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 <sched.h>
  18 #include <time.h>
  19 #include "pthread.h"
  20 #include "internals.h"
  21 #include "spinlock.h"
  22 #include "restart.h"
  23
  24 /* The status field of a fastlock has the following meaning:
  25      0: fastlock is free
  26      1: fastlock is taken, no thread is waiting on it
  27   ADDR: fastlock is taken, ADDR is address of thread descriptor for
  28         first waiting thread, other waiting threads are linked via
  29         their p_nextwaiting field.
  30    The waiting list is not sorted by priority order.
  31    Actually, we always insert at top of list (sole insertion mode
  32    that can be performed without locking).
  33    For __pthread_unlock, we perform a linear search in the list
  34    to find the highest-priority, oldest waiting thread.
  35    This is safe because there are no concurrent __pthread_unlock
  36    operations -- only the thread that locked the mutex can unlock it. */
  37
  38 void __pthread_lock(struct _pthread_fastlock * lock)
  39 {
  40   long oldstatus, newstatus;
  41   pthread_descr self = NULL;
  42
  43   do {
  44     oldstatus = lock->status;
  45     if (oldstatus == 0) {
  46       newstatus = 1;
  47     } else {
  48       self = thread_self();
  49       self->p_nextwaiting = (pthread_descr) oldstatus;
  50       newstatus = (long) self;
  51     }
  52   } while(! compare_and_swap(&lock->status, oldstatus, newstatus,
  53                              &lock->spinlock));
  54   if (oldstatus != 0) suspend(self);
  55 }
  56
  57 int __pthread_trylock(struct _pthread_fastlock * lock)
  58 {
  59   long oldstatus;
  60
  61   do {
  62     oldstatus = lock->status;
  63     if (oldstatus != 0) return EBUSY;
  64   } while(! compare_and_swap(&lock->status, 0, 1, &lock->spinlock));
  65   return 0;
  66 }
  67
  68 void __pthread_unlock(struct _pthread_fastlock * lock)
  69 {
  70   long oldstatus;
  71   pthread_descr thr, * ptr, * maxptr;
  72   int maxprio;
  73
  74 again:
  75   oldstatus = lock->status;
  76   if (oldstatus == 1) {
  77     /* No threads are waiting for this lock */
  78     if (! compare_and_swap(&lock->status, 1, 0, &lock->spinlock)) goto again;
  79     return;
  80   }
  81   /* Find thread in waiting queue with maximal priority */
  82   ptr = (pthread_descr *) &lock->status;
  83   thr = (pthread_descr) oldstatus;
  84   maxprio = 0;
  85   maxptr = ptr;
  86   while (thr != (pthread_descr) 1) {
  87     if (thr->p_priority >= maxprio) {
  88       maxptr = ptr;
  89       maxprio = thr->p_priority;
  90     }
  91     ptr = &(thr->p_nextwaiting);
  92     thr = *ptr;
  93   }
  94   /* Remove max prio thread from waiting list. */
  95   if (maxptr == (pthread_descr *) &lock->status) {
  96     /* If max prio thread is at head, remove it with compare-and-swap
  97        to guard against concurrent lock operation */
  98     thr = (pthread_descr) oldstatus;
  99     if (! compare_and_swap(&lock->status,
 100                            oldstatus, (long)(thr->p_nextwaiting),
 101                            &lock->spinlock))
 102       goto again;
 103   } else {
 104     /* No risk of concurrent access, remove max prio thread normally */
 105     thr = *maxptr;
 106     *maxptr = thr->p_nextwaiting;
 107   }
 108   /* Wake up the selected waiting thread */
 109   thr->p_nextwaiting = NULL;
 110   restart(thr);
 111 }
 112
 113 /* Compare-and-swap emulation with a spinlock */
 114
 115 #ifdef TEST_FOR_COMPARE_AND_SWAP
 116 int __pthread_has_cas = 0;
 117 #endif
 118
 119 #if !defined HAS_COMPARE_AND_SWAP || defined TEST_FOR_COMPARE_AND_SWAP
 120
 121 static void __pthread_acquire(int * spinlock);
 122
 123 int __pthread_compare_and_swap(long * ptr, long oldval, long newval,
 124                                int * spinlock)
 125 {
 126   int res;
 127   if (testandset(spinlock)) __pthread_acquire(spinlock);
 128   if (*ptr == oldval) {
 129     *ptr = newval; res = 1;
 130   } else {
 131     res = 0;
 132   }
 133   *spinlock = 0;
 134   return res;
 135 }
 136
 137 /* This function is called if the inlined test-and-set
 138    in __pthread_compare_and_swap() failed */
 139
 140 /* The retry strategy is as follows:
 141    - We test and set the spinlock MAX_SPIN_COUNT times, calling
 142      sched_yield() each time.  This gives ample opportunity for other
 143      threads with priority >= our priority to make progress and
 144      release the spinlock.
 145    - If a thread with priority < our priority owns the spinlock,
 146      calling sched_yield() repeatedly is useless, since we're preventing
 147      the owning thread from making progress and releasing the spinlock.
 148      So, after MAX_SPIN_LOCK attemps, we suspend the calling thread
 149      using nanosleep().  This again should give time to the owning thread
 150      for releasing the spinlock.
 151      Notice that the nanosleep() interval must not be too small,
 152      since the kernel does busy-waiting for short intervals in a realtime
 153      process (!).  The smallest duration that guarantees thread
 154      suspension is currently 2ms.
 155    - When nanosleep() returns, we try again, doing MAX_SPIN_COUNT
 156      sched_yield(), then sleeping again if needed. */
 157
 158 static void __pthread_acquire(int * spinlock)
 159 {
 160   int cnt = 0;
 161   struct timespec tm;
 162
 163   while (testandset(spinlock)) {
 164     if (cnt < MAX_SPIN_COUNT) {
 165       sched_yield();
 166       cnt++;
 167     } else {
 168       tm.tv_sec = 0;
 169       tm.tv_nsec = SPIN_SLEEP_DURATION;
 170       nanosleep(&tm, NULL);
 171       cnt = 0;
 172     }
 173   }
 174 }
 175
 176 #endif