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_nextwaiting 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       THREAD_SETMEM(self, p_nextwaiting, (pthread_descr) oldstatus);
  55   } while(! compare_and_swap(&lock->__status, oldstatus, newstatus,
  56                              &lock->__spinlock));
  57   if (oldstatus != 0) suspend(self);
  58 }
  59
  60 void internal_function __pthread_unlock(struct _pthread_fastlock * lock)
  61 {
  62   long oldstatus;
  63   pthread_descr thr, * ptr, * maxptr;
  64   int maxprio;
  65
  66 again:
  67   oldstatus = lock->__status;
  68   if (oldstatus == 0 || oldstatus == 1) {
  69     /* No threads are waiting for this lock.  Please note that we also
  70        enter this case if the lock is not taken at all.  If this wouldn't
  71        be done here we would crash further down.  */
  72     if (! compare_and_swap(&lock->__status, oldstatus, 0, &lock->__spinlock))
  73       goto again;
  74     return;
  75   }
  76   /* Find thread in waiting queue with maximal priority */
  77   ptr = (pthread_descr *) &lock->__status;
  78   thr = (pthread_descr) oldstatus;
  79   maxprio = 0;
  80   maxptr = ptr;
  81   while (thr != (pthread_descr) 1) {
  82     if (thr->p_priority >= maxprio) {
  83       maxptr = ptr;
  84       maxprio = thr->p_priority;
  85     }
  86     ptr = &(thr->p_nextwaiting);
  87     thr = *ptr;
  88   }
  89   /* Remove max prio thread from waiting list. */
  90   if (maxptr == (pthread_descr *) &lock->__status) {
  91     /* If max prio thread is at head, remove it with compare-and-swap
  92        to guard against concurrent lock operation */
  93     thr = (pthread_descr) oldstatus;
  94     if (! compare_and_swap(&lock->__status,
  95                            oldstatus, (long)(thr->p_nextwaiting),
  96                            &lock->__spinlock))
  97       goto again;
  98   } else {
  99     /* No risk of concurrent access, remove max prio thread normally */
 100     thr = *maxptr;
 101     *maxptr = thr->p_nextwaiting;
 102   }
 103   /* Wake up the selected waiting thread */
 104   thr->p_nextwaiting = NULL;
 105   restart(thr);
 106 }
 107
 108 /* Compare-and-swap emulation with a spinlock */
 109
 110 #ifdef TEST_FOR_COMPARE_AND_SWAP
 111 int __pthread_has_cas = 0;
 112 #endif
 113
 114 #if !defined HAS_COMPARE_AND_SWAP || defined TEST_FOR_COMPARE_AND_SWAP
 115
 116 static void __pthread_acquire(int * spinlock);
 117
 118 int __pthread_compare_and_swap(long * ptr, long oldval, long newval,
 119                                int * spinlock)
 120 {
 121   int res;
 122   if (testandset(spinlock)) __pthread_acquire(spinlock);
 123   if (*ptr == oldval) {
 124     *ptr = newval; res = 1;
 125   } else {
 126     res = 0;
 127   }
 128   *spinlock = 0;
 129   return res;
 130 }
 131
 132 /* This function is called if the inlined test-and-set
 133    in __pthread_compare_and_swap() failed */
 134
 135 /* The retry strategy is as follows:
 136    - We test and set the spinlock MAX_SPIN_COUNT times, calling
 137      sched_yield() each time.  This gives ample opportunity for other
 138      threads with priority >= our priority to make progress and
 139      release the spinlock.
 140    - If a thread with priority < our priority owns the spinlock,
 141      calling sched_yield() repeatedly is useless, since we're preventing
 142      the owning thread from making progress and releasing the spinlock.
 143      So, after MAX_SPIN_LOCK attemps, we suspend the calling thread
 144      using nanosleep().  This again should give time to the owning thread
 145      for releasing the spinlock.
 146      Notice that the nanosleep() interval must not be too small,
 147      since the kernel does busy-waiting for short intervals in a realtime
 148      process (!).  The smallest duration that guarantees thread
 149      suspension is currently 2ms.
 150    - When nanosleep() returns, we try again, doing MAX_SPIN_COUNT
 151      sched_yield(), then sleeping again if needed. */
 152
 153 static void __pthread_acquire(int * spinlock)
 154 {
 155   int cnt = 0;
 156   struct timespec tm;
 157
 158   while (testandset(spinlock)) {
 159     if (cnt < MAX_SPIN_COUNT) {
 160       sched_yield();
 161       cnt++;
 162     } else {
 163       tm.tv_sec = 0;
 164       tm.tv_nsec = SPIN_SLEEP_DURATION;
 165       nanosleep(&tm, NULL);
 166       cnt = 0;
 167     }
 168   }
 169 }
 170
 171 #endif