libitm/config/linux/rwlock.h

   1 /* Copyright (C) 2011-2021 Free Software Foundation, Inc.
   2    Contributed by Torvald Riegel <triegel@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 "local_atomic"
  29 #include "common.h"
  30
  31 namespace GTM HIDDEN {
  32
  33 struct gtm_thread;
  34
  35 // This datastructure is the blocking, futex-based version of the Dekker-style
  36 // reader-writer lock used to provide mutual exclusion between active and
  37 // serial transactions.
  38 // See libitm's documentation for further details.
  39 //
  40 // In this implementation, writers are given highest priority access but
  41 // read-to-write upgrades do not have a higher priority than writers.
  42 //
  43 // Do not change the layout of this class; it must remain a POD type with
  44 // standard layout, and the writers field must be first (i.e., so the
  45 // assembler code can assume that its address is equal to the address of the
  46 // respective instance of the class), and htm_fastpath must be second.
  47
  48 class gtm_rwlock
  49 {
  50   std::atomic<int> writers;       // Writers' futex.
  51   // We put the HTM fastpath control variable here so that HTM fastpath
  52   // transactions can check efficiently whether they are allowed to run.
  53   // This must be accessed atomically because threads can load this value
  54   // when they are neither a registered reader nor writer (i.e., when they
  55   // attempt to execute the HTM fastpath).
  56   std::atomic<uint32_t> htm_fastpath;
  57   // TODO Put these futexes on different cachelines?  (writers and htm_fastpath
  58   // should remain on the same cacheline.
  59   std::atomic<int> writer_readers;// A confirmed writer waits here for readers.
  60   std::atomic<int> readers;       // Readers wait here for writers (iff true).
  61
  62  public:
  63   gtm_rwlock() : writers(0), htm_fastpath(0), writer_readers(0), readers(0)
  64   { }
  65
  66   void read_lock (gtm_thread *tx);
  67   void read_unlock (gtm_thread *tx);
  68
  69   void write_lock ();
  70   void write_unlock ();
  71
  72   bool write_upgrade (gtm_thread *tx);
  73   void write_upgrade_finish (gtm_thread *tx);
  74
  75   // Returns true iff there is a concurrent active or waiting writer, or
  76   // htm_fastpath is zero. This is primarily useful for simple HyTM
  77   // approaches, and the values being checked are loaded with
  78   // memory_order_relaxed.
  79   bool htm_fastpath_disabled ()
  80   {
  81     return writers.load (memory_order_relaxed) != 0
  82         || htm_fastpath.load (memory_order_relaxed) == 0;
  83   }
  84
  85   // This does not need to return an exact value, hence relaxed MO is
  86   // sufficient.
  87   uint32_t get_htm_fastpath ()
  88   {
  89     return htm_fastpath.load (memory_order_relaxed);
  90   }
  91   // This must only be called while having acquired the write lock, and other
  92   // threads do not need to load an exact value; hence relaxed MO is
  93   // sufficient.
  94   void set_htm_fastpath (uint32_t val)
  95   {
  96     htm_fastpath.store (val, memory_order_relaxed);
  97   }
  98
  99  protected:
 100   bool write_lock_generic (gtm_thread *tx);
 101 };
 102
 103 } // namespace GTM
 104
 105 #endif // GTM_RWLOCK_H