nptl/sysdeps/generic/lowlevellock.h

   1 /* Copyright (C) 2002 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3    Contributed by Ulrich Drepper <drepper@redhat.com>, 2002.
   4
   5    The GNU C Library is free software; you can redistribute it and/or
   6    modify it under the terms of the GNU Lesser General Public
   7    License as published by the Free Software Foundation; either
   8    version 2.1 of the License, or (at your option) any later version.
   9
  10    The GNU C Library 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 GNU
  13    Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public
  16    License along with the GNU C Library; if not, write to the Free
  17    Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
  18    02111-1307 USA.  */
  19
  20 #include <atomic.h>
  21
  22
  23 /* Implement generic mutex.  Basic futex syscall support is required:
  24
  25      lll_futex_wait(futex, value) - call sys_futex with FUTEX_WAIT
  26                                     and third parameter VALUE
  27
  28      lll_futex_wake(futex, value) - call sys_futex with FUTEX_WAKE
  29                                     and third parameter VALUE
  30 */
  31
  32
  33 /* Mutex lock counter:
  34    bit 31 clear means unlocked;
  35    bit 31 set means locked.
  36
  37    All code that looks at bit 31 first increases the 'number of
  38    interested threads' usage counter, which is in bits 0-30.
  39
  40    All negative mutex values indicate that the mutex is still locked.  */
  41
  42
  43 static inline void
  44 __generic_mutex_lock (int *mutex)
  45 {
  46   unsigned int v;
  47
  48   /* Bit 31 was clear, we got the mutex.  (this is the fastpath).  */
  49   if (atomic_bit_test_set (mutex, 31) == 0)
  50     return;
  51
  52   atomic_increment (mutex);
  53
  54   while (1)
  55     {
  56       if (atomic_bit_test_set (mutex, 31) == 0)
  57         {
  58           atomic_decrement (mutex);
  59           return;
  60         }
  61
  62       /* We have to wait now. First make sure the futex value we are
  63          monitoring is truly negative (i.e. locked). */
  64       v = *mutex;
  65       if (v >= 0)
  66         continue;
  67
  68       lll_futex_wait (mutex, v);
  69     }
  70 }
  71
  72
  73 static inline void
  74 __generic_mutex_unlock (int *mutex)
  75 {
  76   /* Adding 0x80000000 to the counter results in 0 if and only if
  77      there are not other interested threads - we can return (this is
  78      the fastpath).  */
  79   if (atomic_add_zero (0x80000000, mutex))
  80     return;
  81
  82   /* There are other threads waiting for this mutex, wake one of them
  83      up.  */
  84   lll_futex_wake (mutex, 1);
  85 }
  86
  87
  88 #define lll_mutex_lock(futex) __generic_mutex_lock (&(futex))
  89 #define lll_mutex_unlock(futex) __generic_mutex_unlock (&(futex))