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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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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();
}
}
}