1  /* Copyright (C) 2009-2023 Free Software Foundation, Inc.
       2     Contributed by Richard Henderson <rth@redhat.com>.
       3  
       4     This file is part of the GNU Transactional Memory Library (libitm).
       5  
       6     Libitm is free software; you can redistribute it and/or modify it
       7     under the terms of the GNU General Public License as published by
       8     the Free Software Foundation; either version 3 of the License, or
       9     (at your option) any later version.
      10  
      11     Libitm is distributed in the hope that it will be useful, but WITHOUT ANY
      12     WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
      13     FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
      14     more details.
      15  
      16     Under Section 7 of GPL version 3, you are granted additional
      17     permissions described in the GCC Runtime Library Exception, version
      18     3.1, as published by the Free Software Foundation.
      19  
      20     You should have received a copy of the GNU General Public License and
      21     a copy of the GCC Runtime Library Exception along with this program;
      22     see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
      23     <http://www.gnu.org/licenses/>.  */
      24  
      25  #ifndef GTM_RWLOCK_H
      26  #define GTM_RWLOCK_H
      27  
      28  #include <pthread.h>
      29  #include "local_atomic"
      30  
      31  namespace GTM HIDDEN {
      32  
      33  struct gtm_thread;
      34  
      35  // This datastructure is the blocking, mutex-based side of the Dekker-style
      36  // reader-writer lock used to provide mutual exclusion between active and
      37  // serial transactions. It has similarities to POSIX pthread_rwlock_t except
      38  // that we also provide for upgrading a reader->writer lock, with a
      39  // positive indication of failure (another writer acquired the lock
      40  // before we were able to acquire). While the writer flag (a_writer below) is
      41  // global and protected by the mutex, there are per-transaction reader flags,
      42  // which are stored in a transaction's shared state.
      43  // See libitm's documentation for further details.
      44  //
      45  // In this implementation, writers are given highest priority access but
      46  // read-to-write upgrades do not have a higher priority than writers.
      47  //
      48  // Do not change the layout of this class; it must remain a POD type with
      49  // standard layout, and the summary field must be first (i.e., so the
      50  // assembler code can assume that its address is equal to the address of the
      51  // respective instance of the class), and htm_fastpath must be second.
      52  
      53  class gtm_rwlock
      54  {
      55    static const unsigned a_writer  = 1;	// An active writer.
      56    static const unsigned w_writer  = 2;	// The w_writers field != 0
      57    static const unsigned w_reader  = 4;  // The w_readers field != 0
      58  
      59    std::atomic<unsigned int> summary;	// Bitmask of the above.
      60  
      61    // We put the HTM fastpath control variable here so that HTM fastpath
      62    // transactions can check efficiently whether they are allowed to run.
      63    // This must be accessed atomically because threads can load this value
      64    // when they are neither a registered reader nor writer (i.e., when they
      65    // attempt to execute the HTM fastpath).
      66    std::atomic<uint32_t> htm_fastpath;
      67  
      68    pthread_mutex_t mutex;	        // Held if manipulating any field.
      69    pthread_cond_t c_readers;	        // Readers wait here
      70    pthread_cond_t c_writers;	        // Writers wait here for writers
      71    pthread_cond_t c_confirmed_writers;	// Writers wait here for readers
      72  
      73    unsigned int a_readers;	// Nr active readers as observed by a writer
      74    unsigned int w_readers;	// Nr waiting readers
      75    unsigned int w_writers;	// Nr waiting writers
      76  
      77   public:
      78    gtm_rwlock();
      79    ~gtm_rwlock();
      80  
      81    void read_lock (gtm_thread *tx);
      82    void read_unlock (gtm_thread *tx);
      83  
      84    void write_lock ();
      85    void write_unlock ();
      86  
      87    bool write_upgrade (gtm_thread *tx);
      88    void write_upgrade_finish (gtm_thread *tx);
      89  
      90    // Returns true iff there is a concurrent active or waiting writer, or
      91    // htm_fastpath is zero. This is primarily useful for simple HyTM
      92    // approaches, and the values being checked are loaded with
      93    // memory_order_relaxed.
      94    bool htm_fastpath_disabled ()
      95    {
      96      return (summary.load (memory_order_relaxed) & (a_writer | w_writer))
      97  	|| htm_fastpath.load (memory_order_relaxed) == 0;
      98    }
      99  
     100    // This does not need to return an exact value, hence relaxed MO is
     101    // sufficient.
     102    uint32_t get_htm_fastpath ()
     103    {
     104      return htm_fastpath.load (memory_order_relaxed);
     105    }
     106    // This must only be called while having acquired the write lock, and other
     107    // threads do not need to load an exact value; hence relaxed MO is
     108    // sufficient.
     109    void set_htm_fastpath (uint32_t val)
     110    {
     111      htm_fastpath.store (val, memory_order_relaxed);
     112    }
     113  
     114   protected:
     115    bool write_lock_generic (gtm_thread *tx);
     116  };
     117  
     118  } // namespace GTM
     119  
     120  #endif // GTM_RWLOCK_H