nptl/lowlevellock.h

   1 /* Low level locking macros used in NPTL implementation.  Stub version.
   2    Copyright (C) 2002-2014 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, see
  18    <http://www.gnu.org/licenses/>.  */
  19
  20 #include <atomic.h>
  21
  22
  23 /* Mutex lock counter:
  24    bit 31 clear means unlocked;
  25    bit 31 set means locked.
  26
  27    All code that looks at bit 31 first increases the 'number of
  28    interested threads' usage counter, which is in bits 0-30.
  29
  30    All negative mutex values indicate that the mutex is still locked.  */
  31
  32
  33 static inline void
  34 __generic_mutex_lock (int *mutex)
  35 {
  36   unsigned int v;
  37
  38   /* Bit 31 was clear, we got the mutex.  (this is the fastpath).  */
  39   if (atomic_bit_test_set (mutex, 31) == 0)
  40     return;
  41
  42   atomic_increment (mutex);
  43
  44   while (1)
  45     {
  46       if (atomic_bit_test_set (mutex, 31) == 0)
  47         {
  48           atomic_decrement (mutex);
  49           return;
  50         }
  51
  52       /* We have to wait now. First make sure the futex value we are
  53          monitoring is truly negative (i.e. locked). */
  54       v = *mutex;
  55       if (v >= 0)
  56         continue;
  57
  58       lll_futex_wait (mutex, v,
  59                       // XYZ check mutex flag
  60                       LLL_SHARED);
  61     }
  62 }
  63
  64
  65 static inline void
  66 __generic_mutex_unlock (int *mutex)
  67 {
  68   /* Adding 0x80000000 to the counter results in 0 if and only if
  69      there are not other interested threads - we can return (this is
  70      the fastpath).  */
  71   if (atomic_add_zero (mutex, 0x80000000))
  72     return;
  73
  74   /* There are other threads waiting for this mutex, wake one of them
  75      up.  */
  76   lll_futex_wake (mutex, 1,
  77                   // XYZ check mutex flag
  78                   LLL_SHARED);
  79 }
  80
  81
  82 #define lll_mutex_lock(futex) __generic_mutex_lock (&(futex))
  83 #define lll_mutex_unlock(futex) __generic_mutex_unlock (&(futex))