nptl/lowlevellock.h

   1 /* Low level locking macros used in NPTL implementation.  Stub version.
   2    Copyright (C) 2002 Free Software Foundation, Inc.
   3    This file is part of the GNU C Library.
   4    Contributed by Ulrich Drepper <drepper@redhat.com>, 2002.
   5
   6    The GNU C Library is free software; you can redistribute it and/or
   7    modify it under the terms of the GNU Lesser General Public
   8    License as published by the Free Software Foundation; either
   9    version 2.1 of the License, or (at your option) any later version.
  10
  11    The GNU C Library is distributed in the hope that it will be useful,
  12    but WITHOUT ANY WARRANTY; without even the implied warranty of
  13    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  14    Lesser General Public License for more details.
  15
  16    You should have received a copy of the GNU Lesser General Public
  17    License along with the GNU C Library; if not, write to the Free
  18    Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
  19    02111-1307 USA.  */
  20
  21 #include <atomic.h>
  22
  23
  24 /* Implement generic mutex.  Basic futex syscall support is required:
  25
  26      lll_futex_wait(futex, value) - call sys_futex with FUTEX_WAIT
  27                                     and third parameter VALUE
  28
  29      lll_futex_wake(futex, value) - call sys_futex with FUTEX_WAKE
  30                                     and third parameter VALUE
  31 */
  32
  33
  34 /* Mutex lock counter:
  35    bit 31 clear means unlocked;
  36    bit 31 set means locked.
  37
  38    All code that looks at bit 31 first increases the 'number of
  39    interested threads' usage counter, which is in bits 0-30.
  40
  41    All negative mutex values indicate that the mutex is still locked.  */
  42
  43
  44 static inline void
  45 __generic_mutex_lock (int *mutex)
  46 {
  47   unsigned int v;
  48
  49   /* Bit 31 was clear, we got the mutex.  (this is the fastpath).  */
  50   if (atomic_bit_test_set (mutex, 31) == 0)
  51     return;
  52
  53   atomic_increment (mutex);
  54
  55   while (1)
  56     {
  57       if (atomic_bit_test_set (mutex, 31) == 0)
  58         {
  59           atomic_decrement (mutex);
  60           return;
  61         }
  62
  63       /* We have to wait now. First make sure the futex value we are
  64          monitoring is truly negative (i.e. locked). */
  65       v = *mutex;
  66       if (v >= 0)
  67         continue;
  68
  69       lll_futex_wait (mutex, v);
  70     }
  71 }
  72
  73
  74 static inline void
  75 __generic_mutex_unlock (int *mutex)
  76 {
  77   /* Adding 0x80000000 to the counter results in 0 if and only if
  78      there are not other interested threads - we can return (this is
  79      the fastpath).  */
  80   if (atomic_add_zero (mutex, 0x80000000))
  81     return;
  82
  83   /* There are other threads waiting for this mutex, wake one of them
  84      up.  */
  85   lll_futex_wake (mutex, 1);
  86 }
  87
  88
  89 #define lll_mutex_lock(futex) __generic_mutex_lock (&(futex))
  90 #define lll_mutex_unlock(futex) __generic_mutex_unlock (&(futex))