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Kernel: Don't ref/unref the holder thread in Mutex

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There was a whole bunch of ref counting churn coming from Mutex, which
had a RefPtr<Thread> m_holder to (mostly) point at the thread holding
the mutex.

Since we never actually dereference the m_holder value, but only use it
for identity checks against thread pointers, we can store it as an
uintptr_t and skip the ref counting entirely.

Threads can't die while holding a mutex anyway, so there's no risk of
them going missing on us.
This commit is contained in:
Andreas Kling 2023-04-02 21:36:39 +02:00
parent c3915e4058
commit ed1253ab90
Notes: sideshowbarker 2024-07-17 07:20:57 +09:00 
Author: https://github.com/awesomekling
Commit: https://github.com/SerenityOS/serenity/commit/ed1253ab90
Pull-request: https://github.com/SerenityOS/serenity/pull/18163
Reviewed-by: https://github.com/Hendiadyoin1
2 changed files with 26 additions and 31 deletions
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44
Kernel/Locking/Mutex.cpp
View file

@ -39,12 +39,12 @@ void Mutex::lock(Mode mode, [[maybe_unused]] LockLocation const& location)
VERIFY(!m_holder);
VERIFY(m_shared_holders == 0);
if (mode == Mode::Exclusive) {
m_holder = current_thread;
m_holder = bit_cast<uintptr_t>(current_thread);
} else {
VERIFY(mode == Mode::Shared);
++m_shared_holders;
#if LOCK_SHARED_UPGRADE_DEBUG
m_shared_holders_map.set(current_thread, 1);
m_shared_holders_map.set(bit_cast<uintptr_t>(current_thread), 1);
#endif
}
VERIFY(m_times_locked == 0);
@ -59,7 +59,7 @@ void Mutex::lock(Mode mode, [[maybe_unused]] LockLocation const& location)
}
case Mode::Exclusive: {
VERIFY(m_holder);
if (m_holder != current_thread) {
if (m_holder != bit_cast<uintptr_t>(current_thread)) {
block(*current_thread, mode, lock, 1);
did_block = true;
// If we blocked then m_mode should have been updated to what we requested
@ -67,7 +67,7 @@ void Mutex::lock(Mode mode, [[maybe_unused]] LockLocation const& location)
}
if (m_mode == Mode::Exclusive) {
VERIFY(m_holder == current_thread);
VERIFY(m_holder == bit_cast<uintptr_t>(current_thread));
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
@ -100,7 +100,7 @@ void Mutex::lock(Mode mode, [[maybe_unused]] LockLocation const& location)
if (mode == Mode::Exclusive) {
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);
VERIFY(m_shared_holders_map.size() != 1 || m_shared_holders_map.begin()->key != bit_cast<uintptr_t>(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
@ -119,7 +119,7 @@ void Mutex::lock(Mode mode, [[maybe_unused]] LockLocation const& location)
VERIFY(!did_block);
} else if (did_block) {
VERIFY(mode == Mode::Exclusive);
VERIFY(m_holder == current_thread);
VERIFY(m_holder == bit_cast<uintptr_t>(current_thread));
VERIFY(m_shared_holders == 0);
}
@ -130,11 +130,7 @@ void Mutex::lock(Mode mode, [[maybe_unused]] LockLocation const& location)
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_map.set(current_thread, 1);
m_shared_holders_map.ensure(bit_cast<uintptr_t>(current_thread), [] { return 0; })++;
#endif
}
@ -175,17 +171,17 @@ void Mutex::unlock()
switch (current_mode) {
case Mode::Exclusive:
VERIFY(m_holder == current_thread);
VERIFY(m_holder == bit_cast<uintptr_t>(current_thread));
VERIFY(m_shared_holders == 0);
if (m_times_locked == 0)
m_holder = nullptr;
m_holder = 0;
break;
case Mode::Shared: {
VERIFY(!m_holder);
VERIFY(m_shared_holders > 0);
--m_shared_holders;
#if LOCK_SHARED_UPGRADE_DEBUG
auto it = m_shared_holders_map.find(current_thread);
auto it = m_shared_holders_map.find(bit_cast<uintptr_t>(current_thread));
if (it->value > 1)
it->value--;
else
@ -262,7 +258,7 @@ void Mutex::unblock_waiters(Mode previous_mode)
auto requested_locks = thread.unblock_from_mutex(*this);
m_shared_holders += requested_locks;
#if LOCK_SHARED_UPGRADE_DEBUG
auto set_result = m_shared_holders_map.set(&thread, requested_locks);
auto set_result = m_shared_holders_map.set(bit_cast<uintptr_t>(&thread), requested_locks);
VERIFY(set_result == AK::HashSetResult::InsertedNewEntry);
#endif
m_times_locked += requested_locks;
@ -273,7 +269,7 @@ void Mutex::unblock_waiters(Mode previous_mode)
if (auto* next_exclusive_thread = list.first()) {
m_mode = Mode::Exclusive;
m_times_locked = next_exclusive_thread->unblock_from_mutex(*this);
m_holder = next_exclusive_thread;
m_holder = bit_cast<uintptr_t>(next_exclusive_thread);
return true;
}
return false;
@ -303,16 +299,16 @@ auto Mutex::force_unlock_exclusive_if_locked(u32& lock_count_to_restore) -> Mode
auto current_mode = m_mode;
switch (current_mode) {
case Mode::Exclusive: {
if (m_holder != current_thread) {
if (m_holder != bit_cast<uintptr_t>(current_thread)) {
lock_count_to_restore = 0;
return Mode::Unlocked;
}
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, {});
current_thread->holding_lock(*this, -(int)m_times_locked, {});
#endif
m_holder = nullptr;
m_holder = 0;
VERIFY(m_times_locked > 0);
lock_count_to_restore = m_times_locked;
m_times_locked = 0;
@ -340,7 +336,7 @@ void Mutex::restore_exclusive_lock(u32 lock_count, [[maybe_unused]] LockLocation
bool did_block = false;
SpinlockLocker lock(m_lock);
[[maybe_unused]] auto previous_mode = m_mode;
if (m_mode == Mode::Exclusive && m_holder != current_thread) {
if (m_mode == Mode::Exclusive && m_holder != bit_cast<uintptr_t>(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
@ -353,24 +349,24 @@ void Mutex::restore_exclusive_lock(u32 lock_count, [[maybe_unused]] LockLocation
VERIFY(m_shared_holders == 0);
if (did_block) {
VERIFY(m_times_locked > 0);
VERIFY(m_holder == current_thread);
VERIFY(m_holder == bit_cast<uintptr_t>(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;
m_holder = bit_cast<uintptr_t>(current_thread);
} else {
VERIFY(m_mode == Mode::Exclusive);
VERIFY(m_holder == current_thread);
VERIFY(m_holder == bit_cast<uintptr_t>(current_thread));
VERIFY(m_times_locked > 0);
m_times_locked += lock_count;
}
}
#if LOCK_DEBUG
m_holder->holding_lock(*this, (int)lock_count, location);
current_thread->holding_lock(*this, (int)lock_count, location);
#endif
}

13
Kernel/Locking/Mutex.h
View file

@ -57,7 +57,7 @@ public:
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();
return m_holder == bit_cast<uintptr_t>(Thread::current());
}
[[nodiscard]] StringView name() const { return m_name; }
@ -96,12 +96,11 @@ private:
// lock it again. When locked in shared mode, any thread can do that.
u32 m_times_locked { 0 };
// One of the threads that hold this lock, or nullptr. When locked in shared
// mode, this is stored on best effort basis: nullptr value does *not* mean
// The address of one of the threads that hold this lock, or 0.
// When locked in shared mode, this is stored on best effort basis: 0 does *not* mean
// the lock is unlocked, it just means we don't know which threads hold it.
// When locked exclusively, this is always the one thread that holds the
// lock.
RefPtr<Thread> m_holder;
// When locked exclusively, this is always the one thread that holds the lock.
uintptr_t m_holder { 0 };
size_t m_shared_holders { 0 };
struct BlockedThreadLists {
@ -124,7 +123,7 @@ private:
mutable Spinlock<LockRank::None> m_lock {};
#if LOCK_SHARED_UPGRADE_DEBUG
HashMap<Thread*, u32> m_shared_holders_map;
HashMap<uintptr_t, u32> m_shared_holders_map;
#endif
};