1  /* Unlock a rwlock.  Generic version.
       2     Copyright (C) 2000-2023 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  
      21  #include <pt-internal.h>
      22  
      23  /* Unlock *RWLOCK, rescheduling a waiting writer thread or, if there
      24     are no threads waiting for a write lock, rescheduling the reader
      25     threads.  */
      26  int
      27  __pthread_rwlock_unlock (pthread_rwlock_t *rwlock)
      28  {
      29    struct __pthread *wakeup;
      30  
      31    __pthread_spin_wait (&rwlock->__lock);
      32  
      33    assert (__pthread_spin_trylock (&rwlock->__held) == EBUSY);
      34  
      35    if (rwlock->__readers > 1)
      36      /* There are other readers.  */
      37      {
      38        rwlock->__readers--;
      39        __pthread_spin_unlock (&rwlock->__lock);
      40        return 0;
      41      }
      42  
      43    if (rwlock->__readers == 1)
      44      /* Last reader.  */
      45      rwlock->__readers = 0;
      46  
      47  
      48    /* Wake someone else up.  Try the writer queue first, then the
      49       reader queue if that is empty.  */
      50  
      51    if (rwlock->__writerqueue)
      52      {
      53        wakeup = rwlock->__writerqueue;
      54        __pthread_dequeue (wakeup);
      55  
      56        /* We do not unlock RWLOCK->held: we are transferring the ownership
      57           to the thread that we are waking up.  */
      58  
      59        __pthread_spin_unlock (&rwlock->__lock);
      60        __pthread_wakeup (wakeup);
      61  
      62        return 0;
      63      }
      64  
      65    if (rwlock->__readerqueue)
      66      {
      67        unsigned n = 0;
      68  
      69        __pthread_queue_iterate (rwlock->__readerqueue, wakeup)
      70  	n++;
      71  
      72        {
      73  	struct __pthread *wakeups[n];
      74  	unsigned i = 0;
      75  
      76  	__pthread_dequeuing_iterate (rwlock->__readerqueue, wakeup)
      77  	  wakeups[i++] = wakeup;
      78  
      79  	rwlock->__readers += n;
      80  	rwlock->__readerqueue = 0;
      81  
      82  	__pthread_spin_unlock (&rwlock->__lock);
      83  
      84  	for (i = 0; i < n; i++)
      85  	  __pthread_wakeup (wakeups[i]);
      86        }
      87  
      88        return 0;
      89      }
      90  
      91  
      92    /* No one is waiting.  Just unlock it.  */
      93  
      94    __pthread_spin_unlock (&rwlock->__held);
      95    __pthread_spin_unlock (&rwlock->__lock);
      96    return 0;
      97  }
      98  weak_alias (__pthread_rwlock_unlock, pthread_rwlock_unlock);