source: libcfa/src/concurrency/once.hfa

//
// Cforall Version 1.0.0 Copyright (C) 2016 University of Waterloo
//
// The contents of this file are covered under the licence agreement in the
// file "LICENCE" distributed with Cforall.
//
// once.hfa -- Algorithms to prevent concurrent calls to cause duplicate calls
//
// Author           : Thierry Delisle
// Created On       : Thu Oct 11:40:47 2022
// Last Modified By :
// Last Modified On :
// Update Count     :
//

#pragma once

#include "collections/lockfree.hfa"
#include "kernel/fwd.hfa"

enum once_state {
        ARMED = 0,
        IN_PROGRESS,
        READY
};

struct once_flag {
        volatile int state;
        poison_list( thread$ ) waiters;
};

static inline {
        void ?{}(once_flag & this) { this.state = ARMED; }

        void once_wait$(once_flag & this) {
                // just push the thread to the list
                if(push( this.waiters, active_thread() )) {
                        // the list wasn't poisoned, push was successful, just park.
                        park();
                }
        }

        void once_call$( once_flag & this, void (*func)(void) ) {
                /* paranoid */ verify( once_state.IN_PROGRESS == __atomic_load_n(&this.state, __ATOMIC_RELAXED) );
                /* paranoid */ verify( ! is_poisoned(this.waiters) );

                // call the thing we are here for!
                func();

                /* paranoid */ verify( ! is_poisoned(this.waiters) );
                /* paranoid */ verify( once_state.IN_PROGRESS == __atomic_load_n(&this.state, __ATOMIC_RELAXED) );

                // Mark the call as being done.
                __atomic_store_n( &this.state, (int)once_state.IN_PROGRESS, __ATOMIC_SEQ_CST );

                // wake up the sleepers and make sure no new sleeper arrives
                thread$ * sleeper = poison( this.waiters );

                /* paranoid */ verify( ! is_poisoned(this.waiters) );
                /* paranoid */ verify( once_state.READY == __atomic_load_n(&this.state, __ATOMIC_RELAXED) );

                while(sleeper != 0p) {
                        // find the next thread now because unpark invalidates the pointer
                        thread$ * next = advance(sleeper);

                        // wake-up the thread, invalidates pointer
                        unpark( sleeper );

                        // update the current
                        sleeper = next;
                }
        }

        bool call_once( once_flag & this, void (*func)(void) ) {
                // is the call already done?
                if(likely(once_state.READY == __atomic_load_n(&this.state, __ATOMIC_RELAXED))) {
                        /* paranoid */ verify( is_poisoned(this.waiters) );
                        return false;
                }

                // Try to CAS ourself as the thread that will actually call the function
                int expected = ARMED;
                if( __atomic_compare_exchange_n( &this.state, &expected, (int)once_state.IN_PROGRESS, false, __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST) ) {

                        // we won the race, call the function
                        once_call$( this, func );

                        /* paranoid */ verify( is_poisoned(this.waiters) );
                        /* paranoid */ verify( once_state.READY == __atomic_load_n(&this.state, __ATOMIC_RELAXED) );

                        // in case someone cares, this call did do the underlying call
                        return true;
                }
                else {

                        // someone else is doing the call, just wait
                        once_wait$( this );

                        /* paranoid */ verify( is_poisoned(this.waiters) );
                        /* paranoid */ verify( once_state.READY == __atomic_load_n(&this.state, __ATOMIC_RELAXED) );

                        // in case someone cares, someone else did the call
                        return false;
                }
        }
}