nptl/pthread_rwlock_rdlock.c

   1 /* Copyright (C) 2003-2016 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 <stap-probe.h>
  26 #include <elide.h>
  27 #include <stdbool.h>
  28
  29
  30 /* Acquire read lock for RWLOCK.  Slow path.  */
  31 static int __attribute__((noinline))
  32 __pthread_rwlock_rdlock_slow (pthread_rwlock_t *rwlock)
  33 {
  34   int result = 0;
  35   bool wake = false;
  36   int futex_shared =
  37       rwlock->__data.__shared == LLL_PRIVATE ? FUTEX_PRIVATE : FUTEX_SHARED;
  38
  39   /* Lock is taken in caller.  */
  40
  41   while (1)
  42     {
  43       /* Make sure we are not holding the rwlock as a writer.  This is
  44          a deadlock situation we recognize and report.  */
  45       if (__builtin_expect (rwlock->__data.__writer
  46                             == THREAD_GETMEM (THREAD_SELF, tid), 0))
  47         {
  48           result = EDEADLK;
  49           break;
  50         }
  51
  52       /* Remember that we are a reader.  */
  53       if (__glibc_unlikely (++rwlock->__data.__nr_readers_queued == 0))
  54         {
  55           /* Overflow on number of queued readers.  */
  56           --rwlock->__data.__nr_readers_queued;
  57           result = EAGAIN;
  58           break;
  59         }
  60
  61       int waitval = rwlock->__data.__readers_wakeup;
  62
  63       /* Free the lock.  */
  64       lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
  65
  66       /* Wait for the writer to finish.  We do not check the return value
  67          because we decide how to continue based on the state of the rwlock.  */
  68       futex_wait_simple (&rwlock->__data.__readers_wakeup, waitval,
  69                          futex_shared);
  70
  71       /* Get the lock.  */
  72       lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
  73
  74       --rwlock->__data.__nr_readers_queued;
  75
  76       /* Get the rwlock if there is no writer...  */
  77       if (rwlock->__data.__writer == 0
  78           /* ...and if either no writer is waiting or we prefer readers.  */
  79           && (!rwlock->__data.__nr_writers_queued
  80               || PTHREAD_RWLOCK_PREFER_READER_P (rwlock)))
  81         {
  82           /* Increment the reader counter.  Avoid overflow.  */
  83           if (__glibc_unlikely (++rwlock->__data.__nr_readers == 0))
  84             {
  85               /* Overflow on number of readers.  */
  86               --rwlock->__data.__nr_readers;
  87               result = EAGAIN;
  88             }
  89           else
  90             {
  91               LIBC_PROBE (rdlock_acquire_read, 1, rwlock);
  92               /* See pthread_rwlock_rdlock.  */
  93               if (rwlock->__data.__nr_readers == 1
  94                   && rwlock->__data.__nr_readers_queued > 0
  95                   && rwlock->__data.__nr_writers_queued > 0)
  96                 {
  97                   ++rwlock->__data.__readers_wakeup;
  98                   wake = true;
  99                 }
 100             }
 101
 102           break;
 103         }
 104     }
 105
 106   /* We are done, free the lock.  */
 107   lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
 108
 109   if (wake)
 110     futex_wake (&rwlock->__data.__readers_wakeup, INT_MAX, futex_shared);
 111
 112   return result;
 113 }
 114
 115
 116 /* Fast path of acquiring read lock on RWLOCK.  */
 117
 118 int
 119 __pthread_rwlock_rdlock (pthread_rwlock_t *rwlock)
 120 {
 121   int result = 0;
 122   bool wake = false;
 123   int futex_shared =
 124       rwlock->__data.__shared == LLL_PRIVATE ? FUTEX_PRIVATE : FUTEX_SHARED;
 125
 126   LIBC_PROBE (rdlock_entry, 1, rwlock);
 127
 128   if (ELIDE_LOCK (rwlock->__data.__rwelision,
 129                   rwlock->__data.__lock == 0
 130                   && rwlock->__data.__writer == 0
 131                   && rwlock->__data.__nr_readers == 0))
 132     return 0;
 133
 134   /* Make sure we are alone.  */
 135   lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
 136
 137   /* Get the rwlock if there is no writer...  */
 138   if (rwlock->__data.__writer == 0
 139       /* ...and if either no writer is waiting or we prefer readers.  */
 140       && (!rwlock->__data.__nr_writers_queued
 141           || PTHREAD_RWLOCK_PREFER_READER_P (rwlock)))
 142     {
 143       /* Increment the reader counter.  Avoid overflow.  */
 144       if (__glibc_unlikely (++rwlock->__data.__nr_readers == 0))
 145         {
 146           /* Overflow on number of readers.      */
 147           --rwlock->__data.__nr_readers;
 148           result = EAGAIN;
 149         }
 150       else
 151         {
 152           LIBC_PROBE (rdlock_acquire_read, 1, rwlock);
 153           /* If we are the first reader, and there are blocked readers and
 154              writers (which we don't prefer, see above), then it can be the
 155              case that we stole the lock from a writer that was already woken
 156              to acquire it.  That means that we need to take over the writer's
 157              responsibility to wake all readers (see pthread_rwlock_unlock).
 158              Thus, wake all readers in this case.  */
 159           if (rwlock->__data.__nr_readers == 1
 160               && rwlock->__data.__nr_readers_queued > 0
 161               && rwlock->__data.__nr_writers_queued > 0)
 162             {
 163               ++rwlock->__data.__readers_wakeup;
 164               wake = true;
 165             }
 166         }
 167
 168       /* We are done, free the lock.  */
 169       lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
 170
 171       if (wake)
 172         futex_wake (&rwlock->__data.__readers_wakeup, INT_MAX, futex_shared);
 173
 174       return result;
 175     }
 176
 177   return __pthread_rwlock_rdlock_slow (rwlock);
 178 }
 179
 180 weak_alias (__pthread_rwlock_rdlock, pthread_rwlock_rdlock)
 181 hidden_def (__pthread_rwlock_rdlock)