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       newstatus = (long) self;
  51     }
  52     if (self != NULL)
  53       THREAD_SETMEM(self, p_nextwaiting, (pthread_descr) oldstatus);
  54   } while(! compare_and_swap(&lock->__status, oldstatus, newstatus,
  55                              &lock->__spinlock));
  56   if (oldstatus != 0) suspend(self);
  57 }
  58
  59 int __pthread_trylock(struct _pthread_fastlock * lock)
  60 {
  61   long oldstatus;
  62
  63   do {
  64     oldstatus = lock->__status;
  65     if (oldstatus != 0) return EBUSY;
  66   } while(! compare_and_swap(&lock->__status, 0, 1, &lock->__spinlock));
  67   return 0;
  68 }
  69
  70 void __pthread_unlock(struct _pthread_fastlock * lock)
  71 {
  72   long oldstatus;
  73   pthread_descr thr, * ptr, * maxptr;
  74   int maxprio;
  75
  76 again:
  77   oldstatus = lock->__status;
  78   if (oldstatus == 1) {
  79     /* No threads are waiting for this lock */
  80     if (! compare_and_swap(&lock->__status, 1, 0, &lock->__spinlock))
  81       goto again;
  82     return;
  83   }
  84   /* Find thread in waiting queue with maximal priority */
  85   ptr = (pthread_descr *) &lock->__status;
  86   thr = (pthread_descr) oldstatus;
  87   maxprio = 0;
  88   maxptr = ptr;
  89   while (thr != (pthread_descr) 1) {
  90     if (thr->p_priority >= maxprio) {
  91       maxptr = ptr;
  92       maxprio = thr->p_priority;
  93     }
  94     ptr = &(thr->p_nextwaiting);
  95     thr = *ptr;
  96   }
  97   /* Remove max prio thread from waiting list. */
  98   if (maxptr == (pthread_descr *) &lock->__status) {
  99     /* If max prio thread is at head, remove it with compare-and-swap
 100        to guard against concurrent lock operation */
 101     thr = (pthread_descr) oldstatus;
 102     if (! compare_and_swap(&lock->__status,
 103                            oldstatus, (long)(thr->p_nextwaiting),
 104                            &lock->__spinlock))
 105       goto again;
 106   } else {
 107     /* No risk of concurrent access, remove max prio thread normally */
 108     thr = *maxptr;
 109     *maxptr = thr->p_nextwaiting;
 110   }
 111   /* Wake up the selected waiting thread */
 112   thr->p_nextwaiting = NULL;
 113   restart(thr);
 114 }
 115
 116 /* Compare-and-swap emulation with a spinlock */
 117
 118 #ifdef TEST_FOR_COMPARE_AND_SWAP
 119 int __pthread_has_cas = 0;
 120 #endif
 121
 122 #if !defined HAS_COMPARE_AND_SWAP || defined TEST_FOR_COMPARE_AND_SWAP
 123
 124 static void __pthread_acquire(int * spinlock);
 125
 126 int __pthread_compare_and_swap(long * ptr, long oldval, long newval,
 127                                int * spinlock)
 128 {
 129   int res;
 130   if (testandset(spinlock)) __pthread_acquire(spinlock);
 131   if (*ptr == oldval) {
 132     *ptr = newval; res = 1;
 133   } else {
 134     res = 0;
 135   }
 136   *spinlock = 0;
 137   return res;
 138 }
 139
 140 /* This function is called if the inlined test-and-set
 141    in __pthread_compare_and_swap() failed */
 142
 143 /* The retry strategy is as follows:
 144    - We test and set the spinlock MAX_SPIN_COUNT times, calling
 145      sched_yield() each time.  This gives ample opportunity for other
 146      threads with priority >= our priority to make progress and
 147      release the spinlock.
 148    - If a thread with priority < our priority owns the spinlock,
 149      calling sched_yield() repeatedly is useless, since we're preventing
 150      the owning thread from making progress and releasing the spinlock.
 151      So, after MAX_SPIN_LOCK attemps, we suspend the calling thread
 152      using nanosleep().  This again should give time to the owning thread
 153      for releasing the spinlock.
 154      Notice that the nanosleep() interval must not be too small,
 155      since the kernel does busy-waiting for short intervals in a realtime
 156      process (!).  The smallest duration that guarantees thread
 157      suspension is currently 2ms.
 158    - When nanosleep() returns, we try again, doing MAX_SPIN_COUNT
 159      sched_yield(), then sleeping again if needed. */
 160
 161 static void __pthread_acquire(int * spinlock)
 162 {
 163   int cnt = 0;
 164   struct timespec tm;
 165
 166   while (testandset(spinlock)) {
 167     if (cnt < MAX_SPIN_COUNT) {
 168       sched_yield();
 169       cnt++;
 170     } else {
 171       tm.tv_sec = 0;
 172       tm.tv_nsec = SPIN_SLEEP_DURATION;
 173       nanosleep(&tm, NULL);
 174       cnt = 0;
 175     }
 176   }
 177 }
 178
 179 #endif