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