ladybird/Kernel/Lock.cpp
Sergey Bugaev 05ba4295e9 Kernel: Introduce shared locking mode
A Lock can now be held either in shared or exclusive mode. Multiple threads can
hold the same lock in shared mode at one time, but if any thread holds the lock
in exclusive mode, no other thread can hold it at the same time in either mode.
2020-04-18 13:58:29 +02:00

121 lines
4.1 KiB
C++

/*
* Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are met:
*
* 1. Redistributions of source code must retain the above copyright notice, this
* list of conditions and the following disclaimer.
*
* 2. Redistributions in binary form must reproduce the above copyright notice,
* this list of conditions and the following disclaimer in the documentation
* and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
* DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
* SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
* CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
* OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/
#include <Kernel/KSyms.h>
#include <Kernel/Lock.h>
#include <Kernel/Thread.h>
namespace Kernel {
static bool modes_conflict(Lock::Mode mode1, Lock::Mode mode2)
{
if (mode1 == Lock::Mode::Unlocked || mode2 == Lock::Mode::Unlocked)
return false;
if (mode1 == Lock::Mode::Shared && mode2 == Lock::Mode::Shared)
return false;
return true;
}
void Lock::lock(Mode mode)
{
ASSERT(mode != Mode::Unlocked);
ASSERT(!Scheduler::is_active());
if (!are_interrupts_enabled()) {
klog() << "Interrupts disabled when trying to take Lock{" << m_name << "}";
dump_backtrace();
hang();
}
for (;;) {
bool expected = false;
if (m_lock.compare_exchange_strong(expected, true, AK::memory_order_acq_rel)) {
// FIXME: Do not add new readers if writers are queued.
bool modes_dont_conflict = !modes_conflict(m_mode, mode);
bool already_hold_exclusive_lock = m_mode == Mode::Exclusive && m_holder == Thread::current;
if (modes_dont_conflict || already_hold_exclusive_lock) {
// We got the lock!
if (!already_hold_exclusive_lock)
m_mode = mode;
m_holder = Thread::current;
m_times_locked++;
m_lock.store(false, AK::memory_order_release);
return;
}
Thread::current->wait_on(m_queue, &m_lock, m_holder, m_name);
}
}
}
void Lock::unlock()
{
for (;;) {
bool expected = false;
if (m_lock.compare_exchange_strong(expected, true, AK::memory_order_acq_rel)) {
ASSERT(m_times_locked);
--m_times_locked;
ASSERT(m_mode != Mode::Unlocked);
if (m_mode == Mode::Exclusive)
ASSERT(m_holder == Thread::current);
if (m_holder == Thread::current && (m_mode == Mode::Shared || m_times_locked == 0))
m_holder = nullptr;
if (m_times_locked > 0) {
m_lock.store(false, AK::memory_order_release);
return;
}
m_mode = Mode::Unlocked;
m_queue.wake_one(&m_lock);
return;
}
// I don't know *who* is using "m_lock", so just yield.
Scheduler::yield();
}
}
bool Lock::force_unlock_if_locked()
{
ASSERT(m_mode != Mode::Shared);
InterruptDisabler disabler;
if (m_holder != Thread::current)
return false;
ASSERT(m_times_locked == 1);
m_holder = nullptr;
m_mode = Mode::Unlocked;
m_times_locked = 0;
m_queue.wake_one();
return true;
}
void Lock::clear_waiters()
{
ASSERT(m_mode != Mode::Shared);
InterruptDisabler disabler;
m_queue.clear();
}
}