mariadb/util/frwlock.cc

/* -*- mode: C++; c-basic-offset: 4; indent-tabs-mode: nil -*- */
// vim: ft=cpp:expandtab:ts=8:sw=4:softtabstop=4:
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  TokuDB, Tokutek Fractal Tree Indexing Library.
  Copyright (C) 2007-2013 Tokutek, Inc.

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  The technology is licensed by the Massachusetts Institute of
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  Foundation of State University of New York at Stony Brook under
  United States of America Serial No. 11/760379 and to the patents
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  This software is covered by US Patent No. 8,185,551.
  This software is covered by US Patent No. 8,489,638.

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#ident "Copyright (c) 2007-2013 Tokutek Inc.  All rights reserved."
#ident "The technology is licensed by the Massachusetts Institute of Technology, Rutgers State University of New Jersey, and the Research Foundation of State University of New York at Stony Brook under United States of America Serial No. 11/760379 and to the patents and/or patent applications resulting from it."

#include <toku_assert.h>

#include <util/context.h>
#include <util/frwlock.h>

namespace toku {

static __thread int thread_local_tid = -1;
static int get_local_tid() {
    if (thread_local_tid == -1) {
        thread_local_tid = toku_os_gettid();
    }
    return thread_local_tid;
}

void frwlock::init(toku_mutex_t *const mutex) {
    m_mutex = mutex;

    m_num_readers = 0;
    m_num_writers = 0;
    m_num_want_write = 0;
    m_num_want_read = 0;
    m_num_signaled_readers = 0;
    m_num_expensive_want_write = 0;
    
    toku_cond_init(&m_wait_read, nullptr);
    m_queue_item_read.cond = &m_wait_read;
    m_queue_item_read.next = nullptr;
    m_wait_read_is_in_queue = false;
    m_current_writer_expensive = false;
    m_read_wait_expensive = false;
    m_current_writer_tid = -1;
    m_blocking_writer_context_id = CTX_INVALID;

    m_wait_head = nullptr;
    m_wait_tail = nullptr;
}

void frwlock::deinit(void) {
    toku_cond_destroy(&m_wait_read);
}

bool frwlock::queue_is_empty(void) const {
    return m_wait_head == nullptr;
}

void frwlock::enq_item(queue_item *const item) {
    paranoid_invariant_null(item->next);
    if (m_wait_tail != nullptr) {
        m_wait_tail->next = item;
    } else {
        paranoid_invariant_null(m_wait_head);
        m_wait_head = item;
    }
    m_wait_tail = item;
}

toku_cond_t *frwlock::deq_item(void) {
    paranoid_invariant_notnull(m_wait_head);
    paranoid_invariant_notnull(m_wait_tail);
    queue_item *item = m_wait_head;
    m_wait_head = m_wait_head->next;
    if (m_wait_tail == item) {
        m_wait_tail = nullptr;
    }
    return item->cond;
}

// Prerequisite: Holds m_mutex.
void frwlock::write_lock(bool expensive) {
    toku_mutex_assert_locked(m_mutex);
    if (this->try_write_lock(expensive)) {
        return;
    }

    toku_cond_t cond = TOKU_COND_INITIALIZER;
    queue_item item = { .cond = &cond, .next = nullptr };
    this->enq_item(&item);

    // Wait for our turn.
    ++m_num_want_write;
    if (expensive) {
        ++m_num_expensive_want_write;
    }
    if (m_num_writers == 0 && m_num_want_write == 1) {
        // We are the first to want a write lock. No new readers can get the lock.
        // Set our thread id and context for proper instrumentation.
        // see: toku_context_note_frwlock_contention()
        m_current_writer_tid = get_local_tid();
        m_blocking_writer_context_id = toku_thread_get_context()->get_id();
    }
    toku_cond_wait(&cond, m_mutex);
    toku_cond_destroy(&cond);

    // Now it's our turn.
    paranoid_invariant(m_num_want_write > 0);
    paranoid_invariant_zero(m_num_readers);
    paranoid_invariant_zero(m_num_writers);
    paranoid_invariant_zero(m_num_signaled_readers);

    // Not waiting anymore; grab the lock.
    --m_num_want_write;
    if (expensive) {
        --m_num_expensive_want_write;
    }
    m_num_writers = 1;
    m_current_writer_expensive = expensive;
    m_current_writer_tid = get_local_tid();
    m_blocking_writer_context_id = toku_thread_get_context()->get_id();
}

bool frwlock::try_write_lock(bool expensive) {
    toku_mutex_assert_locked(m_mutex);
    if (m_num_readers > 0 || m_num_writers > 0 || m_num_signaled_readers > 0 || m_num_want_write > 0) {
        return false;
    }
    // No one holds the lock.  Grant the write lock.
    paranoid_invariant_zero(m_num_want_write);
    paranoid_invariant_zero(m_num_want_read);
    m_num_writers = 1;
    m_current_writer_expensive = expensive;
    m_current_writer_tid = get_local_tid();
    m_blocking_writer_context_id = toku_thread_get_context()->get_id();
    return true;
}

void frwlock::read_lock(void) {
    toku_mutex_assert_locked(m_mutex);
    if (m_num_writers > 0 || m_num_want_write > 0) {
        if (!m_wait_read_is_in_queue) {
            // Throw the read cond_t onto the queue.
            paranoid_invariant(m_num_signaled_readers == m_num_want_read);
            m_queue_item_read.next = nullptr;
            this->enq_item(&m_queue_item_read);
            m_wait_read_is_in_queue = true;
            paranoid_invariant(!m_read_wait_expensive);
            m_read_wait_expensive = (
                m_current_writer_expensive || 
                (m_num_expensive_want_write > 0)
                );
        }

        // Note this contention event in engine status.
        toku_context_note_frwlock_contention(
            toku_thread_get_context()->get_id(),
            m_blocking_writer_context_id
            );

        // Wait for our turn.
        ++m_num_want_read;
        toku_cond_wait(&m_wait_read, m_mutex);

        // Now it's our turn.
        paranoid_invariant_zero(m_num_writers);
        paranoid_invariant(m_num_want_read > 0);
        paranoid_invariant(m_num_signaled_readers > 0);

        // Not waiting anymore; grab the lock.
        --m_num_want_read;
        --m_num_signaled_readers;
    }
    ++m_num_readers;
}

bool frwlock::try_read_lock(void) {
    toku_mutex_assert_locked(m_mutex);
    if (m_num_writers > 0 || m_num_want_write > 0) {
        return false;
    }
    // No writer holds the lock.
    // No writers are waiting.
    // Grant the read lock.
    ++m_num_readers;
    return true;
}

void frwlock::maybe_signal_next_writer(void) {
    if (m_num_want_write > 0 && m_num_signaled_readers == 0 && m_num_readers == 0) {
        toku_cond_t *cond = this->deq_item();
        paranoid_invariant(cond != &m_wait_read);
        // Grant write lock to waiting writer.
        paranoid_invariant(m_num_want_write > 0);
        toku_cond_signal(cond);
    }
}

void frwlock::read_unlock(void) {
    toku_mutex_assert_locked(m_mutex);
    paranoid_invariant(m_num_writers == 0);
    paranoid_invariant(m_num_readers > 0);
    --m_num_readers;
    this->maybe_signal_next_writer();
}

bool frwlock::read_lock_is_expensive(void) {
    toku_mutex_assert_locked(m_mutex);
    if (m_wait_read_is_in_queue) {
        return m_read_wait_expensive;
    }
    else {
        return m_current_writer_expensive || (m_num_expensive_want_write > 0);
    }
}


void frwlock::maybe_signal_or_broadcast_next(void) {
    paranoid_invariant(m_num_signaled_readers == 0);

    if (this->queue_is_empty()) {
        paranoid_invariant(m_num_want_write == 0);
        paranoid_invariant(m_num_want_read == 0);
        return;
    }
    toku_cond_t *cond = this->deq_item();
    if (cond == &m_wait_read) {
        // Grant read locks to all waiting readers
        paranoid_invariant(m_wait_read_is_in_queue);
        paranoid_invariant(m_num_want_read > 0);
        m_num_signaled_readers = m_num_want_read;
        m_wait_read_is_in_queue = false;
        m_read_wait_expensive = false;
        toku_cond_broadcast(cond);
    }
    else {
        // Grant write lock to waiting writer.
        paranoid_invariant(m_num_want_write > 0);
        toku_cond_signal(cond);
    }
}

void frwlock::write_unlock(void) {
    toku_mutex_assert_locked(m_mutex);
    paranoid_invariant(m_num_writers == 1);
    m_num_writers = 0;
    m_current_writer_expensive = false;
    m_current_writer_tid = -1;
    m_blocking_writer_context_id = CTX_INVALID;
    this->maybe_signal_or_broadcast_next();
}
bool frwlock::write_lock_is_expensive(void) {
    toku_mutex_assert_locked(m_mutex);
    return (m_num_expensive_want_write > 0) || (m_current_writer_expensive);
}


uint32_t frwlock::users(void) const {
    toku_mutex_assert_locked(m_mutex);
    return m_num_readers + m_num_writers + m_num_want_read + m_num_want_write;
}
uint32_t frwlock::blocked_users(void) const {
    toku_mutex_assert_locked(m_mutex);
    return m_num_want_read + m_num_want_write;
}
uint32_t frwlock::writers(void) const {
    // this is sometimes called as "assert(lock->writers())" when we
    // assume we have the write lock.  if that's the assumption, we may
    // not own the mutex, so we don't assert_locked here
    return m_num_writers;
}
uint32_t frwlock::blocked_writers(void) const {
    toku_mutex_assert_locked(m_mutex);
    return m_num_want_write;
}
uint32_t frwlock::readers(void) const {
    toku_mutex_assert_locked(m_mutex);
    return m_num_readers;
}
uint32_t frwlock::blocked_readers(void) const {
    toku_mutex_assert_locked(m_mutex);
    return m_num_want_read;
}

} // namespace toku