/*
    Copyright 2005-2015 Intel Corporation.  All Rights Reserved.

    This file is part of Threading Building Blocks. Threading Building Blocks is free software;
    you can redistribute it and/or modify it under the terms of the GNU General Public License
    version 2  as  published  by  the  Free Software Foundation.  Threading Building Blocks is
    distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the
    implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
    See  the GNU General Public License for more details.   You should have received a copy of
    the  GNU General Public License along with Threading Building Blocks; if not, write to the
    Free Software Foundation, Inc.,  51 Franklin St,  Fifth Floor,  Boston,  MA 02110-1301 USA

    As a special exception,  you may use this file  as part of a free software library without
    restriction.  Specifically,  if other files instantiate templates  or use macros or inline
    functions from this file, or you compile this file and link it with other files to produce
    an executable,  this file does not by itself cause the resulting executable to be covered
    by the GNU General Public License. This exception does not however invalidate any other
    reasons why the executable file might be covered by the GNU General Public License.
*/

#include "spin_rw_mutex_v2.h"
#include "tbb/tbb_machine.h"
#include "../tbb/itt_notify.h"
#include "tbb/atomic.h"

namespace tbb {

using namespace internal;

static inline bool CAS(volatile uintptr_t &addr, uintptr_t newv, uintptr_t oldv) {
    return as_atomic(addr).compare_and_swap(newv, oldv) == oldv;
}

//! Signal that write lock is released
void spin_rw_mutex::internal_itt_releasing(spin_rw_mutex *mutex) {
    __TBB_ASSERT_EX(mutex, NULL); // To prevent compiler warnings
    ITT_NOTIFY(sync_releasing, mutex);
}

//! Acquire write (exclusive) lock on the given mutex.
bool spin_rw_mutex::internal_acquire_writer(spin_rw_mutex *mutex)
{
    ITT_NOTIFY(sync_prepare, mutex);
    for( atomic_backoff backoff;;backoff.pause() ) {
        state_t s = mutex->state;
        if( !(s & BUSY) ) { // no readers, no writers
            if( CAS(mutex->state, WRITER, 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(&mutex->state, WRITER_PENDING);
        }
    }
    ITT_NOTIFY(sync_acquired, mutex);
    __TBB_ASSERT( (mutex->state & BUSY)==WRITER, "invalid state of a write lock" );
    return false;
}

//! Release write lock on the given mutex
void spin_rw_mutex::internal_release_writer(spin_rw_mutex *mutex) {
    __TBB_ASSERT( (mutex->state & BUSY)==WRITER, "invalid state of a write lock" );
    ITT_NOTIFY(sync_releasing, mutex);
    mutex->state = 0; 
}

//! Acquire read (shared) lock on the given mutex.
void spin_rw_mutex::internal_acquire_reader(spin_rw_mutex *mutex) {
    ITT_NOTIFY(sync_prepare, mutex);
    for( atomic_backoff backoff;;backoff.pause() ) {
        state_t s = mutex->state;
        if( !(s & (WRITER|WRITER_PENDING)) ) { // no writer or write requests
            if( CAS(mutex->state, s+ONE_READER, s) )
                break; // successfully stored increased number of readers
            backoff.reset(); // we could be very close to complete op.
        }
    }
    ITT_NOTIFY(sync_acquired, mutex);
    __TBB_ASSERT( mutex->state & READERS, "invalid state of a read lock: no readers" );
    __TBB_ASSERT( !(mutex->state & WRITER), "invalid state of a read lock: active writer" );
}

//! Upgrade reader to become a writer.
/** Returns whether the upgrade happened without releasing and re-acquiring the lock */
bool spin_rw_mutex::internal_upgrade(spin_rw_mutex *mutex) {
    state_t s = mutex->state;
    __TBB_ASSERT( s & READERS, "invalid state before upgrade: no readers " );
    __TBB_ASSERT( !(s & WRITER), "invalid state before upgrade: active writer " );
    // 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) ) {
        if( CAS(mutex->state, s | WRITER_PENDING, s) )
        {
            ITT_NOTIFY(sync_prepare, mutex);
            atomic_backoff backoff;
            while( (mutex->state & READERS) != ONE_READER ) backoff.pause();
            __TBB_ASSERT(mutex->state == (ONE_READER | WRITER_PENDING),"invalid state when upgrading to writer");
            // both new readers and writers are blocked at this time
            mutex->state = WRITER;
            ITT_NOTIFY(sync_acquired, mutex);
            __TBB_ASSERT( (mutex->state & BUSY) == WRITER, "invalid state after upgrade" );
            return true; // successfully upgraded
        } else {
            s = mutex->state; // re-read
        }
    }
    // slow reacquire
    internal_release_reader(mutex);
    return internal_acquire_writer(mutex); // always returns false
}

//! Downgrade writer to a reader
void spin_rw_mutex::internal_downgrade(spin_rw_mutex *mutex) {
    __TBB_ASSERT( (mutex->state & BUSY) == WRITER, "invalid state before downgrade" );
    ITT_NOTIFY(sync_releasing, mutex);
    mutex->state = ONE_READER;
    __TBB_ASSERT( mutex->state & READERS, "invalid state after downgrade: no readers" );
    __TBB_ASSERT( !(mutex->state & WRITER), "invalid state after downgrade: active writer" );
}

//! Release read lock on the given mutex
void spin_rw_mutex::internal_release_reader(spin_rw_mutex *mutex)
{
    __TBB_ASSERT( mutex->state & READERS, "invalid state of a read lock: no readers" );
    __TBB_ASSERT( !(mutex->state & WRITER), "invalid state of a read lock: active writer" );
    ITT_NOTIFY(sync_releasing, mutex); // release reader
    __TBB_FetchAndAddWrelease((volatile void *)&(mutex->state),-(intptr_t)ONE_READER);
}

//! Try to acquire write lock on the given mutex
bool spin_rw_mutex::internal_try_acquire_writer( spin_rw_mutex * mutex )
{
    // for a writer: only possible to acquire if no active readers or writers
    state_t s = mutex->state; // on IA-64 architecture, this volatile load has acquire semantic
    if( !(s & BUSY) ) // no readers, no writers; mask is 1..1101
        if( CAS(mutex->state, WRITER, s) ) {
            ITT_NOTIFY(sync_acquired, mutex);
            return true; // successfully stored writer flag
        }
    return false;
}

//! Try to acquire read lock on the given mutex
bool spin_rw_mutex::internal_try_acquire_reader( spin_rw_mutex * mutex )
{
    // for a reader: acquire if no active or waiting writers
    state_t s = mutex->state;    // on IA-64 architecture, a load of volatile variable has acquire semantic
    while( !(s & (WRITER|WRITER_PENDING)) ) // no writers
        if( CAS(mutex->state, s+ONE_READER, s) ) {
            ITT_NOTIFY(sync_acquired, mutex);
            return true; // successfully stored increased number of readers
        }
    return false;
}

} // namespace tbb
