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 <pthread.h>
  23 #include <pthreadP.h>
  24 #include <sys/time.h>
  25 #include <kernel-features.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 (rwlock, abstime)
  32      pthread_rwlock_t *rwlock;
  33      const struct timespec *abstime;
  34 {
  35   int result = 0;
  36   bool wake_readers = false;
  37
  38   /* Make sure we are alone.  */
  39   lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
  40
  41   while (1)
  42     {
  43       int err;
  44
  45       /* Get the rwlock if there is no writer and no reader.  */
  46       if (rwlock->__data.__writer == 0 && rwlock->__data.__nr_readers == 0)
  47         {
  48           /* Mark self as writer.  */
  49           rwlock->__data.__writer = THREAD_GETMEM (THREAD_SELF, tid);
  50           break;
  51         }
  52
  53       /* Make sure we are not holding the rwlock as a writer.  This is
  54          a deadlock situation we recognize and report.  */
  55       if (__builtin_expect (rwlock->__data.__writer
  56                             == THREAD_GETMEM (THREAD_SELF, tid), 0))
  57         {
  58           result = EDEADLK;
  59           break;
  60         }
  61
  62       /* Make sure the passed in timeout value is valid.  Ideally this
  63          test would be executed once.  But since it must not be
  64          performed if we would not block at all simply moving the test
  65          to the front is no option.  Replicating all the code is
  66          costly while this test is not.  */
  67       if (__builtin_expect (abstime->tv_nsec >= 1000000000
  68                             || abstime->tv_nsec < 0, 0))
  69         {
  70           result = EINVAL;
  71           break;
  72         }
  73
  74       /* Work around the fact that the kernel rejects negative timeout values
  75          despite them being valid.  */
  76       if (__glibc_unlikely (abstime->tv_sec < 0))
  77         {
  78           result = ETIMEDOUT;
  79           break;
  80         }
  81
  82 #if (!defined __ASSUME_FUTEX_CLOCK_REALTIME \
  83      || !defined lll_futex_timed_wait_bitset)
  84       /* Get the current time.  So far we support only one clock.  */
  85       struct timeval tv;
  86       (void) __gettimeofday (&tv, NULL);
  87
  88       /* Convert the absolute timeout value to a relative timeout.  */
  89       struct timespec rt;
  90       rt.tv_sec = abstime->tv_sec - tv.tv_sec;
  91       rt.tv_nsec = abstime->tv_nsec - tv.tv_usec * 1000;
  92       if (rt.tv_nsec < 0)
  93         {
  94           rt.tv_nsec += 1000000000;
  95           --rt.tv_sec;
  96         }
  97       /* Did we already time out?  */
  98       if (rt.tv_sec < 0)
  99         {
 100           result = ETIMEDOUT;
 101           break;
 102         }
 103 #endif
 104
 105       /* Remember that we are a writer.  */
 106       if (++rwlock->__data.__nr_writers_queued == 0)
 107         {
 108           /* Overflow on number of queued writers.  */
 109           --rwlock->__data.__nr_writers_queued;
 110           result = EAGAIN;
 111           break;
 112         }
 113
 114       int waitval = rwlock->__data.__writer_wakeup;
 115
 116       /* Free the lock.  */
 117       lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
 118
 119       /* Wait for the writer or reader(s) to finish.  */
 120 #if (!defined __ASSUME_FUTEX_CLOCK_REALTIME \
 121      || !defined lll_futex_timed_wait_bitset)
 122       err = lll_futex_timed_wait (&rwlock->__data.__writer_wakeup,
 123                                   waitval, &rt, rwlock->__data.__shared);
 124 #else
 125       err = lll_futex_timed_wait_bitset (&rwlock->__data.__writer_wakeup,
 126                                          waitval, abstime,
 127                                          FUTEX_CLOCK_REALTIME,
 128                                          rwlock->__data.__shared);
 129 #endif
 130
 131       /* Get the lock.  */
 132       lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
 133
 134       /* To start over again, remove the thread from the writer list.  */
 135       --rwlock->__data.__nr_writers_queued;
 136
 137       /* Did the futex call time out?  */
 138       if (err == -ETIMEDOUT)
 139         {
 140           result = ETIMEDOUT;
 141           /* If we prefer writers, it can have happened that readers blocked
 142              for us to acquire the lock first.  If we have timed out, we need
 143              to wake such readers if there are any, and if there is no writer
 144              currently (otherwise, the writer will take care of wake-up).
 145              Likewise, even if we prefer readers, we can be responsible for
 146              wake-up (see pthread_rwlock_unlock) if no reader or writer has
 147              acquired the lock.  We have timed out and thus not consumed a
 148              futex wake-up; therefore, if there is no other blocked writer
 149              that would consume the wake-up and thus take over responsibility,
 150              we need to wake blocked readers.  */
 151           if ((!PTHREAD_RWLOCK_PREFER_READER_P (rwlock)
 152                || ((rwlock->__data.__nr_readers == 0)
 153                    && (rwlock->__data.__nr_writers_queued == 0)))
 154               && (rwlock->__data.__nr_readers_queued > 0)
 155               && (rwlock->__data.__writer == 0))
 156             {
 157               ++rwlock->__data.__readers_wakeup;
 158               wake_readers = true;
 159             }
 160           break;
 161         }
 162     }
 163
 164   /* We are done, free the lock.  */
 165   lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
 166
 167   /* Might be required after timeouts.  */
 168   if (wake_readers)
 169     lll_futex_wake (&rwlock->__data.__readers_wakeup, INT_MAX,
 170         rwlock->__data.__shared);
 171
 172   return result;
 173 }