libpthread/nptl/sysdeps/m68k/pthread_spin_lock.c

   1 /* pthread_spin_lock -- lock a spin lock.  Generic version.
   2    Copyright (C) 2012-2016 Free Software Foundation, Inc.
   3
   4    The GNU C Library is free software; you can redistribute it and/or
   5    modify it under the terms of the GNU Lesser General Public
   6    License as published by the Free Software Foundation; either
   7    version 2.1 of the License, or (at your option) any later version.
   8
   9    The GNU C Library is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12    Lesser General Public License for more details.
  13
  14    You should have received a copy of the GNU Lesser General Public
  15    License along with the GNU C Library; if not, see
  16    <http://www.gnu.org/licenses/>.  */
  17
  18 #include <atomic.h>
  19 #include "pthreadP.h"
  20
  21 /* A machine-specific version can define SPIN_LOCK_READS_BETWEEN_CMPXCHG
  22   to the number of plain reads that it's optimal to spin on between uses
  23   of atomic_compare_and_exchange_val_acq.  If spinning forever is optimal
  24   then use -1.  If no plain reads here would ever be optimal, use 0.  */
  25 #define SPIN_LOCK_READS_BETWEEN_CMPXCHG 1000
  26
  27 int
  28 pthread_spin_lock (pthread_spinlock_t *lock)
  29 {
  30   /* atomic_exchange usually takes less instructions than
  31      atomic_compare_and_exchange.  On the other hand,
  32      atomic_compare_and_exchange potentially generates less bus traffic
  33      when the lock is locked.
  34      We assume that the first try mostly will be successful, and we use
  35      atomic_exchange.  For the subsequent tries we use
  36      atomic_compare_and_exchange.  */
  37   if (atomic_exchange_acq (lock, 1) == 0)
  38     return 0;
  39
  40   do
  41     {
  42       /* The lock is contended and we need to wait.  Going straight back
  43          to cmpxchg is not a good idea on many targets as that will force
  44          expensive memory synchronizations among processors and penalize other
  45          running threads.
  46          On the other hand, we do want to update memory state on the local core
  47          once in a while to avoid spinning indefinitely until some event that
  48          will happen to update local memory as a side-effect.  */
  49       if (SPIN_LOCK_READS_BETWEEN_CMPXCHG >= 0)
  50         {
  51           int wait = SPIN_LOCK_READS_BETWEEN_CMPXCHG;
  52
  53           while (*lock != 0 && wait > 0)
  54             --wait;
  55         }
  56       else
  57         {
  58           while (*lock != 0)
  59             ;
  60         }
  61     }
  62   while (atomic_compare_and_exchange_val_acq (lock, 1, 0) != 0);
  63
  64   return 0;
  65 }