nptl/pthread_rwlock_timedwrlock.c

   1 /* Copyright (C) 2003-2015 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3    Contributed by Martin Schwidefsky <schwidefsky@de.ibm.com>, 2003.
   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 #include <errno.h>
  20 #include <sysdep.h>
  21 #include <lowlevellock.h>
  22 #include <futex-internal.h>
  23 #include <pthread.h>
  24 #include <pthreadP.h>
  25 #include <sys/time.h>
  26 #include <stdbool.h>
  27
  28
  29 /* Try to acquire write lock for RWLOCK or return after specfied time.  */
  30 int
  31 pthread_rwlock_timedwrlock (pthread_rwlock_t *rwlock,
  32                             const struct timespec *abstime)
  33 {
  34   int result = 0;
  35   bool wake_readers = false;
  36   int futex_shared =
  37       rwlock->__data.__shared == LLL_PRIVATE ? FUTEX_PRIVATE : FUTEX_SHARED;
  38
  39   /* Make sure we are alone.  */
  40   lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
  41
  42   while (1)
  43     {
  44       int err;
  45
  46       /* Get the rwlock if there is no writer and no reader.  */
  47       if (rwlock->__data.__writer == 0 && rwlock->__data.__nr_readers == 0)
  48         {
  49           /* Mark self as writer.  */
  50           rwlock->__data.__writer = THREAD_GETMEM (THREAD_SELF, tid);
  51           break;
  52         }
  53
  54       /* Make sure we are not holding the rwlock as a writer.  This is
  55          a deadlock situation we recognize and report.  */
  56       if (__builtin_expect (rwlock->__data.__writer
  57                             == THREAD_GETMEM (THREAD_SELF, tid), 0))
  58         {
  59           result = EDEADLK;
  60           break;
  61         }
  62
  63       /* Make sure the passed in timeout value is valid.  Ideally this
  64          test would be executed once.  But since it must not be
  65          performed if we would not block at all simply moving the test
  66          to the front is no option.  Replicating all the code is
  67          costly while this test is not.  */
  68       if (__builtin_expect (abstime->tv_nsec >= 1000000000
  69                             || abstime->tv_nsec < 0, 0))
  70         {
  71           result = EINVAL;
  72           break;
  73         }
  74
  75       /* Remember that we are a writer.  */
  76       if (++rwlock->__data.__nr_writers_queued == 0)
  77         {
  78           /* Overflow on number of queued writers.  */
  79           --rwlock->__data.__nr_writers_queued;
  80           result = EAGAIN;
  81           break;
  82         }
  83
  84       int waitval = rwlock->__data.__writer_wakeup;
  85
  86       /* Free the lock.  */
  87       lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
  88
  89       /* Wait for the writer or reader(s) to finish.  We handle ETIMEDOUT
  90          below; on other return values, we decide how to continue based on
  91          the state of the rwlock.  */
  92       err = futex_abstimed_wait (&rwlock->__data.__writer_wakeup, waitval,
  93                                  abstime, futex_shared);
  94
  95       /* Get the lock.  */
  96       lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
  97
  98       /* To start over again, remove the thread from the writer list.  */
  99       --rwlock->__data.__nr_writers_queued;
 100
 101       /* Did the futex call time out?  */
 102       if (err == ETIMEDOUT)
 103         {
 104           result = ETIMEDOUT;
 105           /* If we prefer writers, it can have happened that readers blocked
 106              for us to acquire the lock first.  If we have timed out, we need
 107              to wake such readers if there are any, and if there is no writer
 108              currently (otherwise, the writer will take care of wake-up).
 109              Likewise, even if we prefer readers, we can be responsible for
 110              wake-up (see pthread_rwlock_unlock) if no reader or writer has
 111              acquired the lock.  We have timed out and thus not consumed a
 112              futex wake-up; therefore, if there is no other blocked writer
 113              that would consume the wake-up and thus take over responsibility,
 114              we need to wake blocked readers.  */
 115           if ((!PTHREAD_RWLOCK_PREFER_READER_P (rwlock)
 116                || ((rwlock->__data.__nr_readers == 0)
 117                    && (rwlock->__data.__nr_writers_queued == 0)))
 118               && (rwlock->__data.__nr_readers_queued > 0)
 119               && (rwlock->__data.__writer == 0))
 120             {
 121               ++rwlock->__data.__readers_wakeup;
 122               wake_readers = true;
 123             }
 124           break;
 125         }
 126     }
 127
 128   /* We are done, free the lock.  */
 129   lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
 130
 131   /* Might be required after timeouts.  */
 132   if (wake_readers)
 133     futex_wake (&rwlock->__data.__readers_wakeup, INT_MAX, futex_shared);
 134
 135   return result;
 136 }