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8d9254fc | 1 | /* Copyright (C) 2011-2020 Free Software Foundation, Inc. |
0a35513e AH |
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 | ||
799142bf | 28 | #include "local_atomic" |
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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. | |
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42 | // |
43 | // Do not change the layout of this class; it must remain a POD type with | |
6041f70a | 44 | // standard layout, and the writers field must be first (i.e., so the |
bec9ec3f | 45 | // assembler code can assume that its address is equal to the address of the |
6041f70a | 46 | // respective instance of the class), and htm_fastpath must be second. |
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47 | |
48 | class gtm_rwlock | |
49 | { | |
799142bf | 50 | std::atomic<int> writers; // Writers' futex. |
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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. | |
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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). | |
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61 | |
62 | public: | |
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63 | gtm_rwlock() : writers(0), htm_fastpath(0), writer_readers(0), readers(0) |
64 | { } | |
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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); | |
610e3901 | 73 | void write_upgrade_finish (gtm_thread *tx); |
0a35513e | 74 | |
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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 () | |
64fbcc74 | 80 | { |
6041f70a TR |
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); | |
64fbcc74 TR |
97 | } |
98 | ||
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99 | protected: |
100 | bool write_lock_generic (gtm_thread *tx); | |
101 | }; | |
102 | ||
103 | } // namespace GTM | |
104 | ||
105 | #endif // GTM_RWLOCK_H |