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 __pthread_lock(struct _pthread_fastlock * lock)
  40 {
  41   long oldstatus, newstatus;
  42   pthread_descr self = NULL;
  43
  44   do {
  45     oldstatus = lock->status;
  46     if (oldstatus == 0) {
  47       newstatus = 1;
  48     } else {
  49       self = thread_self();
  50       self->p_nextwaiting = (pthread_descr) oldstatus;
  51       newstatus = (long) self;
  52     }
  53   } while(! compare_and_swap(&lock->status, oldstatus, newstatus,
  54                              &lock->spinlock));
  55   if (oldstatus != 0) suspend(self);
  56 }
  57
  58 int __pthread_trylock(struct _pthread_fastlock * lock)
  59 {
  60   long oldstatus;
  61
  62   do {
  63     oldstatus = lock->status;
  64     if (oldstatus != 0) return EBUSY;
  65   } while(! compare_and_swap(&lock->status, 0, 1, &lock->spinlock));
  66   return 0;
  67 }
  68
  69 void __pthread_unlock(struct _pthread_fastlock * lock)
  70 {
  71   long oldstatus;
  72   pthread_descr thr, * ptr, * maxptr;
  73   int maxprio;
  74
  75 again:
  76   oldstatus = lock->status;
  77   if (oldstatus == 1) {
  78     /* No threads are waiting for this lock */
  79     if (! compare_and_swap(&lock->status, 1, 0, &lock->spinlock)) goto again;
  80     return;
  81   }
  82   /* Find thread in waiting queue with maximal priority */
  83   ptr = (pthread_descr *) &lock->status;
  84   thr = (pthread_descr) oldstatus;
  85   maxprio = 0;
  86   maxptr = ptr;
  87   while (thr != (pthread_descr) 1) {
  88     if (thr->p_priority >= maxprio) {
  89       maxptr = ptr;
  90       maxprio = thr->p_priority;
  91     }
  92     ptr = &(thr->p_nextwaiting);
  93     thr = *ptr;
  94   }
  95   /* Remove max prio thread from waiting list. */
  96   if (maxptr == (pthread_descr *) &lock->status) {
  97     /* If max prio thread is at head, remove it with compare-and-swap
  98        to guard against concurrent lock operation */
  99     thr = (pthread_descr) oldstatus;
 100     if (! compare_and_swap(&lock->status,
 101                            oldstatus, (long)(thr->p_nextwaiting),
 102                            &lock->spinlock))
 103       goto again;
 104   } else {
 105     /* No risk of concurrent access, remove max prio thread normally */
 106     thr = *maxptr;
 107     *maxptr = thr->p_nextwaiting;
 108   }
 109   /* Wake up the selected waiting thread */
 110   thr->p_nextwaiting = NULL;
 111   restart(thr);
 112 }
 113
 114 /* Compare-and-swap emulation with a spinlock */
 115
 116 #ifdef TEST_FOR_COMPARE_AND_SWAP
 117 int __pthread_has_cas = 0;
 118 #endif
 119
 120 #if !defined HAS_COMPARE_AND_SWAP || defined TEST_FOR_COMPARE_AND_SWAP
 121
 122 static void __pthread_acquire(int * spinlock);
 123
 124 int __pthread_compare_and_swap(long * ptr, long oldval, long newval,
 125                                int * spinlock)
 126 {
 127   int res;
 128   if (testandset(spinlock)) __pthread_acquire(spinlock);
 129   if (*ptr == oldval) {
 130     *ptr = newval; res = 1;
 131   } else {
 132     res = 0;
 133   }
 134   *spinlock = 0;
 135   return res;
 136 }
 137
 138 /* This function is called if the inlined test-and-set
 139    in __pthread_compare_and_swap() failed */
 140
 141 /* The retry strategy is as follows:
 142    - We test and set the spinlock MAX_SPIN_COUNT times, calling
 143      sched_yield() each time.  This gives ample opportunity for other
 144      threads with priority >= our priority to make progress and
 145      release the spinlock.
 146    - If a thread with priority < our priority owns the spinlock,
 147      calling sched_yield() repeatedly is useless, since we're preventing
 148      the owning thread from making progress and releasing the spinlock.
 149      So, after MAX_SPIN_LOCK attemps, we suspend the calling thread
 150      using nanosleep().  This again should give time to the owning thread
 151      for releasing the spinlock.
 152      Notice that the nanosleep() interval must not be too small,
 153      since the kernel does busy-waiting for short intervals in a realtime
 154      process (!).  The smallest duration that guarantees thread
 155      suspension is currently 2ms.
 156    - When nanosleep() returns, we try again, doing MAX_SPIN_COUNT
 157      sched_yield(), then sleeping again if needed. */
 158
 159 static void __pthread_acquire(int * spinlock)
 160 {
 161   int cnt = 0;
 162   struct timespec tm;
 163
 164   while (testandset(spinlock)) {
 165     if (cnt < MAX_SPIN_COUNT) {
 166       sched_yield();
 167       cnt++;
 168     } else {
 169       tm.tv_sec = 0;
 170       tm.tv_nsec = SPIN_SLEEP_DURATION;
 171       nanosleep(&tm, NULL);
 172       cnt = 0;
 173     }
 174   }
 175 }
 176
 177 #endif