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/*
* 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 <AK/TemporaryChange.h>
#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);
if (!are_interrupts_enabled()) {
klog() << "Interrupts disabled when trying to take Lock{" << m_name << "}";
dump_backtrace();
Processor::halt();
}
auto current_thread = Thread::current();
for (;;) {
bool expected = false;
if (m_lock.compare_exchange_strong(expected, true, AK::memory_order_acq_rel)) {
do {
// 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 == current_thread;
if (modes_dont_conflict || already_hold_exclusive_lock) {
// We got the lock!
if (!already_hold_exclusive_lock)
m_mode = mode;
m_holder = current_thread;
m_times_locked++;
m_lock.store(false, AK::memory_order_release);
return;
}
} while (current_thread->wait_on(m_queue, m_name, nullptr, &m_lock, m_holder) == Thread::BlockResult::NotBlocked);
} else if (Processor::current().in_critical()) {
// If we're in a critical section and trying to lock, no context
// switch will happen, so yield.
// The assumption is that if we call this from a critical section
// that we DO want to temporarily leave it
u32 prev_flags;
u32 prev_crit = Processor::current().clear_critical(prev_flags, !Processor::current().in_irq());
Scheduler::yield();
// Note, we may now be on a different CPU!
Processor::current().restore_critical(prev_crit, prev_flags);
}
}
}
void Lock::unlock()
{
auto current_thread = Thread::current();
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 == current_thread);
if (m_holder == current_thread && (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.
// The assumption is that if we call this from a critical section
// that we DO want to temporarily leave it
u32 prev_flags;
u32 prev_crit = Processor::current().clear_critical(prev_flags, false);
Scheduler::yield();
// Note, we may now be on a different CPU!
Processor::current().restore_critical(prev_crit, prev_flags);
}
}
bool Lock::force_unlock_if_locked()
{
ASSERT(m_mode != Mode::Shared);
ScopedCritical critical;
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);
ScopedCritical critical;
m_queue.clear();
}
}
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