nptl/pthread_rwlock_init.c

   1 /* Copyright (C) 2002,2007,2009,2011 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 "pthreadP.h"
  21 #include <string.h>
  22 #include <kernel-features.h>
  23
  24
  25 static const struct pthread_rwlockattr default_attr =
  26   {
  27     .lockkind = PTHREAD_RWLOCK_DEFAULT_NP,
  28     .pshared = PTHREAD_PROCESS_PRIVATE
  29   };
  30
  31
  32 int
  33 __pthread_rwlock_init (rwlock, attr)
  34      pthread_rwlock_t *rwlock;
  35      const pthread_rwlockattr_t *attr;
  36 {
  37   const struct pthread_rwlockattr *iattr;
  38
  39   iattr = ((const struct pthread_rwlockattr *) attr) ?: &default_attr;
  40
  41   memset (rwlock, '\0', sizeof (*rwlock));
  42
  43   rwlock->__data.__flags
  44     = iattr->lockkind == PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP;
  45
  46   /* The __SHARED field is computed to minimize the work that needs to
  47      be done while handling the futex.  There are two inputs: the
  48      availability of private futexes and whether the rwlock is shared
  49      or private.  Unfortunately the value of a private rwlock is
  50      fixed: it must be zero.  The PRIVATE_FUTEX flag has the value
  51      0x80 in case private futexes are available and zero otherwise.
  52      This leads to the following table:
  53
  54                  |     pshared     |     result
  55                  | shared  private | shared  private |
  56      ------------+-----------------+-----------------+
  57      !avail 0    |     0       0   |     0       0   |
  58       avail 0x80 |  0x80       0   |     0    0x80   |
  59
  60      If the pshared value is in locking functions XORed with avail
  61      we get the expected result.  */
  62 #ifdef __ASSUME_PRIVATE_FUTEX
  63   rwlock->__data.__shared = (iattr->pshared == PTHREAD_PROCESS_PRIVATE
  64                              ? 0 : FUTEX_PRIVATE_FLAG);
  65 #else
  66   rwlock->__data.__shared = (iattr->pshared == PTHREAD_PROCESS_PRIVATE
  67                              ? 0
  68                              : THREAD_GETMEM (THREAD_SELF,
  69                                               header.private_futex));
  70 #endif
  71
  72   return 0;
  73 }
  74 strong_alias (__pthread_rwlock_init, pthread_rwlock_init)