_lock);
  334                 if (sx->sx_cnt <= 0 &&
  335                     (what == SX_SLOCKED || sx->sx_xholder != curthread))
  336                         panic("Lock %s not %slocked @ %s:%d\n",
  337                             sx->sx_object.lo_name, (what == SX_SLOCKED) ?
  338                             "share " : "", file, line);
  339                 mtx_unlock(sx->sx_lock);
  340 #endif
  341                 break;
  342         case SX_XLOCKED:
  343                 mtx_lock(sx->sx_lock);
  344                 if (sx->sx_xholder != curthread)
  345                         panic("Lock %s not exclusively locked @ %s:%d\n",
  346                             sx->sx_object.lo_name, file, line);
  347                 mtx_unlock(sx->sx_lock);
  348                 break;
  349         case SX_UNLOCKED:
  350 #ifdef WITNESS
  351                 witness_assert(&sx->sx_object, what, file, line);
  352 #else
  353                 /*
  354                  * We are able to check only exclusive lock here,
  355                  * we cannot assert that *this* thread owns slock.
  356                  */
  357                 mtx_lock(sx->sx_lock);
  358                 if (sx->sx_xholder == curthread)
  359                         panic("Lock %s exclusively locked @ %s:%d\n",
  360                             sx->sx_object.lo_name, file, line);
  361                 mtx_unlock(sx->sx_lock);
  362 #endif
  363                 break;
  364         default:
  365                 panic("Unknown sx lock assertion: %d @ %s:%d", what, file,
  366                     line);
  367         }
  368 }
  369 #endif  /* INVARIANT_SUPPORT */

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