nptl/pthread_rwlock_init.c

   1 /* Copyright (C) 2002, 2007 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 <kernel-features.h>
  22
  23
  24 static const struct pthread_rwlockattr default_attr =
  25   {
  26     .lockkind = PTHREAD_RWLOCK_DEFAULT_NP,
  27     .pshared = PTHREAD_PROCESS_PRIVATE
  28   };
  29
  30
  31 int
  32 __pthread_rwlock_init (rwlock, attr)
  33      pthread_rwlock_t *rwlock;
  34      const pthread_rwlockattr_t *attr;
  35 {
  36   const struct pthread_rwlockattr *iattr;
  37
  38   iattr = ((const struct pthread_rwlockattr *) attr) ?: &default_attr;
  39
  40   rwlock->__data.__lock = 0;
  41   rwlock->__data.__nr_readers = 0;
  42   rwlock->__data.__readers_wakeup = 0;
  43   rwlock->__data.__writer_wakeup = 0;
  44   rwlock->__data.__nr_readers_queued = 0;
  45   rwlock->__data.__nr_writers_queued = 0;
  46   rwlock->__data.__writer = 0;
  47
  48   rwlock->__data.__flags
  49     = iattr->lockkind == PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP;
  50
  51   /* The __SHARED field is computed to minimize the work that needs to
  52      be done while handling the futex.  There are two inputs: the
  53      availability of private futexes and whether the rwlock is shared
  54      or private.  Unfortunately the value of a private rwlock is
  55      fixed: it must be zero.  The PRIVATE_FUTEX flag has the value
  56      0x80 in case private futexes are available and zero otherwise.
  57      This leads to the following table:
  58
  59                  |     pshared     |     result
  60                  | shared  private | shared  private |
  61      ------------+-----------------+-----------------+
  62      !avail 0    |     0       0   |     0       0   |
  63       avail 0x80 |  0x80       0   |     0    0x80   |
  64
  65      If the pshared value is in locking functions XORed with avail
  66      we get the expected result.  */
  67 #ifdef __ASSUME_PRIVATE_FUTEX
  68   rwlock->__data.__shared = (iattr->pshared == PTHREAD_PROCESS_PRIVATE
  69                              ? 0 : FUTEX_PRIVATE_FLAG);
  70 #else
  71   rwlock->__data.__shared = (iattr->pshared == PTHREAD_PROCESS_PRIVATE
  72                              ? 0
  73                              : THREAD_GETMEM (THREAD_SELF,
  74                                               header.private_futex));
  75 #endif
  76
  77   rwlock->__data.__pad1 = 0;
  78   rwlock->__data.__pad2 = 0;
  79
  80   return 0;
  81 }
  82 strong_alias (__pthread_rwlock_init, pthread_rwlock_init)