1 /* Copyright (C) 2011-2013 Free Software Foundation, Inc.
2 Contributed by Torvald Riegel <triegel@redhat.com>.
4 This file is part of the GNU Transactional Memory Library (libitm).
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.
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
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.
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/>. */
28 #include "local_atomic"
31 namespace GTM HIDDEN
{
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.
40 // In this implementation, writers are given highest priority access but
41 // read-to-write upgrades do not have a higher priority than writers.
45 // TODO Put futexes on different cachelines?
46 std::atomic
<int> writers
; // Writers' futex.
47 std::atomic
<int> writer_readers
;// A confirmed writer waits here for readers.
48 std::atomic
<int> readers
; // Readers wait here for writers (iff true).
51 gtm_rwlock() : writers(0), writer_readers(0), readers(0) {};
53 void read_lock (gtm_thread
*tx
);
54 void read_unlock (gtm_thread
*tx
);
59 bool write_upgrade (gtm_thread
*tx
);
60 void write_upgrade_finish (gtm_thread
*tx
);
62 // Returns true iff there is a concurrent active or waiting writer.
63 // This is primarily useful for simple HyTM approaches, and the value being
64 // checked is loaded with memory_order_relaxed.
65 bool is_write_locked()
67 return writers
.load (memory_order_relaxed
) != 0;
71 bool write_lock_generic (gtm_thread
*tx
);
76 #endif // GTM_RWLOCK_H