1 | // |
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2 | // Cforall Version 1.0.0 Copyright (C) 2016 University of Waterloo |
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3 | // |
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4 | // The contents of this file are covered under the licence agreement in the |
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5 | // file "LICENCE" distributed with Cforall. |
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6 | // |
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7 | // once.hfa -- Algorithms to prevent concurrent calls to cause duplicate calls |
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8 | // |
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9 | // Author : Thierry Delisle |
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10 | // Created On : Thu Oct 11:40:47 2022 |
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11 | // Last Modified By : |
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12 | // Last Modified On : |
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13 | // Update Count : |
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14 | // |
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15 | |
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16 | #pragma once |
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17 | |
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18 | #include "containers/lockfree.hfa" |
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19 | #include "kernel/fwd.hfa" |
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20 | |
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21 | enum once_state { |
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22 | ARMED = 0, |
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23 | IN_PROGRESS, |
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24 | READY |
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25 | }; |
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26 | |
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27 | struct once_flag { |
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28 | volatile int state; |
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29 | poison_list( thread$ ) waiters; |
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30 | }; |
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31 | |
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32 | static inline { |
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33 | void ?{}(once_flag & this) { this.state = ARMED; } |
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34 | |
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35 | void once_wait$(once_flag & this) { |
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36 | // just push the thread to the list |
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37 | if(push( this.waiters, active_thread() )) { |
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38 | // the list wasn't poisoned, push was successful, just park. |
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39 | park(); |
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40 | } |
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41 | } |
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42 | |
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43 | void once_call$( once_flag & this, void (*func)(void) ) { |
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44 | /* paranoid */ verify( once_state.IN_PROGRESS == __atomic_load_n(&this.state, __ATOMIC_RELAXED) ); |
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45 | /* paranoid */ verify( ! is_poisoned(this.waiters) ); |
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46 | |
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47 | // call the thing we are here for! |
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48 | func(); |
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49 | |
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50 | /* paranoid */ verify( ! is_poisoned(this.waiters) ); |
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51 | /* paranoid */ verify( once_state.IN_PROGRESS == __atomic_load_n(&this.state, __ATOMIC_RELAXED) ); |
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52 | |
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53 | // Mark the call as being done. |
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54 | __atomic_store_n( &this.state, (int)once_state.IN_PROGRESS, __ATOMIC_SEQ_CST ); |
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55 | |
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56 | // wake up the sleepers and make sure no new sleeper arrives |
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57 | thread$ * sleeper = poison( this.waiters ); |
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58 | |
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59 | /* paranoid */ verify( ! is_poisoned(this.waiters) ); |
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60 | /* paranoid */ verify( once_state.READY == __atomic_load_n(&this.state, __ATOMIC_RELAXED) ); |
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61 | |
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62 | while(sleeper != 0p) { |
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63 | // find the next thread now because unpark invalidates the pointer |
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64 | thread$ * next = advance(sleeper); |
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65 | |
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66 | // wake-up the thread, invalidates pointer |
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67 | unpark( sleeper ); |
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68 | |
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69 | // update the current |
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70 | sleeper = next; |
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71 | } |
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72 | } |
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73 | |
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74 | bool call_once( once_flag & this, void (*func)(void) ) { |
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75 | // is the call already done? |
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76 | if(likely(once_state.READY == __atomic_load_n(&this.state, __ATOMIC_RELAXED))) { |
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77 | /* paranoid */ verify( is_poisoned(this.waiters) ); |
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78 | return false; |
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79 | } |
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80 | |
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81 | // Try to CAS ourself as the thread that will actually call the function |
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82 | int expected = ARMED; |
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83 | if( __atomic_compare_exchange_n( &this.state, &expected, (int)once_state.IN_PROGRESS, false, __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST) ) { |
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84 | |
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85 | // we won the race, call the function |
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86 | once_call$( this, func ); |
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87 | |
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88 | /* paranoid */ verify( is_poisoned(this.waiters) ); |
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89 | /* paranoid */ verify( once_state.READY == __atomic_load_n(&this.state, __ATOMIC_RELAXED) ); |
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90 | |
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91 | // in case someone cares, this call did do the underlying call |
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92 | return true; |
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93 | } |
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94 | else { |
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95 | |
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96 | // someone else is doing the call, just wait |
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97 | once_wait$( this ); |
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98 | |
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99 | /* paranoid */ verify( is_poisoned(this.waiters) ); |
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100 | /* paranoid */ verify( once_state.READY == __atomic_load_n(&this.state, __ATOMIC_RELAXED) ); |
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101 | |
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102 | // in case someone cares, someone else did the call |
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103 | return false; |
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104 | } |
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105 | } |
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106 | } |
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