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Kernel: Stop using HashMap in Mutex

Browse source 
This commit removes the usage of HashMap in Mutex, thereby making Mutex
be allocation-free.

In order to achieve this several simplifications were made to Mutex,
removing unused code-paths and extra VERIFYs:
 * We no longer support 'upgrading' a shared lock holder to an
   exclusive holder when it is the only shared holder and it did not
   unlock the lock before relocking it as exclusive. NOTE: Unlike the
   rest of these changes, this scenario is not VERIFY-able in an
   allocation-free way, as a result the new LOCK_SHARED_UPGRADE_DEBUG
   debug flag was added, this flag lets Mutex allocate in order to
   detect such cases when debugging a deadlock.
 * We no longer support checking if a Mutex is locked by the current
   thread when the Mutex was not locked exclusively, the shared version
   of this check was not used anywhere.
 * We no longer support force unlocking/relocking a Mutex if the Mutex
   was not locked exclusively, the shared version of these functions
   was not used anywhere.
This commit is contained in:
Idan Horowitz 2022-01-29 01:47:18 +02:00 committed by Andreas Kling
parent 60aa4152e9
commit e28af4a2fc
9 changed files with 96 additions and 168 deletions
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4
Kernel/Debug.h.in
View file

@ -182,6 +182,10 @@
#cmakedefine01 LOCK_RESTORE_DEBUG
#endif
#ifndef LOCK_SHARED_UPGRADE_DEBUG
#cmakedefine01 LOCK_SHARED_UPGRADE_DEBUG
#endif
#ifndef LOCK_TRACE_DEBUG
#cmakedefine01 LOCK_TRACE_DEBUG
#endif

218
Kernel/Locking/Mutex.cpp
View file

@ -1,5 +1,6 @@
/*
* Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
* Copyright (c) 2022, Idan Horowitz <idan.horowitz@serenityos.org>
*
* SPDX-License-Identifier: BSD-2-Clause
*/
@ -31,12 +32,15 @@ void Mutex::lock(Mode mode, [[maybe_unused]] LockLocation const& location)
dbgln_if(LOCK_TRACE_DEBUG, "Mutex::lock @ ({}) {}: acquire {}, currently unlocked", this, m_name, mode_to_string(mode));
m_mode = mode;
VERIFY(!m_holder);
VERIFY(m_shared_holders.is_empty());
VERIFY(m_shared_holders == 0);
if (mode == Mode::Exclusive) {
m_holder = current_thread;
} else {
VERIFY(mode == Mode::Shared);
m_shared_holders.set(current_thread, 1);
++m_shared_holders;
#if LOCK_SHARED_UPGRADE_DEBUG
m_shared_holders_map.set(current_thread, 1);
#endif
}
VERIFY(m_times_locked == 0);
m_times_locked++;
@ -59,12 +63,11 @@ void Mutex::lock(Mode mode, [[maybe_unused]] LockLocation const& location)
if (m_mode == Mode::Exclusive) {
VERIFY(m_holder == current_thread);
VERIFY(m_shared_holders.is_empty());
VERIFY(m_shared_holders == 0);
} else if (did_block && mode == Mode::Shared) {
// Only if we blocked trying to acquire a shared lock the lock would have been converted
VERIFY(!m_holder);
VERIFY(!m_shared_holders.is_empty());
VERIFY(m_shared_holders.find(current_thread) != m_shared_holders.end());
VERIFY(m_shared_holders > 0);
}
if constexpr (LOCK_TRACE_DEBUG) {
@ -89,19 +92,14 @@ void Mutex::lock(Mode mode, [[maybe_unused]] LockLocation const& location)
case Mode::Shared: {
VERIFY(!m_holder);
if (mode == Mode::Exclusive) {
if (m_shared_holders.size() == 1) {
auto it = m_shared_holders.begin();
if (it->key == current_thread) {
it->value++;
m_times_locked++;
m_mode = Mode::Exclusive;
m_holder = current_thread;
m_shared_holders.clear();
dbgln_if(LOCK_TRACE_DEBUG, "Mutex::lock @ {} ({}): acquire {}, converted shared to exclusive lock, locks held {}", this, m_name, mode_to_string(mode), m_times_locked);
return;
}
}
dbgln_if(LOCK_TRACE_DEBUG, "Mutex::lock @ {} ({}): blocking for exclusive access, currently shared, locks held {}", this, m_name, m_times_locked);
#if LOCK_SHARED_UPGRADE_DEBUG
VERIFY(m_shared_holders_map.size() != 1 || m_shared_holders_map.begin()->key != current_thread);
#endif
// WARNING: The following block will deadlock if the current thread is the only shared locker of this Mutex
// and is asking to upgrade the lock to be exclusive without first releasing the shared lock. We have no
// allocation-free way to detect such a scenario, so if you suspect that this is the cause of your deadlock,
// try turning on LOCK_SHARED_UPGRADE_DEBUG.
block(*current_thread, mode, lock, 1);
did_block = true;
VERIFY(m_mode == mode);
@ -112,23 +110,26 @@ void Mutex::lock(Mode mode, [[maybe_unused]] LockLocation const& location)
VERIFY(m_times_locked > 0);
if (m_mode == Mode::Shared) {
VERIFY(!m_holder);
VERIFY(!did_block || m_shared_holders.contains(current_thread));
VERIFY(!did_block);
} else if (did_block) {
VERIFY(mode == Mode::Exclusive);
VERIFY(m_holder == current_thread);
VERIFY(m_shared_holders.is_empty());
VERIFY(m_shared_holders == 0);
}
if (!did_block) {
// if we didn't block we must still be a shared lock
VERIFY(m_mode == Mode::Shared);
m_times_locked++;
VERIFY(!m_shared_holders.is_empty());
auto it = m_shared_holders.find(current_thread);
if (it != m_shared_holders.end())
VERIFY(m_shared_holders > 0);
++m_shared_holders;
#if LOCK_SHARED_UPGRADE_DEBUG
auto it = m_shared_holders_map.find(current_thread);
if (it != m_shared_holders_map.end())
it->value++;
else
m_shared_holders.set(current_thread, 1);
m_shared_holders_map.set(current_thread, 1);
#endif
}
#if LOCK_DEBUG
@ -166,20 +167,21 @@ void Mutex::unlock()
switch (current_mode) {
case Mode::Exclusive:
VERIFY(m_holder == current_thread);
VERIFY(m_shared_holders.is_empty());
VERIFY(m_shared_holders == 0);
if (m_times_locked == 0)
m_holder = nullptr;
break;
case Mode::Shared: {
VERIFY(!m_holder);
auto it = m_shared_holders.find(current_thread);
VERIFY(it != m_shared_holders.end());
if (it->value > 1) {
VERIFY(m_shared_holders > 0);
--m_shared_holders;
#if LOCK_SHARED_UPGRADE_DEBUG
auto it = m_shared_holders_map.find(current_thread);
if (it->value > 1)
it->value--;
} else {
VERIFY(it->value > 0);
m_shared_holders.remove(it);
}
else
m_shared_holders_map.remove(it);
#endif
break;
}
default:
@ -193,7 +195,7 @@ void Mutex::unlock()
#endif
if (m_times_locked == 0) {
VERIFY(current_mode == Mode::Exclusive ? !m_holder : m_shared_holders.is_empty());
VERIFY(current_mode == Mode::Exclusive ? !m_holder : m_shared_holders == 0);
m_mode = Mode::Unlocked;
unblock_waiters(current_mode);
@ -230,8 +232,11 @@ void Mutex::unblock_waiters(Mode previous_mode)
m_mode = Mode::Shared;
for (auto& thread : m_blocked_threads_list_shared) {
auto requested_locks = thread.unblock_from_lock(*this);
auto set_result = m_shared_holders.set(&thread, requested_locks);
m_shared_holders += requested_locks;
#if LOCK_SHARED_UPGRADE_DEBUG
auto set_result = m_shared_holders_map.set(&thread, requested_locks);
VERIFY(set_result == AK::HashSetResult::InsertedNewEntry);
#endif
m_times_locked += requested_locks;
}
return true;
@ -255,7 +260,7 @@ void Mutex::unblock_waiters(Mode previous_mode)
}
}
auto Mutex::force_unlock_if_locked(u32& lock_count_to_restore) -> Mode
auto Mutex::force_unlock_exclusive_if_locked(u32& lock_count_to_restore) -> Mode
{
// NOTE: This may be called from an interrupt handler (not an IRQ handler)
// and also from within critical sections!
@ -270,7 +275,7 @@ auto Mutex::force_unlock_if_locked(u32& lock_count_to_restore) -> Mode
return Mode::Unlocked;
}
dbgln_if(LOCK_RESTORE_DEBUG, "Mutex::force_unlock_if_locked @ {}: unlocking exclusive with lock count: {}", this, m_times_locked);
dbgln_if(LOCK_RESTORE_DEBUG, "Mutex::force_unlock_exclusive_if_locked @ {}: unlocking exclusive with lock count: {}", this, m_times_locked);
#if LOCK_DEBUG
m_holder->holding_lock(*this, -(int)m_times_locked, {});
#endif
@ -282,32 +287,6 @@ auto Mutex::force_unlock_if_locked(u32& lock_count_to_restore) -> Mode
unblock_waiters(Mode::Exclusive);
break;
}
case Mode::Shared: {
VERIFY(!m_holder);
auto it = m_shared_holders.find(current_thread);
if (it == m_shared_holders.end()) {
lock_count_to_restore = 0;
return Mode::Unlocked;
}
dbgln_if(LOCK_RESTORE_DEBUG, "Mutex::force_unlock_if_locked @ {}: unlocking exclusive with lock count: {}, total locks: {}",
this, it->value, m_times_locked);
VERIFY(it->value > 0);
lock_count_to_restore = it->value;
VERIFY(lock_count_to_restore > 0);
#if LOCK_DEBUG
m_holder->holding_lock(*this, -(int)lock_count_to_restore, {});
#endif
m_shared_holders.remove(it);
VERIFY(m_times_locked >= lock_count_to_restore);
m_times_locked -= lock_count_to_restore;
if (m_times_locked == 0) {
m_mode = Mode::Unlocked;
unblock_waiters(Mode::Shared);
}
break;
}
case Mode::Unlocked: {
lock_count_to_restore = 0;
break;
@ -318,107 +297,46 @@ auto Mutex::force_unlock_if_locked(u32& lock_count_to_restore) -> Mode
return current_mode;
}
void Mutex::restore_lock(Mode mode, u32 lock_count, [[maybe_unused]] LockLocation const& location)
void Mutex::restore_exclusive_lock(u32 lock_count, [[maybe_unused]] LockLocation const& location)
{
VERIFY(mode != Mode::Unlocked);
VERIFY(lock_count > 0);
VERIFY(!Processor::current_in_irq());
auto* current_thread = Thread::current();
bool did_block = false;
SpinlockLocker lock(m_lock);
switch (mode) {
case Mode::Exclusive: {
auto previous_mode = m_mode;
if ((m_mode == Mode::Exclusive && m_holder != current_thread)
|| (m_mode == Mode::Shared && (m_shared_holders.size() != 1 || !m_shared_holders.contains(current_thread)))) {
block(*current_thread, Mode::Exclusive, lock, lock_count);
did_block = true;
// If we blocked then m_mode should have been updated to what we requested
[[maybe_unused]] auto previous_mode = m_mode;
if (m_mode == Mode::Exclusive && m_holder != current_thread) {
block(*current_thread, Mode::Exclusive, lock, lock_count);
did_block = true;
// If we blocked then m_mode should have been updated to what we requested
VERIFY(m_mode == Mode::Exclusive);
}
dbgln_if(LOCK_RESTORE_DEBUG, "Mutex::restore_exclusive_lock @ {}: restoring exclusive with lock count {}, was {}", this, lock_count, mode_to_string(previous_mode));
VERIFY(m_mode != Mode::Shared);
VERIFY(m_shared_holders == 0);
if (did_block) {
VERIFY(m_times_locked > 0);
VERIFY(m_holder == current_thread);
} else {
if (m_mode == Mode::Unlocked) {
m_mode = Mode::Exclusive;
VERIFY(m_times_locked == 0);
m_times_locked = lock_count;
VERIFY(!m_holder);
m_holder = current_thread;
} else {
VERIFY(m_mode == Mode::Exclusive);
}
dbgln_if(LOCK_RESTORE_DEBUG, "Mutex::restore_lock @ {}: restoring {} with lock count {}, was {}", this, mode_to_string(mode), lock_count, mode_to_string(previous_mode));
VERIFY(m_mode != Mode::Shared);
VERIFY(m_shared_holders.is_empty());
if (did_block) {
VERIFY(m_times_locked > 0);
VERIFY(m_holder == current_thread);
} else {
if (m_mode == Mode::Unlocked) {
m_mode = Mode::Exclusive;
VERIFY(m_times_locked == 0);
m_times_locked = lock_count;
VERIFY(!m_holder);
m_holder = current_thread;
} else if (m_mode == Mode::Shared) {
// Upgrade the shared lock to an exclusive lock
VERIFY(!m_holder);
VERIFY(m_shared_holders.size() == 1);
VERIFY(m_shared_holders.contains(current_thread));
m_mode = Mode::Exclusive;
m_holder = current_thread;
m_shared_holders.clear();
} else {
VERIFY(m_mode == Mode::Exclusive);
VERIFY(m_holder == current_thread);
VERIFY(m_times_locked > 0);
m_times_locked += lock_count;
}
}
#if LOCK_DEBUG
m_holder->holding_lock(*this, (int)lock_count, location);
#endif
return;
}
case Mode::Shared: {
auto previous_mode = m_mode;
if (m_mode == Mode::Exclusive && m_holder != current_thread) {
block(*current_thread, Mode::Shared, lock, lock_count);
did_block = true;
// If we blocked then m_mode should have been updated to what we requested
VERIFY(m_mode == Mode::Shared);
}
dbgln_if(LOCK_RESTORE_DEBUG, "Mutex::restore_lock @ {}: restoring {} with lock count {}, was {}",
this, mode_to_string(mode), lock_count, mode_to_string(previous_mode));
VERIFY(!m_holder);
if (did_block) {
VERIFY(m_times_locked > 0);
VERIFY(m_shared_holders.contains(current_thread));
} else {
if (m_mode == Mode::Unlocked) {
m_mode = Mode::Shared;
m_times_locked += lock_count;
auto set_result = m_shared_holders.set(current_thread, lock_count);
// There may be other shared lock holders already, but we should not have an entry yet
VERIFY(set_result == AK::HashSetResult::InsertedNewEntry);
} else if (m_mode == Mode::Shared) {
m_times_locked += lock_count;
if (auto it = m_shared_holders.find(current_thread); it != m_shared_holders.end()) {
it->value += lock_count;
} else {
auto set_result = m_shared_holders.set(current_thread, lock_count);
// There may be other shared lock holders already, but we should not have an entry yet
VERIFY(set_result == AK::HashSetResult::InsertedNewEntry);
}
} else {
VERIFY(m_mode == Mode::Exclusive);
VERIFY(m_holder == current_thread);
m_times_locked += lock_count;
}
m_times_locked += lock_count;
}
}
#if LOCK_DEBUG
m_holder->holding_lock(*this, (int)lock_count, location);
m_holder->holding_lock(*this, (int)lock_count, location);
#endif
return;
}
default:
VERIFY_NOT_REACHED();
}
}
}

23
Kernel/Locking/Mutex.h
View file

@ -1,5 +1,6 @@
/*
* Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
* Copyright (c) 2022, Idan Horowitz <idan.horowitz@serenityos.org>
*
* SPDX-License-Identifier: BSD-2-Clause
*/
@ -33,23 +34,23 @@ public:
~Mutex() = default;
void lock(Mode mode = Mode::Exclusive, LockLocation const& location = LockLocation::current());
void restore_lock(Mode, u32, LockLocation const& location = LockLocation::current());
void restore_exclusive_lock(u32, LockLocation const& location = LockLocation::current());
void unlock();
[[nodiscard]] Mode force_unlock_if_locked(u32&);
[[nodiscard]] Mode force_unlock_exclusive_if_locked(u32&);
[[nodiscard]] bool is_locked() const
{
SpinlockLocker lock(m_lock);
return m_mode != Mode::Unlocked;
}
[[nodiscard]] bool is_locked_by_current_thread() const
[[nodiscard]] bool is_exclusively_locked_by_current_thread() const
{
SpinlockLocker lock(m_lock);
if (m_mode == Mode::Exclusive)
return m_holder == Thread::current();
if (m_mode == Mode::Shared)
return m_shared_holders.contains(Thread::current());
return false;
VERIFY(m_mode != Mode::Shared); // This method should only be used on exclusively-held locks
if (m_mode == Mode::Unlocked)
return false;
return m_holder == Thread::current();
}
[[nodiscard]] StringView name() const { return m_name; }
@ -93,12 +94,16 @@ private:
// When locked exclusively, this is always the one thread that holds the
// lock.
RefPtr<Thread> m_holder;
HashMap<Thread*, u32> m_shared_holders;
size_t m_shared_holders { 0 };
BlockedThreadList m_blocked_threads_list_exclusive;
BlockedThreadList m_blocked_threads_list_shared;
mutable Spinlock m_lock;
#if LOCK_SHARED_UPGRADE_DEBUG
HashMap<Thread*, u32> m_shared_holders_map;
#endif
};
class MutexLocker {

2
Kernel/Net/Socket.h
View file

@ -132,7 +132,7 @@ protected:
ErrorOr<void> so_error() const
{
VERIFY(m_mutex.is_locked_by_current_thread());
VERIFY(m_mutex.is_exclusively_locked_by_current_thread());
return m_so_error;
}

2
Kernel/Process.cpp
View file

@ -105,7 +105,7 @@ void Process::kill_threads_except_self()
});
u32 dropped_lock_count = 0;
if (big_lock().force_unlock_if_locked(dropped_lock_count) != LockMode::Unlocked)
if (big_lock().force_unlock_exclusive_if_locked(dropped_lock_count) != LockMode::Unlocked)
dbgln("Process {} big lock had {} locks", *this, dropped_lock_count);
}

4
Kernel/Process.h
View file

@ -978,10 +978,10 @@ inline ProcessID Thread::pid() const
}
#define VERIFY_PROCESS_BIG_LOCK_ACQUIRED(process) \
VERIFY(process->big_lock().is_locked_by_current_thread());
VERIFY(process->big_lock().is_exclusively_locked_by_current_thread());
#define VERIFY_NO_PROCESS_BIG_LOCK(process) \
VERIFY(!process->big_lock().is_locked_by_current_thread());
VERIFY(!process->big_lock().is_exclusively_locked_by_current_thread());
inline static ErrorOr<NonnullOwnPtr<KString>> try_copy_kstring_from_user(const Kernel::Syscall::StringArgument& string)
{

2
Kernel/Syscalls/execve.cpp
View file

@ -630,7 +630,7 @@ ErrorOr<void> Process::do_exec(NonnullRefPtr<OpenFileDescription> main_program_d
}
u32 lock_count_to_restore;
[[maybe_unused]] auto rc = big_lock().force_unlock_if_locked(lock_count_to_restore);
[[maybe_unused]] auto rc = big_lock().force_unlock_exclusive_if_locked(lock_count_to_restore);
VERIFY_INTERRUPTS_DISABLED();
VERIFY(Processor::in_critical());
return {};

8
Kernel/Thread.cpp
View file

@ -193,7 +193,7 @@ void Thread::block(Kernel::Mutex& lock, SpinlockLocker<Spinlock>& lock_lock, u32
// Yield to the scheduler, and wait for us to resume unblocked.
VERIFY(!g_scheduler_lock.is_locked_by_current_processor());
VERIFY(Processor::in_critical());
if (&lock != &big_lock && big_lock.is_locked_by_current_thread()) {
if (&lock != &big_lock && big_lock.is_exclusively_locked_by_current_thread()) {
// We're locking another lock and already hold the big lock...
// We need to release the big lock
yield_and_release_relock_big_lock();
@ -390,7 +390,7 @@ void Thread::exit(void* exit_value)
void Thread::yield_without_releasing_big_lock(VerifyLockNotHeld verify_lock_not_held)
{
VERIFY(!g_scheduler_lock.is_locked_by_current_processor());
VERIFY(verify_lock_not_held == VerifyLockNotHeld::No || !process().big_lock().is_locked_by_current_thread());
VERIFY(verify_lock_not_held == VerifyLockNotHeld::No || !process().big_lock().is_exclusively_locked_by_current_thread());
// Disable interrupts here. This ensures we don't accidentally switch contexts twice
InterruptDisabler disable;
Scheduler::yield(); // flag a switch
@ -415,7 +415,7 @@ void Thread::yield_and_release_relock_big_lock()
LockMode Thread::unlock_process_if_locked(u32& lock_count_to_restore)
{
return process().big_lock().force_unlock_if_locked(lock_count_to_restore);
return process().big_lock().force_unlock_exclusive_if_locked(lock_count_to_restore);
}
void Thread::relock_process(LockMode previous_locked, u32 lock_count_to_restore)
@ -433,7 +433,7 @@ void Thread::relock_process(LockMode previous_locked, u32 lock_count_to_restore)
if (previous_locked != LockMode::Unlocked) {
// We've unblocked, relock the process if needed and carry on.
process().big_lock().restore_lock(previous_locked, lock_count_to_restore);
process().big_lock().restore_exclusive_lock(lock_count_to_restore);
}
}

1
Meta/CMake/all_the_debug_macros.cmake
View file

@ -106,6 +106,7 @@ set(LOCK_DEBUG ON)
set(LOCK_IN_CRITICAL_DEBUG ON)
set(LOCK_RANK_ENFORCEMENT ON)
set(LOCK_RESTORE_DEBUG ON)
set(LOCK_SHARED_UPGRADE_DEBUG ON)
set(LOCK_TRACE_DEBUG ON)
set(LOOKUPSERVER_DEBUG ON)
set(MALLOC_DEBUG ON)