spin_rw_mutex.cpp

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/*
    Copyright (c) 2005-2020 Intel Corporation

    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
    You may obtain a copy of the License at

        http://www.apache.org/licenses/LICENSE-2.0

    Unless required by applicable law or agreed to in writing, software
    distributed under the License is distributed on an "AS IS" BASIS,
    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    See the License for the specific language governing permissions and
    limitations under the License.
*/

#include "tbb/spin_rw_mutex.h"
#include "tbb/tbb_machine.h"
#include "tbb/atomic.h"
#include "itt_notify.h"

#if defined(_MSC_VER) && defined(_Wp64)
    // Workaround for overzealous compiler warnings in /Wp64 mode
    #pragma warning (disable: 4244)
#endif

namespace tbb {

template<typename T> // a template can work with private spin_rw_mutex::state_t
static inline T CAS(volatile T &addr, T newv, T oldv) {
    // ICC (9.1 and 10.1 tried) unable to do implicit conversion
    // from "volatile T*" to "volatile void*", so explicit cast added.
    return tbb::internal::as_atomic(addr).compare_and_swap( newv, oldv );
}

bool spin_rw_mutex_v3::internal_acquire_writer()
{
    ITT_NOTIFY(sync_prepare, this);
    for( internal::atomic_backoff backoff;;backoff.pause() ){
        state_t s = const_cast<volatile state_t&>(state); // ensure reloading
        if( !(s & BUSY) ) { // no readers, no writers
            if( CAS(state, WRITER, s)==s )
                break; // successfully stored writer flag
            backoff.reset(); // we could be very close to complete op.
        } else if( !(s & WRITER_PENDING) ) { // no pending writers
            __TBB_AtomicOR(&state, WRITER_PENDING);
        }
    }
    ITT_NOTIFY(sync_acquired, this);
    return false;
}

void spin_rw_mutex_v3::internal_release_writer()
{
    ITT_NOTIFY(sync_releasing, this);
    __TBB_AtomicAND( &state, READERS );
}

void spin_rw_mutex_v3::internal_acquire_reader()
{
    ITT_NOTIFY(sync_prepare, this);
    for( internal::atomic_backoff b;;b.pause() ){
        state_t s = const_cast<volatile state_t&>(state); // ensure reloading
        if( !(s & (WRITER|WRITER_PENDING)) ) { // no writer or write requests
            state_t t = (state_t)__TBB_FetchAndAddW( &state, (intptr_t) ONE_READER );
            if( !( t&WRITER ))
                break; // successfully stored increased number of readers
            // writer got there first, undo the increment
            __TBB_FetchAndAddW( &state, -(intptr_t)ONE_READER );
        }
    }

    ITT_NOTIFY(sync_acquired, this);
    __TBB_ASSERT( state & READERS, "invalid state of a read lock: no readers" );
}


bool spin_rw_mutex_v3::internal_upgrade()
{
    state_t s = state;
    __TBB_ASSERT( s & READERS, "invalid state before upgrade: no readers " );
    // check and set writer-pending flag
    // required conditions: either no pending writers, or we are the only reader
    // (with multiple readers and pending writer, another upgrade could have been requested)
    while( (s & READERS)==ONE_READER || !(s & WRITER_PENDING) ) {
        state_t old_s = s;
        if( (s=CAS(state, s | WRITER | WRITER_PENDING, s))==old_s ) {
            ITT_NOTIFY(sync_prepare, this);
            internal::atomic_backoff backoff;
            while( (state & READERS) != ONE_READER ) backoff.pause();
            __TBB_ASSERT((state&(WRITER_PENDING|WRITER))==(WRITER_PENDING|WRITER),"invalid state when upgrading to writer");
            // both new readers and writers are blocked at this time
            __TBB_FetchAndAddW( &state,  - (intptr_t)(ONE_READER+WRITER_PENDING));
            ITT_NOTIFY(sync_acquired, this);
            return true; // successfully upgraded
        }
    }
    // slow reacquire
    internal_release_reader();
    return internal_acquire_writer(); // always returns false
}

void spin_rw_mutex_v3::internal_downgrade() {
    ITT_NOTIFY(sync_releasing, this);
    __TBB_FetchAndAddW( &state, (intptr_t)(ONE_READER-WRITER));
    __TBB_ASSERT( state & READERS, "invalid state after downgrade: no readers" );
}

void spin_rw_mutex_v3::internal_release_reader()
{
    __TBB_ASSERT( state & READERS, "invalid state of a read lock: no readers" );
    ITT_NOTIFY(sync_releasing, this); // release reader
    __TBB_FetchAndAddWrelease( &state,-(intptr_t)ONE_READER);
}

bool spin_rw_mutex_v3::internal_try_acquire_writer()
{
    // for a writer: only possible to acquire if no active readers or writers
    state_t s = state;
    if( !(s & BUSY) ) // no readers, no writers; mask is 1..1101
        if( CAS(state, WRITER, s)==s ) {
            ITT_NOTIFY(sync_acquired, this);
            return true; // successfully stored writer flag
        }
    return false;
}

bool spin_rw_mutex_v3::internal_try_acquire_reader()
{
    // for a reader: acquire if no active or waiting writers
    state_t s = state;
    if( !(s & (WRITER|WRITER_PENDING)) ) { // no writers
        state_t t = (state_t)__TBB_FetchAndAddW( &state, (intptr_t) ONE_READER );
        if( !( t&WRITER )) {  // got the lock
            ITT_NOTIFY(sync_acquired, this);
            return true; // successfully stored increased number of readers
        }
        // writer got there first, undo the increment
        __TBB_FetchAndAddW( &state, -(intptr_t)ONE_READER );
    }
    return false;
}

void spin_rw_mutex_v3::internal_construct() {
    ITT_SYNC_CREATE(this, _T("tbb::spin_rw_mutex"), _T(""));
}
} // namespace tbb