sysdeps/htl/pt-rwlock-timedrdlock.c

   1 /* Acquire a rwlock for reading.  Generic version.
   2    Copyright (C) 2002-2019 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    <https://www.gnu.org/licenses/>.  */
  18
  19 #include <pthread.h>
  20 #include <assert.h>
  21
  22 #include <pt-internal.h>
  23
  24 /* Acquire the rwlock *RWLOCK for reading blocking until *ABSTIME if
  25    it is already held.  As a GNU extension, if TIMESPEC is NULL then
  26    wait forever.  */
  27 int
  28 __pthread_rwlock_timedrdlock_internal (struct __pthread_rwlock *rwlock,
  29                                        const struct timespec *abstime)
  30 {
  31   error_t err;
  32   int drain;
  33   struct __pthread *self;
  34
  35   __pthread_spin_lock (&rwlock->__lock);
  36   if (__pthread_spin_trylock (&rwlock->__held) == 0)
  37     /* Successfully acquired the lock.  */
  38     {
  39       assert (rwlock->__readerqueue == 0);
  40       assert (rwlock->__writerqueue == 0);
  41       assert (rwlock->__readers == 0);
  42
  43       rwlock->__readers = 1;
  44       __pthread_spin_unlock (&rwlock->__lock);
  45       return 0;
  46     }
  47   else
  48     /* Lock is held, but is held by a reader?  */
  49   if (rwlock->__readers > 0)
  50     /* Just add ourself to number of readers.  */
  51     {
  52       assert (rwlock->__readerqueue == 0);
  53       rwlock->__readers++;
  54       __pthread_spin_unlock (&rwlock->__lock);
  55       return 0;
  56     }
  57
  58   /* The lock is busy.  */
  59
  60   /* Better be blocked by a writer.  */
  61   assert (rwlock->__readers == 0);
  62
  63   if (abstime != NULL && (abstime->tv_nsec < 0 || abstime->tv_nsec >= 1000000000))
  64     return EINVAL;
  65
  66   self = _pthread_self ();
  67
  68   /* Add ourself to the queue.  */
  69   __pthread_enqueue (&rwlock->__readerqueue, self);
  70   __pthread_spin_unlock (&rwlock->__lock);
  71
  72   /* Block the thread.  */
  73   if (abstime != NULL)
  74     err = __pthread_timedblock (self, abstime, CLOCK_REALTIME);
  75   else
  76     {
  77       err = 0;
  78       __pthread_block (self);
  79     }
  80
  81   __pthread_spin_lock (&rwlock->__lock);
  82   if (self->prevp == NULL)
  83     /* Another thread removed us from the queue, which means a wakeup message
  84        has been sent.  It was either consumed while we were blocking, or
  85        queued after we timed out and before we acquired the rwlock lock, in
  86        which case the message queue must be drained.  */
  87     drain = err ? 1 : 0;
  88   else
  89     {
  90       /* We're still in the queue.  Noone attempted to wake us up, i.e. we
  91          timed out.  */
  92       __pthread_dequeue (self);
  93       drain = 0;
  94     }
  95   __pthread_spin_unlock (&rwlock->__lock);
  96
  97   if (drain)
  98     __pthread_block (self);
  99
 100   if (err)
 101     {
 102       assert (err == ETIMEDOUT);
 103       return err;
 104     }
 105
 106   /* The reader count has already been increment by whoever woke us
 107      up.  */
 108
 109   assert (rwlock->__readers > 0);
 110
 111   return 0;
 112 }
 113
 114 int
 115 __pthread_rwlock_timedrdlock (struct __pthread_rwlock *rwlock,
 116                               const struct timespec *abstime)
 117 {
 118   return __pthread_rwlock_timedrdlock_internal (rwlock, abstime);
 119 }
 120 weak_alias (__pthread_rwlock_timedrdlock, pthread_rwlock_timedrdlock)