linuxthreads/spinlock.c

   1 /* Linuxthreads - a simple clone()-based implementation of Posix        */
   2 /* threads for Linux.                                                   */
   3 /* Copyright (C) 1998 Xavier Leroy (Xavier.Leroy@inria.fr)              */
   4 /*                                                                      */
   5 /* This program is free software; you can redistribute it and/or        */
   6 /* modify it under the terms of the GNU Library General Public License  */
   7 /* as published by the Free Software Foundation; either version 2       */
   8 /* of the License, or (at your option) any later version.               */
   9 /*                                                                      */
  10 /* This program is distributed in the hope that it will be useful,      */
  11 /* but WITHOUT ANY WARRANTY; without even the implied warranty of       */
  12 /* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the        */
  13 /* GNU Library General Public License for more details.                 */
  14
  15 /* Internal locks */
  16
  17 #include <errno.h>
  18 #include <sched.h>
  19 #include <time.h>
  20 #include "pthread.h"
  21 #include "internals.h"
  22 #include "spinlock.h"
  23 #include "restart.h"
  24
  25 /* The status field of a fastlock has the following meaning:
  26      0: fastlock is free
  27      1: fastlock is taken, no thread is waiting on it
  28   ADDR: fastlock is taken, ADDR is address of thread descriptor for
  29         first waiting thread, other waiting threads are linked via
  30         their p_nextlock field.
  31    The waiting list is not sorted by priority order.
  32    Actually, we always insert at top of list (sole insertion mode
  33    that can be performed without locking).
  34    For __pthread_unlock, we perform a linear search in the list
  35    to find the highest-priority, oldest waiting thread.
  36    This is safe because there are no concurrent __pthread_unlock
  37    operations -- only the thread that locked the mutex can unlock it. */
  38
  39 void internal_function __pthread_lock(struct _pthread_fastlock * lock,
  40                                       pthread_descr self)
  41 {
  42   long oldstatus, newstatus;
  43
  44   do {
  45     oldstatus = lock->__status;
  46     if (oldstatus == 0) {
  47       newstatus = 1;
  48     } else {
  49       if (self == NULL)
  50         self = thread_self();
  51       newstatus = (long) self;
  52     }
  53     if (self != NULL) {
  54       ASSERT(self->p_nextlock == NULL);
  55       THREAD_SETMEM(self, p_nextlock, (pthread_descr) oldstatus);
  56     }
  57   } while(! compare_and_swap(&lock->__status, oldstatus, newstatus,
  58                              &lock->__spinlock));
  59   if (oldstatus != 0) suspend(self);
  60 }
  61
  62 void internal_function __pthread_unlock(struct _pthread_fastlock * lock)
  63 {
  64   long oldstatus;
  65   pthread_descr thr, * ptr, * maxptr;
  66   int maxprio;
  67
  68 again:
  69   oldstatus = lock->__status;
  70   if (oldstatus == 0 || oldstatus == 1) {
  71     /* No threads are waiting for this lock.  Please note that we also
  72        enter this case if the lock is not taken at all.  If this wouldn't
  73        be done here we would crash further down.  */
  74     if (! compare_and_swap(&lock->__status, oldstatus, 0, &lock->__spinlock))
  75       goto again;
  76     return;
  77   }
  78   /* Find thread in waiting queue with maximal priority */
  79   ptr = (pthread_descr *) &lock->__status;
  80   thr = (pthread_descr) oldstatus;
  81   maxprio = 0;
  82   maxptr = ptr;
  83   while (thr != (pthread_descr) 1) {
  84     if (thr->p_priority >= maxprio) {
  85       maxptr = ptr;
  86       maxprio = thr->p_priority;
  87     }
  88     ptr = &(thr->p_nextlock);
  89     thr = *ptr;
  90   }
  91   /* Remove max prio thread from waiting list. */
  92   if (maxptr == (pthread_descr *) &lock->__status) {
  93     /* If max prio thread is at head, remove it with compare-and-swap
  94        to guard against concurrent lock operation */
  95     thr = (pthread_descr) oldstatus;
  96     if (! compare_and_swap(&lock->__status,
  97                            oldstatus, (long)(thr->p_nextlock),
  98                            &lock->__spinlock))
  99       goto again;
 100   } else {
 101     /* No risk of concurrent access, remove max prio thread normally */
 102     thr = *maxptr;
 103     *maxptr = thr->p_nextlock;
 104   }
 105   /* Wake up the selected waiting thread */
 106   thr->p_nextlock = NULL;
 107   restart(thr);
 108 }
 109
 110 /* Compare-and-swap emulation with a spinlock */
 111
 112 #ifdef TEST_FOR_COMPARE_AND_SWAP
 113 int __pthread_has_cas = 0;
 114 #endif
 115
 116 #if !defined HAS_COMPARE_AND_SWAP || defined TEST_FOR_COMPARE_AND_SWAP
 117
 118 static void __pthread_acquire(int * spinlock);
 119
 120 int __pthread_compare_and_swap(long * ptr, long oldval, long newval,
 121                                int * spinlock)
 122 {
 123   int res;
 124   if (testandset(spinlock)) __pthread_acquire(spinlock);
 125   if (*ptr == oldval) {
 126     *ptr = newval; res = 1;
 127   } else {
 128     res = 0;
 129   }
 130   *spinlock = 0;
 131   return res;
 132 }
 133
 134 /* This function is called if the inlined test-and-set
 135    in __pthread_compare_and_swap() failed */
 136
 137 /* The retry strategy is as follows:
 138    - We test and set the spinlock MAX_SPIN_COUNT times, calling
 139      sched_yield() each time.  This gives ample opportunity for other
 140      threads with priority >= our priority to make progress and
 141      release the spinlock.
 142    - If a thread with priority < our priority owns the spinlock,
 143      calling sched_yield() repeatedly is useless, since we're preventing
 144      the owning thread from making progress and releasing the spinlock.
 145      So, after MAX_SPIN_LOCK attemps, we suspend the calling thread
 146      using nanosleep().  This again should give time to the owning thread
 147      for releasing the spinlock.
 148      Notice that the nanosleep() interval must not be too small,
 149      since the kernel does busy-waiting for short intervals in a realtime
 150      process (!).  The smallest duration that guarantees thread
 151      suspension is currently 2ms.
 152    - When nanosleep() returns, we try again, doing MAX_SPIN_COUNT
 153      sched_yield(), then sleeping again if needed. */
 154
 155 static void __pthread_acquire(int * spinlock)
 156 {
 157   int cnt = 0;
 158   struct timespec tm;
 159
 160   while (testandset(spinlock)) {
 161     if (cnt < MAX_SPIN_COUNT) {
 162       sched_yield();
 163       cnt++;
 164     } else {
 165       tm.tv_sec = 0;
 166       tm.tv_nsec = SPIN_SLEEP_DURATION;
 167       nanosleep(&tm, NULL);
 168       cnt = 0;
 169     }
 170   }
 171 }
 172
 173 #endif