nptl/pthread_rwlock_rdlock.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 <stap-probe.h>
  25 #include <elide.h>
  26 #include <stdbool.h>
  27
  28
  29 /* Acquire read lock for RWLOCK.  Slow path.  */
  30 static int __attribute__((noinline))
  31 __pthread_rwlock_rdlock_slow (pthread_rwlock_t *rwlock)
  32 {
  33   int result = 0;
  34   bool wake = false;
  35
  36   /* Lock is taken in caller.  */
  37
  38   while (1)
  39     {
  40       /* Make sure we are not holding the rwlock as a writer.  This is
  41          a deadlock situation we recognize and report.  */
  42       if (__builtin_expect (rwlock->__data.__writer
  43                             == THREAD_GETMEM (THREAD_SELF, tid), 0))
  44         {
  45           result = EDEADLK;
  46           break;
  47         }
  48
  49       /* Remember that we are a reader.  */
  50       if (__glibc_unlikely (++rwlock->__data.__nr_readers_queued == 0))
  51         {
  52           /* Overflow on number of queued readers.  */
  53           --rwlock->__data.__nr_readers_queued;
  54           result = EAGAIN;
  55           break;
  56         }
  57
  58       int waitval = rwlock->__data.__readers_wakeup;
  59
  60       /* Free the lock.  */
  61       lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
  62
  63       /* Wait for the writer to finish.  */
  64       lll_futex_wait (&rwlock->__data.__readers_wakeup, waitval,
  65                       rwlock->__data.__shared);
  66
  67       /* Get the lock.  */
  68       lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
  69
  70       --rwlock->__data.__nr_readers_queued;
  71
  72       /* Get the rwlock if there is no writer...  */
  73       if (rwlock->__data.__writer == 0
  74           /* ...and if either no writer is waiting or we prefer readers.  */
  75           && (!rwlock->__data.__nr_writers_queued
  76               || PTHREAD_RWLOCK_PREFER_READER_P (rwlock)))
  77         {
  78           /* Increment the reader counter.  Avoid overflow.  */
  79           if (__glibc_unlikely (++rwlock->__data.__nr_readers == 0))
  80             {
  81               /* Overflow on number of readers.  */
  82               --rwlock->__data.__nr_readers;
  83               result = EAGAIN;
  84             }
  85           else
  86             {
  87               LIBC_PROBE (rdlock_acquire_read, 1, rwlock);
  88               /* See pthread_rwlock_rdlock.  */
  89               if (rwlock->__data.__nr_readers == 1
  90                   && rwlock->__data.__nr_readers_queued > 0
  91                   && rwlock->__data.__nr_writers_queued > 0)
  92                 {
  93                   ++rwlock->__data.__readers_wakeup;
  94                   wake = true;
  95                 }
  96             }
  97
  98           break;
  99         }
 100     }
 101
 102   /* We are done, free the lock.  */
 103   lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
 104
 105   if (wake)
 106     lll_futex_wake (&rwlock->__data.__readers_wakeup, INT_MAX,
 107                     rwlock->__data.__shared);
 108
 109   return result;
 110 }
 111
 112
 113 /* Fast path of acquiring read lock on RWLOCK.  */
 114
 115 int
 116 __pthread_rwlock_rdlock (pthread_rwlock_t *rwlock)
 117 {
 118   int result = 0;
 119   bool wake = false;
 120
 121   LIBC_PROBE (rdlock_entry, 1, rwlock);
 122
 123   if (ELIDE_LOCK (rwlock->__data.__rwelision,
 124                   rwlock->__data.__lock == 0
 125                   && rwlock->__data.__writer == 0
 126                   && rwlock->__data.__nr_readers == 0))
 127     return 0;
 128
 129   /* Make sure we are alone.  */
 130   lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
 131
 132   /* Get the rwlock if there is no writer...  */
 133   if (rwlock->__data.__writer == 0
 134       /* ...and if either no writer is waiting or we prefer readers.  */
 135       && (!rwlock->__data.__nr_writers_queued
 136           || PTHREAD_RWLOCK_PREFER_READER_P (rwlock)))
 137     {
 138       /* Increment the reader counter.  Avoid overflow.  */
 139       if (__glibc_unlikely (++rwlock->__data.__nr_readers == 0))
 140         {
 141           /* Overflow on number of readers.      */
 142           --rwlock->__data.__nr_readers;
 143           result = EAGAIN;
 144         }
 145       else
 146         {
 147           LIBC_PROBE (rdlock_acquire_read, 1, rwlock);
 148           /* If we are the first reader, and there are blocked readers and
 149              writers (which we don't prefer, see above), then it can be the
 150              case that we stole the lock from a writer that was already woken
 151              to acquire it.  That means that we need to take over the writer's
 152              responsibility to wake all readers (see pthread_rwlock_unlock).
 153              Thus, wake all readers in this case.  */
 154           if (rwlock->__data.__nr_readers == 1
 155               && rwlock->__data.__nr_readers_queued > 0
 156               && rwlock->__data.__nr_writers_queued > 0)
 157             {
 158               ++rwlock->__data.__readers_wakeup;
 159               wake = true;
 160             }
 161         }
 162
 163       /* We are done, free the lock.  */
 164       lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
 165
 166       if (wake)
 167         lll_futex_wake (&rwlock->__data.__readers_wakeup, INT_MAX,
 168                         rwlock->__data.__shared);
 169
 170       return result;
 171     }
 172
 173   return __pthread_rwlock_rdlock_slow (rwlock);
 174 }
 175
 176 weak_alias (__pthread_rwlock_rdlock, pthread_rwlock_rdlock)
 177 hidden_def (__pthread_rwlock_rdlock)