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) 2007 Attilio Rao <attilio@freebsd.org>
    3  * Copyright (c) 2001 Jason Evans <jasone@freebsd.org>
    4  * All rights reserved.
    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(s), this list of conditions and the following disclaimer as
   11  *    the first lines of this file unmodified other than the possible
   12  *    addition of one or more copyright notices.
   13  * 2. Redistributions in binary form must reproduce the above copyright
   14  *    notice(s), this list of conditions and the following disclaimer in the
   15  *    documentation and/or other materials provided with the distribution.
   16  *
   17  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDER(S) ``AS IS'' AND ANY
   18  * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
   19  * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
   20  * DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT HOLDER(S) BE LIABLE FOR ANY
   21  * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
   22  * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
   23  * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
   24  * 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 SUCH
   27  * DAMAGE.
   28  */
   29 
   30 /*
   31  * Shared/exclusive locks.  This implementation attempts to ensure
   32  * deterministic lock granting behavior, so that slocks and xlocks are
   33  * interleaved.
   34  *
   35  * Priority propagation will not generally raise the priority of lock holders,
   36  * so should not be relied upon in combination with sx locks.
   37  */
   38 
   39 #include "opt_adaptive_sx.h"
   40 #include "opt_ddb.h"
   41 
   42 #include <sys/cdefs.h>
   43 __FBSDID("$FreeBSD$");
   44 
   45 #include <sys/param.h>
   46 #include <sys/ktr.h>
   47 #include <sys/lock.h>
   48 #include <sys/mutex.h>
   49 #include <sys/proc.h>
   50 #include <sys/sleepqueue.h>
   51 #include <sys/sx.h>
   52 #include <sys/systm.h>
   53 
   54 #ifdef ADAPTIVE_SX
   55 #include <machine/cpu.h>
   56 #endif
   57 
   58 #ifdef DDB
   59 #include <ddb/ddb.h>
   60 #endif
   61 
   62 #if !defined(SMP) && defined(ADAPTIVE_SX)
   63 #error "You must have SMP to enable the ADAPTIVE_SX option"
   64 #endif
   65 
   66 CTASSERT(((SX_ADAPTIVESPIN | SX_RECURSE) & LO_CLASSFLAGS) ==
   67     (SX_ADAPTIVESPIN | SX_RECURSE));
   68 
   69 /* Handy macros for sleep queues. */
   70 #define SQ_EXCLUSIVE_QUEUE      0
   71 #define SQ_SHARED_QUEUE         1
   72 
   73 /*
   74  * Variations on DROP_GIANT()/PICKUP_GIANT() for use in this file.  We
   75  * drop Giant anytime we have to sleep or if we adaptively spin.
   76  */
   77 #define GIANT_DECLARE                                                   \
   78         int _giantcnt = 0;                                              \
   79         WITNESS_SAVE_DECL(Giant)                                        \
   80 
   81 #define GIANT_SAVE() do {                                               \
   82         if (mtx_owned(&Giant)) {                                        \
   83                 WITNESS_SAVE(&Giant.lock_object, Giant);                \
   84                 while (mtx_owned(&Giant)) {                             \
   85                         _giantcnt++;                                    \
   86                         mtx_unlock(&Giant);                             \
   87                 }                                                       \
   88         }                                                               \
   89 } while (0)
   90 
   91 #define GIANT_RESTORE() do {                                            \
   92         if (_giantcnt > 0) {                                            \
   93                 mtx_assert(&Giant, MA_NOTOWNED);                        \
   94                 while (_giantcnt--)                                     \
   95                         mtx_lock(&Giant);                               \
   96                 WITNESS_RESTORE(&Giant.lock_object, Giant);             \
   97         }                                                               \
   98 } while (0)
   99 
  100 /*
  101  * Returns true if an exclusive lock is recursed.  It assumes
  102  * curthread currently has an exclusive lock.
  103  */
  104 #define sx_recursed(sx)         ((sx)->sx_recurse != 0)
  105 
  106 #ifdef DDB
  107 static void     db_show_sx(struct lock_object *lock);
  108 #endif
  109 static void     lock_sx(struct lock_object *lock, int how);
  110 static int      unlock_sx(struct lock_object *lock);
  111 
  112 struct lock_class lock_class_sx = {
  113         .lc_name = "sx",
  114         .lc_flags = LC_SLEEPLOCK | LC_SLEEPABLE | LC_RECURSABLE | LC_UPGRADABLE,
  115 #ifdef DDB
  116         .lc_ddb_show = db_show_sx,
  117 #endif
  118         .lc_lock = lock_sx,
  119         .lc_unlock = unlock_sx,
  120 };
  121 
  122 #ifndef INVARIANTS
  123 #define _sx_assert(sx, what, file, line)
  124 #endif
  125 
  126 void
  127 lock_sx(struct lock_object *lock, int how)
  128 {
  129         struct sx *sx;
  130 
  131         sx = (struct sx *)lock;
  132         if (how)
  133                 sx_xlock(sx);
  134         else
  135                 sx_slock(sx);
  136 }
  137 
  138 int
  139 unlock_sx(struct lock_object *lock)
  140 {
  141         struct sx *sx;
  142 
  143         sx = (struct sx *)lock;
  144         sx_assert(sx, SA_LOCKED | SA_NOTRECURSED);
  145         if (sx_xlocked(sx)) {
  146                 sx_xunlock(sx);
  147                 return (1);
  148         } else {
  149                 sx_sunlock(sx);
  150                 return (0);
  151         }
  152 }
  153 
  154 void
  155 sx_sysinit(void *arg)
  156 {
  157         struct sx_args *sargs = arg;
  158 
  159         sx_init(sargs->sa_sx, sargs->sa_desc);
  160 }
  161 
  162 void
  163 sx_init_flags(struct sx *sx, const char *description, int opts)
  164 {
  165         int flags;
  166 
  167         MPASS((opts & ~(SX_QUIET | SX_RECURSE | SX_NOWITNESS | SX_DUPOK |
  168             SX_NOPROFILE | SX_ADAPTIVESPIN)) == 0);
  169 
  170         flags = LO_RECURSABLE | LO_SLEEPABLE | LO_UPGRADABLE;
  171         if (opts & SX_DUPOK)
  172                 flags |= LO_DUPOK;
  173         if (opts & SX_NOPROFILE)
  174                 flags |= LO_NOPROFILE;
  175         if (!(opts & SX_NOWITNESS))
  176                 flags |= LO_WITNESS;
  177         if (opts & SX_QUIET)
  178                 flags |= LO_QUIET;
  179 
  180         flags |= opts & (SX_ADAPTIVESPIN | SX_RECURSE);
  181         sx->sx_lock = SX_LOCK_UNLOCKED;
  182         sx->sx_recurse = 0;
  183         lock_init(&sx->lock_object, &lock_class_sx, description, NULL, flags);
  184 }
  185 
  186 void
  187 sx_destroy(struct sx *sx)
  188 {
  189 
  190         KASSERT(sx->sx_lock == SX_LOCK_UNLOCKED, ("sx lock still held"));
  191         KASSERT(sx->sx_recurse == 0, ("sx lock still recursed"));
  192         sx->sx_lock = SX_LOCK_DESTROYED;
  193         lock_destroy(&sx->lock_object);
  194 }
  195 
  196 int
  197 _sx_slock(struct sx *sx, int opts, const char *file, int line)
  198 {
  199         int error = 0;
  200 
  201         MPASS(curthread != NULL);
  202         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
  203             ("sx_slock() of destroyed sx @ %s:%d", file, line));
  204         WITNESS_CHECKORDER(&sx->lock_object, LOP_NEWORDER, file, line);
  205         error = __sx_slock(sx, opts, file, line);
  206         if (!error) {
  207                 LOCK_LOG_LOCK("SLOCK", &sx->lock_object, 0, 0, file, line);
  208                 WITNESS_LOCK(&sx->lock_object, 0, file, line);
  209                 curthread->td_locks++;
  210         }
  211 
  212         return (error);
  213 }
  214 
  215 int
  216 _sx_try_slock(struct sx *sx, const char *file, int line)
  217 {
  218         uintptr_t x;
  219 
  220         for (;;) {
  221                 x = sx->sx_lock;
  222                 KASSERT(x != SX_LOCK_DESTROYED,
  223                     ("sx_try_slock() of destroyed sx @ %s:%d", file, line));
  224                 if (!(x & SX_LOCK_SHARED))
  225                         break;
  226                 if (atomic_cmpset_acq_ptr(&sx->sx_lock, x, x + SX_ONE_SHARER)) {
  227                         LOCK_LOG_TRY("SLOCK", &sx->lock_object, 0, 1, file, line);
  228                         WITNESS_LOCK(&sx->lock_object, LOP_TRYLOCK, file, line);
  229                         curthread->td_locks++;
  230                         return (1);
  231                 }
  232         }
  233 
  234         LOCK_LOG_TRY("SLOCK", &sx->lock_object, 0, 0, file, line);
  235         return (0);
  236 }
  237 
  238 int
  239 _sx_xlock(struct sx *sx, int opts, const char *file, int line)
  240 {
  241         int error = 0;
  242 
  243         MPASS(curthread != NULL);
  244         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
  245             ("sx_xlock() of destroyed sx @ %s:%d", file, line));
  246         WITNESS_CHECKORDER(&sx->lock_object, LOP_NEWORDER | LOP_EXCLUSIVE, file,
  247             line);
  248         error = __sx_xlock(sx, curthread, opts, file, line);
  249         if (!error) {
  250                 LOCK_LOG_LOCK("XLOCK", &sx->lock_object, 0, sx->sx_recurse,
  251                     file, line);
  252                 WITNESS_LOCK(&sx->lock_object, LOP_EXCLUSIVE, file, line);
  253                 curthread->td_locks++;
  254         }
  255 
  256         return (error);
  257 }
  258 
  259 int
  260 _sx_try_xlock(struct sx *sx, const char *file, int line)
  261 {
  262         int rval;
  263 
  264         MPASS(curthread != NULL);
  265         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
  266             ("sx_try_xlock() of destroyed sx @ %s:%d", file, line));
  267 
  268         if (sx_xlocked(sx) && (sx->lock_object.lo_flags & SX_RECURSE) != 0) {
  269                 sx->sx_recurse++;
  270                 atomic_set_ptr(&sx->sx_lock, SX_LOCK_RECURSED);
  271                 rval = 1;
  272         } else
  273                 rval = atomic_cmpset_acq_ptr(&sx->sx_lock, SX_LOCK_UNLOCKED,
  274                     (uintptr_t)curthread);
  275         LOCK_LOG_TRY("XLOCK", &sx->lock_object, 0, rval, file, line);
  276         if (rval) {
  277                 WITNESS_LOCK(&sx->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
  278                     file, line);
  279                 curthread->td_locks++;
  280         }
  281 
  282         return (rval);
  283 }
  284 
  285 void
  286 _sx_sunlock(struct sx *sx, const char *file, int line)
  287 {
  288 
  289         MPASS(curthread != NULL);
  290         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
  291             ("sx_sunlock() of destroyed sx @ %s:%d", file, line));
  292         _sx_assert(sx, SA_SLOCKED, file, line);
  293         curthread->td_locks--;
  294         WITNESS_UNLOCK(&sx->lock_object, 0, file, line);
  295         LOCK_LOG_LOCK("SUNLOCK", &sx->lock_object, 0, 0, file, line);
  296 #ifdef LOCK_PROFILING_SHARED
  297         if (SX_SHARERS(sx->sx_lock) == 1)
  298                 lock_profile_release_lock(&sx->lock_object);
  299 #endif
  300         __sx_sunlock(sx, file, line);
  301 }
  302 
  303 void
  304 _sx_xunlock(struct sx *sx, const char *file, int line)
  305 {
  306 
  307         MPASS(curthread != NULL);
  308         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
  309             ("sx_xunlock() of destroyed sx @ %s:%d", file, line));
  310         _sx_assert(sx, SA_XLOCKED, file, line);
  311         curthread->td_locks--;
  312         WITNESS_UNLOCK(&sx->lock_object, LOP_EXCLUSIVE, file, line);
  313         LOCK_LOG_LOCK("XUNLOCK", &sx->lock_object, 0, sx->sx_recurse, file,
  314             line);
  315         if (!sx_recursed(sx))
  316                 lock_profile_release_lock(&sx->lock_object);
  317         __sx_xunlock(sx, curthread, file, line);
  318 }
  319 
  320 /*
  321  * Try to do a non-blocking upgrade from a shared lock to an exclusive lock.
  322  * This will only succeed if this thread holds a single shared lock.
  323  * Return 1 if if the upgrade succeed, 0 otherwise.
  324  */
  325 int
  326 _sx_try_upgrade(struct sx *sx, const char *file, int line)
  327 {
  328         uintptr_t x;
  329         int success;
  330 
  331         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
  332             ("sx_try_upgrade() of destroyed sx @ %s:%d", file, line));
  333         _sx_assert(sx, SA_SLOCKED, file, line);
  334 
  335         /*
  336          * Try to switch from one shared lock to an exclusive lock.  We need
  337          * to maintain the SX_LOCK_EXCLUSIVE_WAITERS flag if set so that
  338          * we will wake up the exclusive waiters when we drop the lock.
  339          */
  340         x = sx->sx_lock & SX_LOCK_EXCLUSIVE_WAITERS;
  341         success = atomic_cmpset_ptr(&sx->sx_lock, SX_SHARERS_LOCK(1) | x,
  342             (uintptr_t)curthread | x);
  343         LOCK_LOG_TRY("XUPGRADE", &sx->lock_object, 0, success, file, line);
  344         if (success)
  345                 WITNESS_UPGRADE(&sx->lock_object, LOP_EXCLUSIVE | LOP_TRYLOCK,
  346                     file, line);
  347         return (success);
  348 }
  349 
  350 /*
  351  * Downgrade an unrecursed exclusive lock into a single shared lock.
  352  */
  353 void
  354 _sx_downgrade(struct sx *sx, const char *file, int line)
  355 {
  356         uintptr_t x;
  357         int wakeup_swapper;
  358 
  359         KASSERT(sx->sx_lock != SX_LOCK_DESTROYED,
  360             ("sx_downgrade() of destroyed sx @ %s:%d", file, line));
  361         _sx_assert(sx, SA_XLOCKED | SA_NOTRECURSED, file, line);
  362 #ifndef INVARIANTS
  363         if (sx_recursed(sx))
  364                 panic("downgrade of a recursed lock");
  365 #endif
  366 
  367         WITNESS_DOWNGRADE(&sx->lock_object, 0, file, line);
  368 
  369         /*
  370          * Try to switch from an exclusive lock with no shared waiters
  371          * to one sharer with no shared waiters.  If there are
  372          * exclusive waiters, we don't need to lock the sleep queue so
  373          * long as we preserve the flag.  We do one quick try and if
  374          * that fails we grab the sleepq lock to keep the flags from
  375          * changing and do it the slow way.
  376          *
  377          * We have to lock the sleep queue if there are shared waiters
  378          * so we can wake them up.
  379          */
  380         x = sx->sx_lock;
  381         if (!(x & SX_LOCK_SHARED_WAITERS) &&
  382             atomic_cmpset_rel_ptr(&sx->sx_lock, x, SX_SHARERS_LOCK(1) |
  383             (x & SX_LOCK_EXCLUSIVE_WAITERS))) {
  384                 LOCK_LOG_LOCK("XDOWNGRADE", &sx->lock_object, 0, 0, file, line);
  385                 return;
  386         }
  387 
  388         /*
  389          * Lock the sleep queue so we can read the waiters bits
  390          * without any races and wakeup any shared waiters.
  391          */
  392         sleepq_lock(&sx->lock_object);
  393 
  394         /*
  395          * Preserve SX_LOCK_EXCLUSIVE_WAITERS while downgraded to a single
  396          * shared lock.  If there are any shared waiters, wake them up.
  397          */
  398         wakeup_swapper = 0;
  399         x = sx->sx_lock;
  400         atomic_store_rel_ptr(&sx->sx_lock, SX_SHARERS_LOCK(1) |
  401             (x & SX_LOCK_EXCLUSIVE_WAITERS));
  402         if (x & SX_LOCK_SHARED_WAITERS)
  403                 wakeup_swapper = sleepq_broadcast(&sx->lock_object, SLEEPQ_SX,
  404                     -1, SQ_SHARED_QUEUE);
  405         else
  406                 sleepq_release(&sx->lock_object);
  407 
  408         LOCK_LOG_LOCK("XDOWNGRADE", &sx->lock_object, 0, 0, file, line);
  409 
  410         if (wakeup_swapper)
  411                 kick_proc0();
  412 }
  413 
  414 /*
  415  * This function represents the so-called 'hard case' for sx_xlock
  416  * operation.  All 'easy case' failures are redirected to this.  Note
  417  * that ideally this would be a static function, but it needs to be
  418  * accessible from at least sx.h.
  419  */
  420 int
  421 _sx_xlock_hard(struct sx *sx, uintptr_t tid, int opts, const char *file,
  422     int line)
  423 {
  424         GIANT_DECLARE;
  425 #ifdef ADAPTIVE_SX
  426         volatile struct thread *owner;
  427 #endif
  428         uint64_t waittime = 0;
  429         uintptr_t x;
  430         int contested = 0, error = 0;
  431 
  432         /* If we already hold an exclusive lock, then recurse. */
  433         if (sx_xlocked(sx)) {
  434                 KASSERT((sx->lock_object.lo_flags & SX_RECURSE) != 0,
  435             ("_sx_xlock_hard: recursed on non-recursive sx %s @ %s:%d\n",
  436                     sx->lock_object.lo_name, file, line));
  437                 sx->sx_recurse++;
  438                 atomic_set_ptr(&sx->sx_lock, SX_LOCK_RECURSED);
  439                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
  440                         CTR2(KTR_LOCK, "%s: %p recursing", __func__, sx);
  441                 return (0);
  442         }
  443 
  444         if (LOCK_LOG_TEST(&sx->lock_object, 0))
  445                 CTR5(KTR_LOCK, "%s: %s contested (lock=%p) at %s:%d", __func__,
  446                     sx->lock_object.lo_name, (void *)sx->sx_lock, file, line);
  447 
  448         while (!atomic_cmpset_acq_ptr(&sx->sx_lock, SX_LOCK_UNLOCKED, tid)) {
  449 #ifdef ADAPTIVE_SX
  450                 /*
  451                  * If the lock is write locked and the owner is
  452                  * running on another CPU, spin until the owner stops
  453                  * running or the state of the lock changes.
  454                  */
  455                 x = sx->sx_lock;
  456                 if (!(x & SX_LOCK_SHARED) &&
  457                     (sx->lock_object.lo_flags & SX_ADAPTIVESPIN)) {
  458                         x = SX_OWNER(x);
  459                         owner = (struct thread *)x;
  460                         if (TD_IS_RUNNING(owner)) {
  461                                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
  462                                         CTR3(KTR_LOCK,
  463                                             "%s: spinning on %p held by %p",
  464                                             __func__, sx, owner);
  465                                 GIANT_SAVE();
  466                                 lock_profile_obtain_lock_failed(
  467                                     &sx->lock_object, &contested, &waittime);
  468                                 while (SX_OWNER(sx->sx_lock) == x &&
  469                                     TD_IS_RUNNING(owner))
  470                                         cpu_spinwait();
  471                                 continue;
  472                         }
  473                 }
  474 #endif
  475 
  476                 sleepq_lock(&sx->lock_object);
  477                 x = sx->sx_lock;
  478 
  479                 /*
  480                  * If the lock was released while spinning on the
  481                  * sleep queue chain lock, try again.
  482                  */
  483                 if (x == SX_LOCK_UNLOCKED) {
  484                         sleepq_release(&sx->lock_object);
  485                         continue;
  486                 }
  487 
  488 #ifdef ADAPTIVE_SX
  489                 /*
  490                  * The current lock owner might have started executing
  491                  * on another CPU (or the lock could have changed
  492                  * owners) while we were waiting on the sleep queue
  493                  * chain lock.  If so, drop the sleep queue lock and try
  494                  * again.
  495                  */
  496                 if (!(x & SX_LOCK_SHARED) &&
  497                     (sx->lock_object.lo_flags & SX_ADAPTIVESPIN)) {
  498                         owner = (struct thread *)SX_OWNER(x);
  499                         if (TD_IS_RUNNING(owner)) {
  500                                 sleepq_release(&sx->lock_object);
  501                                 continue;
  502                         }
  503                 }
  504 #endif
  505 
  506                 /*
  507                  * If an exclusive lock was released with both shared
  508                  * and exclusive waiters and a shared waiter hasn't
  509                  * woken up and acquired the lock yet, sx_lock will be
  510                  * set to SX_LOCK_UNLOCKED | SX_LOCK_EXCLUSIVE_WAITERS.
  511                  * If we see that value, try to acquire it once.  Note
  512                  * that we have to preserve SX_LOCK_EXCLUSIVE_WAITERS
  513                  * as there are other exclusive waiters still.  If we
  514                  * fail, restart the loop.
  515                  */
  516                 if (x == (SX_LOCK_UNLOCKED | SX_LOCK_EXCLUSIVE_WAITERS)) {
  517                         if (atomic_cmpset_acq_ptr(&sx->sx_lock,
  518                             SX_LOCK_UNLOCKED | SX_LOCK_EXCLUSIVE_WAITERS,
  519                             tid | SX_LOCK_EXCLUSIVE_WAITERS)) {
  520                                 sleepq_release(&sx->lock_object);
  521                                 CTR2(KTR_LOCK, "%s: %p claimed by new writer",
  522                                     __func__, sx);
  523                                 break;
  524                         }
  525                         sleepq_release(&sx->lock_object);
  526                         continue;
  527                 }
  528 
  529                 /*
  530                  * Try to set the SX_LOCK_EXCLUSIVE_WAITERS.  If we fail,
  531                  * than loop back and retry.
  532                  */
  533                 if (!(x & SX_LOCK_EXCLUSIVE_WAITERS)) {
  534                         if (!atomic_cmpset_ptr(&sx->sx_lock, x,
  535                             x | SX_LOCK_EXCLUSIVE_WAITERS)) {
  536                                 sleepq_release(&sx->lock_object);
  537                                 continue;
  538                         }
  539                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
  540                                 CTR2(KTR_LOCK, "%s: %p set excl waiters flag",
  541                                     __func__, sx);
  542                 }
  543 
  544                 /*
  545                  * Since we have been unable to acquire the exclusive
  546                  * lock and the exclusive waiters flag is set, we have
  547                  * to sleep.
  548                  */
  549                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
  550                         CTR2(KTR_LOCK, "%s: %p blocking on sleep queue",
  551                             __func__, sx);
  552 
  553                 GIANT_SAVE();
  554                 lock_profile_obtain_lock_failed(&sx->lock_object, &contested,
  555                     &waittime);
  556                 sleepq_add(&sx->lock_object, NULL, sx->lock_object.lo_name,
  557                     SLEEPQ_SX | ((opts & SX_INTERRUPTIBLE) ?
  558                     SLEEPQ_INTERRUPTIBLE : 0), SQ_EXCLUSIVE_QUEUE);
  559                 if (!(opts & SX_INTERRUPTIBLE))
  560                         sleepq_wait(&sx->lock_object);
  561                 else
  562                         error = sleepq_wait_sig(&sx->lock_object);
  563 
  564                 if (error) {
  565                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
  566                                 CTR2(KTR_LOCK,
  567                         "%s: interruptible sleep by %p suspended by signal",
  568                                     __func__, sx);
  569                         break;
  570                 }
  571                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
  572                         CTR2(KTR_LOCK, "%s: %p resuming from sleep queue",
  573                             __func__, sx);
  574         }
  575 
  576         GIANT_RESTORE();
  577         if (!error)
  578                 lock_profile_obtain_lock_success(&sx->lock_object, contested,
  579                     waittime, file, line);
  580         return (error);
  581 }
  582 
  583 /*
  584  * This function represents the so-called 'hard case' for sx_xunlock
  585  * operation.  All 'easy case' failures are redirected to this.  Note
  586  * that ideally this would be a static function, but it needs to be
  587  * accessible from at least sx.h.
  588  */
  589 void
  590 _sx_xunlock_hard(struct sx *sx, uintptr_t tid, const char *file, int line)
  591 {
  592         uintptr_t x;
  593         int queue, wakeup_swapper;
  594 
  595         MPASS(!(sx->sx_lock & SX_LOCK_SHARED));
  596 
  597         /* If the lock is recursed, then unrecurse one level. */
  598         if (sx_xlocked(sx) && sx_recursed(sx)) {
  599                 if ((--sx->sx_recurse) == 0)
  600                         atomic_clear_ptr(&sx->sx_lock, SX_LOCK_RECURSED);
  601                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
  602                         CTR2(KTR_LOCK, "%s: %p unrecursing", __func__, sx);
  603                 return;
  604         }
  605         MPASS(sx->sx_lock & (SX_LOCK_SHARED_WAITERS |
  606             SX_LOCK_EXCLUSIVE_WAITERS));
  607         if (LOCK_LOG_TEST(&sx->lock_object, 0))
  608                 CTR2(KTR_LOCK, "%s: %p contested", __func__, sx);
  609 
  610         sleepq_lock(&sx->lock_object);
  611         x = SX_LOCK_UNLOCKED;
  612 
  613         /*
  614          * The wake up algorithm here is quite simple and probably not
  615          * ideal.  It gives precedence to shared waiters if they are
  616          * present.  For this condition, we have to preserve the
  617          * state of the exclusive waiters flag.
  618          */
  619         if (sx->sx_lock & SX_LOCK_SHARED_WAITERS) {
  620                 queue = SQ_SHARED_QUEUE;
  621                 x |= (sx->sx_lock & SX_LOCK_EXCLUSIVE_WAITERS);
  622         } else
  623                 queue = SQ_EXCLUSIVE_QUEUE;
  624 
  625         /* Wake up all the waiters for the specific queue. */
  626         if (LOCK_LOG_TEST(&sx->lock_object, 0))
  627                 CTR3(KTR_LOCK, "%s: %p waking up all threads on %s queue",
  628                     __func__, sx, queue == SQ_SHARED_QUEUE ? "shared" :
  629                     "exclusive");
  630         atomic_store_rel_ptr(&sx->sx_lock, x);
  631         wakeup_swapper = sleepq_broadcast(&sx->lock_object, SLEEPQ_SX, -1,
  632             queue);
  633         if (wakeup_swapper)
  634                 kick_proc0();
  635 }
  636 
  637 /*
  638  * This function represents the so-called 'hard case' for sx_slock
  639  * operation.  All 'easy case' failures are redirected to this.  Note
  640  * that ideally this would be a static function, but it needs to be
  641  * accessible from at least sx.h.
  642  */
  643 int
  644 _sx_slock_hard(struct sx *sx, int opts, const char *file, int line)
  645 {
  646         GIANT_DECLARE;
  647 #ifdef ADAPTIVE_SX
  648         volatile struct thread *owner;
  649 #endif
  650 #ifdef LOCK_PROFILING_SHARED
  651         uint64_t waittime = 0;
  652         int contested = 0;
  653 #endif
  654         uintptr_t x;
  655         int error = 0;
  656 
  657         /*
  658          * As with rwlocks, we don't make any attempt to try to block
  659          * shared locks once there is an exclusive waiter.
  660          */
  661         for (;;) {
  662                 x = sx->sx_lock;
  663 
  664                 /*
  665                  * If no other thread has an exclusive lock then try to bump up
  666                  * the count of sharers.  Since we have to preserve the state
  667                  * of SX_LOCK_EXCLUSIVE_WAITERS, if we fail to acquire the
  668                  * shared lock loop back and retry.
  669                  */
  670                 if (x & SX_LOCK_SHARED) {
  671                         MPASS(!(x & SX_LOCK_SHARED_WAITERS));
  672                         if (atomic_cmpset_acq_ptr(&sx->sx_lock, x,
  673                             x + SX_ONE_SHARER)) {
  674 #ifdef LOCK_PROFILING_SHARED
  675                                 if (SX_SHARERS(x) == 0)
  676                                         lock_profile_obtain_lock_success(
  677                                             &sx->lock_object, contested,
  678                                             waittime, file, line);
  679 #endif
  680                                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
  681                                         CTR4(KTR_LOCK,
  682                                             "%s: %p succeed %p -> %p", __func__,
  683                                             sx, (void *)x,
  684                                             (void *)(x + SX_ONE_SHARER));
  685                                 break;
  686                         }
  687                         continue;
  688                 }
  689 
  690 #ifdef ADAPTIVE_SX
  691                 /*
  692                  * If the owner is running on another CPU, spin until
  693                  * the owner stops running or the state of the lock
  694                  * changes.
  695                  */
  696                 else if (sx->lock_object.lo_flags & SX_ADAPTIVESPIN) {
  697                         x = SX_OWNER(x);
  698                         owner = (struct thread *)x;
  699                         if (TD_IS_RUNNING(owner)) {
  700                                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
  701                                         CTR3(KTR_LOCK,
  702                                             "%s: spinning on %p held by %p",
  703                                             __func__, sx, owner);
  704                                 GIANT_SAVE();
  705 #ifdef LOCK_PROFILING_SHARED
  706                                 lock_profile_obtain_lock_failed(
  707                                     &sx->lock_object, &contested, &waittime);
  708 #endif
  709                                 while (SX_OWNER(sx->sx_lock) == x &&
  710                                     TD_IS_RUNNING(owner))
  711                                         cpu_spinwait();
  712                                 continue;
  713                         }
  714                 }
  715 #endif
  716 
  717                 /*
  718                  * Some other thread already has an exclusive lock, so
  719                  * start the process of blocking.
  720                  */
  721                 sleepq_lock(&sx->lock_object);
  722                 x = sx->sx_lock;
  723 
  724                 /*
  725                  * The lock could have been released while we spun.
  726                  * In this case loop back and retry.
  727                  */
  728                 if (x & SX_LOCK_SHARED) {
  729                         sleepq_release(&sx->lock_object);
  730                         continue;
  731                 }
  732 
  733 #ifdef ADAPTIVE_SX
  734                 /*
  735                  * If the owner is running on another CPU, spin until
  736                  * the owner stops running or the state of the lock
  737                  * changes.
  738                  */
  739                 if (!(x & SX_LOCK_SHARED) &&
  740                     (sx->lock_object.lo_flags & SX_ADAPTIVESPIN)) {
  741                         owner = (struct thread *)SX_OWNER(x);
  742                         if (TD_IS_RUNNING(owner)) {
  743                                 sleepq_release(&sx->lock_object);
  744                                 continue;
  745                         }
  746                 }
  747 #endif
  748 
  749                 /*
  750                  * Try to set the SX_LOCK_SHARED_WAITERS flag.  If we
  751                  * fail to set it drop the sleep queue lock and loop
  752                  * back.
  753                  */
  754                 if (!(x & SX_LOCK_SHARED_WAITERS)) {
  755                         if (!atomic_cmpset_ptr(&sx->sx_lock, x,
  756                             x | SX_LOCK_SHARED_WAITERS)) {
  757                                 sleepq_release(&sx->lock_object);
  758                                 continue;
  759                         }
  760                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
  761                                 CTR2(KTR_LOCK, "%s: %p set shared waiters flag",
  762                                     __func__, sx);
  763                 }
  764 
  765                 /*
  766                  * Since we have been unable to acquire the shared lock,
  767                  * we have to sleep.
  768                  */
  769                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
  770                         CTR2(KTR_LOCK, "%s: %p blocking on sleep queue",
  771                             __func__, sx);
  772 
  773                 GIANT_SAVE();
  774 #ifdef LOCK_PROFILING_SHARED
  775                 lock_profile_obtain_lock_failed(&sx->lock_object, &contested,
  776                     &waittime);
  777 #endif
  778                 sleepq_add(&sx->lock_object, NULL, sx->lock_object.lo_name,
  779                     SLEEPQ_SX | ((opts & SX_INTERRUPTIBLE) ?
  780                     SLEEPQ_INTERRUPTIBLE : 0), SQ_SHARED_QUEUE);
  781                 if (!(opts & SX_INTERRUPTIBLE))
  782                         sleepq_wait(&sx->lock_object);
  783                 else
  784                         error = sleepq_wait_sig(&sx->lock_object);
  785 
  786                 if (error) {
  787                         if (LOCK_LOG_TEST(&sx->lock_object, 0))
  788                                 CTR2(KTR_LOCK,
  789                         "%s: interruptible sleep by %p suspended by signal",
  790                                     __func__, sx);
  791                         break;
  792                 }
  793                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
  794                         CTR2(KTR_LOCK, "%s: %p resuming from sleep queue",
  795                             __func__, sx);
  796         }
  797 
  798         GIANT_RESTORE();
  799         return (error);
  800 }
  801 
  802 /*
  803  * This function represents the so-called 'hard case' for sx_sunlock
  804  * operation.  All 'easy case' failures are redirected to this.  Note
  805  * that ideally this would be a static function, but it needs to be
  806  * accessible from at least sx.h.
  807  */
  808 void
  809 _sx_sunlock_hard(struct sx *sx, const char *file, int line)
  810 {
  811         uintptr_t x;
  812         int wakeup_swapper;
  813 
  814         for (;;) {
  815                 x = sx->sx_lock;
  816 
  817                 /*
  818                  * We should never have sharers while at least one thread
  819                  * holds a shared lock.
  820                  */
  821                 KASSERT(!(x & SX_LOCK_SHARED_WAITERS),
  822                     ("%s: waiting sharers", __func__));
  823 
  824                 /*
  825                  * See if there is more than one shared lock held.  If
  826                  * so, just drop one and return.
  827                  */
  828                 if (SX_SHARERS(x) > 1) {
  829                         if (atomic_cmpset_ptr(&sx->sx_lock, x,
  830                             x - SX_ONE_SHARER)) {
  831                                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
  832                                         CTR4(KTR_LOCK,
  833                                             "%s: %p succeeded %p -> %p",
  834                                             __func__, sx, (void *)x,
  835                                             (void *)(x - SX_ONE_SHARER));
  836                                 break;
  837                         }
  838                         continue;
  839                 }
  840 
  841                 /*
  842                  * If there aren't any waiters for an exclusive lock,
  843                  * then try to drop it quickly.
  844                  */
  845                 if (!(x & SX_LOCK_EXCLUSIVE_WAITERS)) {
  846                         MPASS(x == SX_SHARERS_LOCK(1));
  847                         if (atomic_cmpset_ptr(&sx->sx_lock, SX_SHARERS_LOCK(1),
  848                             SX_LOCK_UNLOCKED)) {
  849                                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
  850                                         CTR2(KTR_LOCK, "%s: %p last succeeded",
  851                                             __func__, sx);
  852                                 break;
  853                         }
  854                         continue;
  855                 }
  856 
  857                 /*
  858                  * At this point, there should just be one sharer with
  859                  * exclusive waiters.
  860                  */
  861                 MPASS(x == (SX_SHARERS_LOCK(1) | SX_LOCK_EXCLUSIVE_WAITERS));
  862 
  863                 sleepq_lock(&sx->lock_object);
  864 
  865                 /*
  866                  * Wake up semantic here is quite simple:
  867                  * Just wake up all the exclusive waiters.
  868                  * Note that the state of the lock could have changed,
  869                  * so if it fails loop back and retry.
  870                  */
  871                 if (!atomic_cmpset_ptr(&sx->sx_lock,
  872                     SX_SHARERS_LOCK(1) | SX_LOCK_EXCLUSIVE_WAITERS,
  873                     SX_LOCK_UNLOCKED)) {
  874                         sleepq_release(&sx->lock_object);
  875                         continue;
  876                 }
  877                 if (LOCK_LOG_TEST(&sx->lock_object, 0))
  878                         CTR2(KTR_LOCK, "%s: %p waking up all thread on"
  879                             "exclusive queue", __func__, sx);
  880                 wakeup_swapper = sleepq_broadcast(&sx->lock_object, SLEEPQ_SX,
  881                     -1, SQ_EXCLUSIVE_QUEUE);
  882                 if (wakeup_swapper)
  883                         kick_proc0();
  884                 break;
  885         }
  886 }
  887 
  888 #ifdef INVARIANT_SUPPORT
  889 #ifndef INVARIANTS
  890 #undef  _sx_assert
  891 #endif
  892 
  893 /*
  894  * In the non-WITNESS case, sx_assert() can only detect that at least
  895  * *some* thread owns an slock, but it cannot guarantee that *this*
  896  * thread owns an slock.
  897  */
  898 void
  899 _sx_assert(struct sx *sx, int what, const char *file, int line)
  900 {
  901 #ifndef WITNESS
  902         int slocked = 0;
  903 #endif
  904 
  905         if (panicstr != NULL)
  906                 return;
  907         switch (what) {
  908         case SA_SLOCKED:
  909         case SA_SLOCKED | SA_NOTRECURSED:
  910         case SA_SLOCKED | SA_RECURSED:
  911 #ifndef WITNESS
  912                 slocked = 1;
  913                 /* FALLTHROUGH */
  914 #endif
  915         case SA_LOCKED:
  916         case SA_LOCKED | SA_NOTRECURSED:
  917         case SA_LOCKED | SA_RECURSED:
  918 #ifdef WITNESS
  919                 witness_assert(&sx->lock_object, what, file, line);
  920 #else
  921                 /*
  922                  * If some other thread has an exclusive lock or we
  923                  * have one and are asserting a shared lock, fail.
  924                  * Also, if no one has a lock at all, fail.
  925                  */
  926                 if (sx->sx_lock == SX_LOCK_UNLOCKED ||
  927                     (!(sx->sx_lock & SX_LOCK_SHARED) && (slocked ||
  928                     sx_xholder(sx) != curthread)))
  929                         panic("Lock %s not %slocked @ %s:%d\n",
  930                             sx->lock_object.lo_name, slocked ? "share " : "",
  931                             file, line);
  932 
  933                 if (!(sx->sx_lock & SX_LOCK_SHARED)) {
  934                         if (sx_recursed(sx)) {
  935                                 if (what & SA_NOTRECURSED)
  936                                         panic("Lock %s recursed @ %s:%d\n",
  937                                             sx->lock_object.lo_name, file,
  938                                             line);
  939                         } else if (what & SA_RECURSED)
  940                                 panic("Lock %s not recursed @ %s:%d\n",
  941                                     sx->lock_object.lo_name, file, line);
  942                 }
  943 #endif
  944                 break;
  945         case SA_XLOCKED:
  946         case SA_XLOCKED | SA_NOTRECURSED:
  947         case SA_XLOCKED | SA_RECURSED:
  948                 if (sx_xholder(sx) != curthread)
  949                         panic("Lock %s not exclusively locked @ %s:%d\n",
  950                             sx->lock_object.lo_name, file, line);
  951                 if (sx_recursed(sx)) {
  952                         if (what & SA_NOTRECURSED)
  953                                 panic("Lock %s recursed @ %s:%d\n",
  954                                     sx->lock_object.lo_name, file, line);
  955                 } else if (what & SA_RECURSED)
  956                         panic("Lock %s not recursed @ %s:%d\n",
  957                             sx->lock_object.lo_name, file, line);
  958                 break;
  959         case SA_UNLOCKED:
  960 #ifdef WITNESS
  961                 witness_assert(&sx->lock_object, what, file, line);
  962 #else
  963                 /*
  964                  * If we hold an exclusve lock fail.  We can't
  965                  * reliably check to see if we hold a shared lock or
  966                  * not.
  967                  */
  968                 if (sx_xholder(sx) == curthread)
  969                         panic("Lock %s exclusively locked @ %s:%d\n",
  970                             sx->lock_object.lo_name, file, line);
  971 #endif
  972                 break;
  973         default:
  974                 panic("Unknown sx lock assertion: %d @ %s:%d", what, file,
  975                     line);
  976         }
  977 }
  978 #endif  /* INVARIANT_SUPPORT */
  979 
  980 #ifdef DDB
  981 static void
  982 db_show_sx(struct lock_object *lock)
  983 {
  984         struct thread *td;
  985         struct sx *sx;
  986 
  987         sx = (struct sx *)lock;
  988 
  989         db_printf(" state: ");
  990         if (sx->sx_lock == SX_LOCK_UNLOCKED)
  991                 db_printf("UNLOCKED\n");
  992         else if (sx->sx_lock == SX_LOCK_DESTROYED) {
  993                 db_printf("DESTROYED\n");
  994                 return;
  995         } else if (sx->sx_lock & SX_LOCK_SHARED)
  996                 db_printf("SLOCK: %ju\n", (uintmax_t)SX_SHARERS(sx->sx_lock));
  997         else {
  998                 td = sx_xholder(sx);
  999                 db_printf("XLOCK: %p (tid %d, pid %d, \"%s\")\n", td,
 1000                     td->td_tid, td->td_proc->p_pid, td->td_proc->p_comm);
 1001                 if (sx_recursed(sx))
 1002                         db_printf(" recursed: %d\n", sx->sx_recurse);
 1003         }
 1004 
 1005         db_printf(" waiters: ");
 1006         switch(sx->sx_lock &
 1007             (SX_LOCK_SHARED_WAITERS | SX_LOCK_EXCLUSIVE_WAITERS)) {
 1008         case SX_LOCK_SHARED_WAITERS:
 1009                 db_printf("shared\n");
 1010                 break;
 1011         case SX_LOCK_EXCLUSIVE_WAITERS:
 1012                 db_printf("exclusive\n");
 1013                 break;
 1014         case SX_LOCK_SHARED_WAITERS | SX_LOCK_EXCLUSIVE_WAITERS:
 1015                 db_printf("exclusive and shared\n");
 1016                 break;
 1017         default:
 1018                 db_printf("none\n");
 1019         }
 1020 }
 1021 
 1022 /*
 1023  * Check to see if a thread that is blocked on a sleep queue is actually
 1024  * blocked on an sx lock.  If so, output some details and return true.
 1025  * If the lock has an exclusive owner, return that in *ownerp.
 1026  */
 1027 int
 1028 sx_chain(struct thread *td, struct thread **ownerp)
 1029 {
 1030         struct sx *sx;
 1031 
 1032         /*
 1033          * Check to see if this thread is blocked on an sx lock.
 1034          * First, we check the lock class.  If that is ok, then we
 1035          * compare the lock name against the wait message.
 1036          */
 1037         sx = td->td_wchan;
 1038         if (LOCK_CLASS(&sx->lock_object) != &lock_class_sx ||
 1039             sx->lock_object.lo_name != td->td_wmesg)
 1040                 return (0);
 1041 
 1042         /* We think we have an sx lock, so output some details. */
 1043         db_printf("blocked on sx \"%s\" ", td->td_wmesg);
 1044         *ownerp = sx_xholder(sx);
 1045         if (sx->sx_lock & SX_LOCK_SHARED)
 1046                 db_printf("SLOCK (count %ju)\n",
 1047                     (uintmax_t)SX_SHARERS(sx->sx_lock));
 1048         else
 1049                 db_printf("XLOCK\n");
 1050         return (1);
 1051 }
 1052 #endif

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