sysdeps/nptl/bits/struct_mutex.h

   1 /* Default mutex implementation struct definitions.
   2    Copyright (C) 2019-2022 Free Software Foundation, Inc.
   3    This file is part of the GNU C Library.
   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, see
  17    <http://www.gnu.org/licenses/>.  */
  18
  19 #ifndef _THREAD_MUTEX_INTERNAL_H
  20 #define _THREAD_MUTEX_INTERNAL_H 1
  21
  22 /* Generic struct for both POSIX and C11 mutexes.  New ports are expected
  23    to use the default layout, however architecture can redefine it to
  24    add arch-specific extension (such as lock-elision).  The struct have
  25    a size of 32 bytes on LP32 and 40 bytes on LP64 architectures.  */
  26
  27 struct __pthread_mutex_s
  28 {
  29   int __lock __LOCK_ALIGNMENT;
  30   unsigned int __count;
  31   int __owner;
  32 #if __WORDSIZE == 64
  33   unsigned int __nusers;
  34 #endif
  35   /* KIND must stay at this position in the structure to maintain
  36      binary compatibility with static initializers.
  37
  38      Concurrency notes:
  39      The __kind of a mutex is initialized either by the static
  40      PTHREAD_MUTEX_INITIALIZER or by a call to pthread_mutex_init.
  41
  42      After a mutex has been initialized, the __kind of a mutex is usually not
  43      changed.  BUT it can be set to -1 in pthread_mutex_destroy or elision can
  44      be enabled.  This is done concurrently in the pthread_mutex_*lock
  45      functions by using the macro FORCE_ELISION. This macro is only defined
  46      for architectures which supports lock elision.
  47
  48      For elision, there are the flags PTHREAD_MUTEX_ELISION_NP and
  49      PTHREAD_MUTEX_NO_ELISION_NP which can be set in addition to the already
  50      set type of a mutex.  Before a mutex is initialized, only
  51      PTHREAD_MUTEX_NO_ELISION_NP can be set with pthread_mutexattr_settype.
  52
  53      After a mutex has been initialized, the functions pthread_mutex_*lock can
  54      enable elision - if the mutex-type and the machine supports it - by
  55      setting the flag PTHREAD_MUTEX_ELISION_NP. This is done concurrently.
  56      Afterwards the lock / unlock functions are using specific elision
  57      code-paths.  */
  58   int __kind;
  59 #if __WORDSIZE != 64
  60   unsigned int __nusers;
  61 #endif
  62 #if __WORDSIZE == 64
  63   int __spins;
  64   __pthread_list_t __list;
  65 # define __PTHREAD_MUTEX_HAVE_PREV      1
  66 #else
  67   __extension__ union
  68   {
  69     int __spins;
  70     __pthread_slist_t __list;
  71   };
  72 # define __PTHREAD_MUTEX_HAVE_PREV      0
  73 #endif
  74 };
  75
  76 #if __PTHREAD_MUTEX_HAVE_PREV == 1
  77 # define __PTHREAD_MUTEX_INITIALIZER(__kind) \
  78   0, 0, 0, 0, __kind, 0, { 0, 0 }
  79 #else
  80 # define __PTHREAD_MUTEX_INITIALIZER(__kind) \
  81   0, 0, 0, __kind, 0, { 0 }
  82 #endif
  83
  84 #endif