nptl/lowlevellock.h

   1 /* Low level locking macros used in NPTL implementation.  Stub version.
   2    Copyright (C) 2002, 2007 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 /* Mutex lock counter:
  25    bit 31 clear means unlocked;
  26    bit 31 set means locked.
  27
  28    All code that looks at bit 31 first increases the 'number of
  29    interested threads' usage counter, which is in bits 0-30.
  30
  31    All negative mutex values indicate that the mutex is still locked.  */
  32
  33
  34 static inline void
  35 __generic_mutex_lock (int *mutex)
  36 {
  37   unsigned int v;
  38
  39   /* Bit 31 was clear, we got the mutex.  (this is the fastpath).  */
  40   if (atomic_bit_test_set (mutex, 31) == 0)
  41     return;
  42
  43   atomic_increment (mutex);
  44
  45   while (1)
  46     {
  47       if (atomic_bit_test_set (mutex, 31) == 0)
  48         {
  49           atomic_decrement (mutex);
  50           return;
  51         }
  52
  53       /* We have to wait now. First make sure the futex value we are
  54          monitoring is truly negative (i.e. locked). */
  55       v = *mutex;
  56       if (v >= 0)
  57         continue;
  58
  59       lll_futex_wait (mutex, v,
  60                       // XYZ check mutex flag
  61                       LLL_SHARED);
  62     }
  63 }
  64
  65
  66 static inline void
  67 __generic_mutex_unlock (int *mutex)
  68 {
  69   /* Adding 0x80000000 to the counter results in 0 if and only if
  70      there are not other interested threads - we can return (this is
  71      the fastpath).  */
  72   if (atomic_add_zero (mutex, 0x80000000))
  73     return;
  74
  75   /* There are other threads waiting for this mutex, wake one of them
  76      up.  */
  77   lll_futex_wake (mutex, 1,
  78                   // XYZ check mutex flag
  79                   LLL_SHARED);
  80 }
  81
  82
  83 #define lll_mutex_lock(futex) __generic_mutex_lock (&(futex))
  84 #define lll_mutex_unlock(futex) __generic_mutex_unlock (&(futex))