Kernel: Allow Lock to block from BlockCondition

This enables the Lock class to block a thread even while the thread is
working on a BlockCondition. A thread can still only be either blocked
by a Lock or a BlockCondition.

This also establishes a linked list of threads that are blocked by a
Lock and unblocking directly unlocks threads and wakes them directly.

Kernel/Lock.cpp

@@ -8,6 +8,7 @@
 #include <Kernel/Debug.h>
 #include <Kernel/KSyms.h>
 #include <Kernel/Lock.h>
+#include <Kernel/SpinLock.h>
 #include <Kernel/Thread.h>
 
 namespace Kernel {
@@ -23,71 +24,83 @@ void Lock::lock(Mode mode)
     VERIFY(!Processor::current().in_irq());
     VERIFY(mode != Mode::Unlocked);
     auto current_thread = Thread::current();
-    ScopedCritical critical; // in case we're not in a critical section already
-    for (;;) {
-        if (m_lock.exchange(true, AK::memory_order_acq_rel) != false) {
-            // I don't know *who* is using "m_lock", so just yield.
-            Scheduler::yield_from_critical();
-            continue;
-        }
 
-        // FIXME: Do not add new readers if writers are queued.
-        Mode current_mode = m_mode;
-        switch (current_mode) {
-        case Mode::Unlocked: {
-            dbgln_if(LOCK_TRACE_DEBUG, "Lock::lock @ ({}) {}: acquire {}, currently unlocked", this, m_name, mode_to_string(mode));
-            m_mode = mode;
-            VERIFY(!m_holder);
-            VERIFY(m_shared_holders.is_empty());
-            if (mode == Mode::Exclusive) {
-                m_holder = current_thread;
-            } else {
-                VERIFY(mode == Mode::Shared);
-                m_shared_holders.set(current_thread, 1);
-            }
-            VERIFY(m_times_locked == 0);
-            m_times_locked++;
-
-#if LOCK_DEBUG
-            if (current_thread) {
-                current_thread->holding_lock(*this, 1, location);
-            }
-#endif
-            m_queue.should_block(true);
-            m_lock.store(false, AK::memory_order_release);
-            return;
-        }
-        case Mode::Exclusive: {
-            VERIFY(m_holder);
-            if (m_holder != current_thread)
-                break;
-            VERIFY(m_shared_holders.is_empty());
-
-            if constexpr (LOCK_TRACE_DEBUG) {
-                if (mode == Mode::Exclusive)
-                    dbgln("Lock::lock @ {} ({}): acquire {}, currently exclusive, holding: {}", this, m_name, mode_to_string(mode), m_times_locked);
-                else
-                    dbgln("Lock::lock @ {} ({}): acquire exclusive (requested {}), currently exclusive, holding: {}", this, m_name, mode_to_string(mode), m_times_locked);
-            }
-
-            VERIFY(mode == Mode::Exclusive || mode == Mode::Shared);
-            VERIFY(m_times_locked > 0);
-            m_times_locked++;
+    ScopedSpinLock lock(m_lock);
+    bool did_block = false;
+    Mode current_mode = m_mode;
+    switch (current_mode) {
+    case Mode::Unlocked: {
+        dbgln_if(LOCK_TRACE_DEBUG, "Lock::lock @ ({}) {}: acquire {}, currently unlocked", this, m_name, mode_to_string(mode));
+        m_mode = mode;
+        VERIFY(!m_holder);
+        VERIFY(m_shared_holders.is_empty());
+        if (mode == Mode::Exclusive) {
+            m_holder = current_thread;
+        } else {
+            VERIFY(mode == Mode::Shared);
+            m_shared_holders.set(current_thread, 1);
+        }
+        VERIFY(m_times_locked == 0);
+        m_times_locked++;
 
 #if LOCK_DEBUG
+        if (current_thread) {
             current_thread->holding_lock(*this, 1, location);
-#endif
-            m_lock.store(false, AK::memory_order_release);
-            return;
         }
-        case Mode::Shared: {
-            VERIFY(!m_holder);
-            if (mode != Mode::Shared)
-                break;
+#endif
+        return;
+    }
+    case Mode::Exclusive: {
+        VERIFY(m_holder);
+        if (m_holder != current_thread) {
+            block(*current_thread, mode, lock, 1);
+            did_block = true;
+        }
 
-            dbgln_if(LOCK_TRACE_DEBUG, "Lock::lock @ {} ({}): acquire {}, currently shared, locks held {}", this, m_name, mode_to_string(mode), m_times_locked);
+        VERIFY(m_shared_holders.is_empty());
 
-            VERIFY(m_times_locked > 0);
+        if constexpr (LOCK_TRACE_DEBUG) {
+            if (mode == Mode::Exclusive)
+                dbgln("Lock::lock @ {} ({}): acquire {}, currently exclusive, holding: {}", this, m_name, mode_to_string(mode), m_times_locked);
+            else
+                dbgln("Lock::lock @ {} ({}): acquire exclusive (requested {}), currently exclusive, holding: {}", this, m_name, mode_to_string(mode), m_times_locked);
+        }
+
+        VERIFY(mode == Mode::Exclusive || mode == Mode::Shared);
+        VERIFY(m_times_locked > 0);
+        if (!did_block)
+            m_times_locked++;
+
+#if LOCK_DEBUG
+        current_thread->holding_lock(*this, 1, location);
+#endif
+        return;
+    }
+    case Mode::Shared: {
+        VERIFY(!m_holder);
+        if (mode == Mode::Exclusive && 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, "Lock::lock @ {} ({}): acquire {}, converted shared to exclusive lock, locks held {}", this, m_name, mode_to_string(mode), m_times_locked);
+                return;
+            }
+        }
+        if (mode != Mode::Shared) {
+            block(*current_thread, mode, lock, 1);
+            did_block = true;
+        }
+
+        dbgln_if(LOCK_TRACE_DEBUG, "Lock::lock @ {} ({}): acquire {}, currently shared, locks held {}", this, m_name, mode_to_string(mode), m_times_locked);
+
+        VERIFY(m_times_locked > 0);
+        if (did_block) {
+            VERIFY(m_shared_holders.contains(current_thread));
+        } else {
             m_times_locked++;
             VERIFY(!m_shared_holders.is_empty());
             auto it = m_shared_holders.find(current_thread);
@@ -95,20 +108,15 @@ void Lock::lock(Mode mode)
                 it->value++;
             else
                 m_shared_holders.set(current_thread, 1);
+        }
 
 #if LOCK_DEBUG
-            current_thread->holding_lock(*this, 1, location);
+        current_thread->holding_lock(*this, 1, location);
 #endif
-            m_lock.store(false, AK::memory_order_release);
-            return;
-        }
-        default:
-            VERIFY_NOT_REACHED();
-        }
-        m_lock.store(false, AK::memory_order_release);
-        dbgln_if(LOCK_TRACE_DEBUG, "Lock::lock @ {} ({}) waiting...", this, m_name);
-        m_queue.wait_forever(m_name);
-        dbgln_if(LOCK_TRACE_DEBUG, "Lock::lock @ {} ({}) waited", this, m_name);
+        return;
+    }
+    default:
+        VERIFY_NOT_REACHED();
     }
 }
 
@@ -118,66 +126,107 @@ void Lock::unlock()
     // and also from within critical sections!
     VERIFY(!Processor::current().in_irq());
     auto current_thread = Thread::current();
-    ScopedCritical critical; // in case we're not in a critical section already
-    for (;;) {
-        if (m_lock.exchange(true, AK::memory_order_acq_rel) == false) {
-            Mode current_mode = m_mode;
-            if constexpr (LOCK_TRACE_DEBUG) {
-                if (current_mode == Mode::Shared)
-                    dbgln("Lock::unlock @ {} ({}): release {}, locks held: {}", this, m_name, mode_to_string(current_mode), m_times_locked);
-                else
-                    dbgln("Lock::unlock @ {} ({}): release {}, holding: {}", this, m_name, mode_to_string(current_mode), m_times_locked);
-            }
+    ScopedSpinLock lock(m_lock);
+    Mode current_mode = m_mode;
+    if constexpr (LOCK_TRACE_DEBUG) {
+        if (current_mode == Mode::Shared)
+            dbgln("Lock::unlock @ {} ({}): release {}, locks held: {}", this, m_name, mode_to_string(current_mode), m_times_locked);
+        else
+            dbgln("Lock::unlock @ {} ({}): release {}, holding: {}", this, m_name, mode_to_string(current_mode), m_times_locked);
+    }
 
-            VERIFY(current_mode != Mode::Unlocked);
+    VERIFY(current_mode != Mode::Unlocked);
 
-            VERIFY(m_times_locked > 0);
-            m_times_locked--;
+    VERIFY(m_times_locked > 0);
+    m_times_locked--;
 
-            switch (current_mode) {
-            case Mode::Exclusive:
-                VERIFY(m_holder == current_thread);
-                VERIFY(m_shared_holders.is_empty());
-                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) {
-                    it->value--;
-                } else {
-                    VERIFY(it->value > 0);
-                    m_shared_holders.remove(it);
-                }
-                break;
-            }
-            default:
-                VERIFY_NOT_REACHED();
-            }
-
-            bool unlocked_last = (m_times_locked == 0);
-            if (unlocked_last) {
-                VERIFY(current_mode == Mode::Exclusive ? !m_holder : m_shared_holders.is_empty());
-                m_mode = Mode::Unlocked;
-                m_queue.should_block(false);
-            }
+    switch (current_mode) {
+    case Mode::Exclusive:
+        VERIFY(m_holder == current_thread);
+        VERIFY(m_shared_holders.is_empty());
+        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) {
+            it->value--;
+        } else {
+            VERIFY(it->value > 0);
+            m_shared_holders.remove(it);
+        }
+        break;
+    }
+    default:
+        VERIFY_NOT_REACHED();
+    }
 
 #if LOCK_DEBUG
-            if (current_thread) {
-                current_thread->holding_lock(*this, -1, {});
-            }
+    if (current_thread) {
+        current_thread->holding_lock(*this, -1, {});
+    }
 #endif
-            m_lock.store(false, AK::memory_order_release);
-            if (unlocked_last) {
-                u32 did_wake = m_queue.wake_one();
-                dbgln_if(LOCK_TRACE_DEBUG, "Lock::unlock @ {} ({})  wake one ({})", this, m_name, did_wake);
-            }
-            return;
+
+    if (m_times_locked == 0) {
+        VERIFY(current_mode == Mode::Exclusive ? !m_holder : m_shared_holders.is_empty());
+
+        m_mode = Mode::Unlocked;
+        unblock_waiters(current_mode);
+    }
+}
+
+void Lock::block(Thread& current_thread, Mode mode, ScopedSpinLock<SpinLock<u8>>& lock, u32 requested_locks)
+{
+    auto& blocked_thread_list = thread_list_for_mode(mode);
+    VERIFY(!blocked_thread_list.contains(current_thread));
+    blocked_thread_list.append(current_thread);
+
+    dbgln_if(LOCK_TRACE_DEBUG, "Lock::lock @ {} ({}) waiting...", this, m_name);
+    current_thread.block(*this, lock, requested_locks);
+    dbgln_if(LOCK_TRACE_DEBUG, "Lock::lock @ {} ({}) waited", this, m_name);
+
+    VERIFY(blocked_thread_list.contains(current_thread));
+    blocked_thread_list.remove(current_thread);
+}
+
+void Lock::unblock_waiters(Mode previous_mode)
+{
+    VERIFY(m_times_locked == 0);
+    VERIFY(m_mode == Mode::Unlocked);
+
+    if (m_blocked_threads_list_exclusive.is_empty() && m_blocked_threads_list_shared.is_empty())
+        return;
+
+    auto unblock_shared = [&]() {
+        if (m_blocked_threads_list_shared.is_empty())
+            return false;
+        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);
+            VERIFY(set_result == AK::HashSetResult::InsertedNewEntry);
+            m_times_locked += requested_locks;
         }
-        // I don't know *who* is using "m_lock", so just yield.
-        Scheduler::yield_from_critical();
+        return true;
+    };
+    auto unblock_exclusive = [&]() {
+        if (auto* next_exclusive_thread = m_blocked_threads_list_exclusive.first()) {
+            m_mode = Mode::Exclusive;
+            m_times_locked = next_exclusive_thread->unblock_from_lock(*this);
+            m_holder = next_exclusive_thread;
+            return true;
+        }
+        return false;
+    };
+
+    if (previous_mode == Mode::Exclusive) {
+        if (!unblock_shared())
+            unblock_exclusive();
+    } else {
+        if (!unblock_exclusive())
+            unblock_shared();
     }
 }
 
@@ -187,77 +236,61 @@ auto Lock::force_unlock_if_locked(u32& lock_count_to_restore) -> Mode
     // and also from within critical sections!
     VERIFY(!Processor::current().in_irq());
     auto current_thread = Thread::current();
-    ScopedCritical critical; // in case we're not in a critical section already
-    for (;;) {
-        if (m_lock.exchange(true, AK::memory_order_acq_rel) == false) {
-            Mode previous_mode;
-            auto current_mode = m_mode.load(AK::MemoryOrder::memory_order_relaxed);
-            switch (current_mode) {
-            case Mode::Exclusive: {
-                if (m_holder != current_thread) {
-                    m_lock.store(false, AK::MemoryOrder::memory_order_release);
-                    lock_count_to_restore = 0;
-                    return Mode::Unlocked;
-                }
-
-                dbgln_if(LOCK_RESTORE_DEBUG, "Lock::force_unlock_if_locked @ {}: unlocking exclusive with lock count: {}", this, m_times_locked);
-#if LOCK_DEBUG
-                m_holder->holding_lock(*this, -(int)m_times_locked, {});
-#endif
-                m_holder = nullptr;
-                VERIFY(m_times_locked > 0);
-                lock_count_to_restore = m_times_locked;
-                m_times_locked = 0;
-                m_mode = Mode::Unlocked;
-                m_queue.should_block(false);
-                m_lock.store(false, AK::memory_order_release);
-                previous_mode = Mode::Exclusive;
-                break;
-            }
-            case Mode::Shared: {
-                VERIFY(!m_holder);
-                auto it = m_shared_holders.find(current_thread);
-                if (it == m_shared_holders.end()) {
-                    m_lock.store(false, AK::MemoryOrder::memory_order_release);
-                    lock_count_to_restore = 0;
-                    return Mode::Unlocked;
-                }
-
-                dbgln_if(LOCK_RESTORE_DEBUG, "Lock::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;
-                    m_queue.should_block(false);
-                }
-                m_lock.store(false, AK::memory_order_release);
-                previous_mode = Mode::Shared;
-                break;
-            }
-            case Mode::Unlocked: {
-                m_lock.store(false, AK::memory_order_relaxed);
-                lock_count_to_restore = 0;
-                previous_mode = Mode::Unlocked;
-                break;
-            }
-            default:
-                VERIFY_NOT_REACHED();
-            }
-            m_queue.wake_one();
-            return previous_mode;
+    ScopedSpinLock lock(m_lock);
+    auto current_mode = m_mode;
+    switch (current_mode) {
+    case Mode::Exclusive: {
+        if (m_holder != current_thread) {
+            lock_count_to_restore = 0;
+            return Mode::Unlocked;
         }
-        // I don't know *who* is using "m_lock", so just yield.
-        Scheduler::yield_from_critical();
+
+        dbgln_if(LOCK_RESTORE_DEBUG, "Lock::force_unlock_if_locked @ {}: unlocking exclusive with lock count: {}", this, m_times_locked);
+#if LOCK_DEBUG
+        m_holder->holding_lock(*this, -(int)m_times_locked, {});
+#endif
+        m_holder = nullptr;
+        VERIFY(m_times_locked > 0);
+        lock_count_to_restore = m_times_locked;
+        m_times_locked = 0;
+        m_mode = Mode::Unlocked;
+        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, "Lock::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;
+    }
+    default:
+        VERIFY_NOT_REACHED();
+    }
+    return current_mode;
 }
 
 #if LOCK_DEBUG
@@ -270,68 +303,66 @@ void Lock::restore_lock(Mode mode, u32 lock_count)
     VERIFY(lock_count > 0);
     VERIFY(!Processor::current().in_irq());
     auto current_thread = Thread::current();
-    ScopedCritical critical; // in case we're not in a critical section already
-    for (;;) {
-        if (m_lock.exchange(true, AK::memory_order_acq_rel) == false) {
-            switch (mode) {
-            case Mode::Exclusive: {
-                auto expected_mode = Mode::Unlocked;
-                if (!m_mode.compare_exchange_strong(expected_mode, Mode::Exclusive))
-                    break;
-
-                dbgln_if(LOCK_RESTORE_DEBUG, "Lock::restore_lock @ {}: restoring {} with lock count {}, was unlocked", this, mode_to_string(mode), lock_count);
-
-                VERIFY(m_times_locked == 0);
-                m_times_locked = lock_count;
-                VERIFY(!m_holder);
-                VERIFY(m_shared_holders.is_empty());
-                m_holder = current_thread;
-                m_queue.should_block(true);
-                m_lock.store(false, AK::memory_order_release);
-
-#if LOCK_DEBUG
-                m_holder->holding_lock(*this, (int)lock_count, location);
-#endif
-                return;
-            }
-            case Mode::Shared: {
-                auto expected_mode = Mode::Unlocked;
-                if (!m_mode.compare_exchange_strong(expected_mode, Mode::Shared) && expected_mode != Mode::Shared)
-                    break;
-
-                dbgln_if(LOCK_RESTORE_DEBUG, "Lock::restore_lock @ {}: restoring {} with lock count {}, was {}",
-                    this, mode_to_string(mode), lock_count, mode_to_string(expected_mode));
-
-                VERIFY(expected_mode == Mode::Shared || m_times_locked == 0);
-                m_times_locked += lock_count;
-                VERIFY(!m_holder);
-                VERIFY((expected_mode == Mode::Unlocked) == m_shared_holders.is_empty());
-                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);
-                m_queue.should_block(true);
-                m_lock.store(false, AK::memory_order_release);
-
-#if LOCK_DEBUG
-                m_holder->holding_lock(*this, (int)lock_count, location);
-#endif
-                return;
-            }
-            default:
-                VERIFY_NOT_REACHED();
-            }
-
-            m_lock.store(false, AK::memory_order_relaxed);
+    bool did_block = false;
+    ScopedSpinLock lock(m_lock);
+    switch (mode) {
+    case Mode::Exclusive: {
+        if (m_mode != Mode::Unlocked) {
+            block(*current_thread, Mode::Exclusive, lock, lock_count);
+            did_block = true;
         }
-        // I don't know *who* is using "m_lock", so just yield.
-        Scheduler::yield_from_critical();
+
+        dbgln_if(LOCK_RESTORE_DEBUG, "Lock::restore_lock @ {}: restoring {} with lock count {}, was unlocked", this, mode_to_string(mode), lock_count);
+
+        VERIFY(m_shared_holders.is_empty());
+        if (did_block) {
+            VERIFY(m_mode == Mode::Exclusive);
+            VERIFY(m_times_locked > 0);
+            VERIFY(m_holder == current_thread);
+        } else {
+            m_mode = Mode::Exclusive;
+            VERIFY(m_times_locked == 0);
+            m_times_locked = lock_count;
+            VERIFY(!m_holder);
+            m_holder = current_thread;
+        }
+
+#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::Unlocked && m_mode != Mode::Shared) {
+            block(*current_thread, Mode::Shared, lock, lock_count);
+            did_block = true;
+        }
+
+        dbgln_if(LOCK_RESTORE_DEBUG, "Lock::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_mode == Mode::Shared);
+            VERIFY(m_times_locked > 0);
+            VERIFY(m_shared_holders.contains(current_thread));
+        } else {
+            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);
+        }
+
+#if LOCK_DEBUG
+        m_holder->holding_lock(*this, (int)lock_count, location);
+#endif
+        return;
+    }
+    default:
+        VERIFY_NOT_REACHED();
     }
 }
 
-void Lock::clear_waiters()
-{
-    VERIFY(m_mode != Mode::Shared);
-    m_queue.wake_all();
-}
-
 }

Kernel/Lock.h

@@ -17,6 +17,8 @@
 namespace Kernel {
 
 class Lock {
+    friend class Thread;
+
     AK_MAKE_NONCOPYABLE(Lock);
     AK_MAKE_NONMOVABLE(Lock);
 
@@ -40,7 +42,6 @@ public:
     void unlock();
     [[nodiscard]] Mode force_unlock_if_locked(u32&);
     [[nodiscard]] bool is_locked() const { return m_mode != Mode::Unlocked; }
-    void clear_waiters();
 
     [[nodiscard]] const char* name() const { return m_name; }
 
@@ -59,10 +60,19 @@ public:
     }
 
 private:
-    Atomic<bool> m_lock { false };
+    typedef IntrusiveList<Thread, RawPtr<Thread>, &Thread::m_blocked_threads_list_node> BlockedThreadList;
+
+    ALWAYS_INLINE BlockedThreadList& thread_list_for_mode(Mode mode)
+    {
+        VERIFY(mode == Mode::Exclusive || mode == Mode::Shared);
+        return mode == Mode::Exclusive ? m_blocked_threads_list_exclusive : m_blocked_threads_list_shared;
+    }
+
+    void block(Thread&, Mode, ScopedSpinLock<SpinLock<u8>>&, u32);
+    void unblock_waiters(Mode);
+
     const char* m_name { nullptr };
-    WaitQueue m_queue;
-    Atomic<Mode, AK::MemoryOrder::memory_order_relaxed> m_mode { Mode::Unlocked };
+    Mode m_mode { Mode::Unlocked };
 
     // When locked exclusively, only the thread already holding the lock can
     // lock it again. When locked in shared mode, any thread can do that.
@@ -75,6 +85,11 @@ private:
     // lock.
     RefPtr<Thread> m_holder;
     HashMap<Thread*, u32> m_shared_holders;
+
+    BlockedThreadList m_blocked_threads_list_exclusive;
+    BlockedThreadList m_blocked_threads_list_shared;
+
+    SpinLock<u8> m_lock;
 };
 
 class Locker {

Kernel/LockMode.h

@@ -8,7 +8,7 @@
 
 namespace Kernel {
 
-enum class LockMode {
+enum class LockMode : u8 {
     Unlocked,
     Shared,
     Exclusive

Kernel/Process.cpp

@@ -117,7 +117,9 @@ void Process::kill_threads_except_self()
         thread.set_should_die();
     });
 
-    big_lock().clear_waiters();
+    u32 dropped_lock_count = 0;
+    if (big_lock().force_unlock_if_locked(dropped_lock_count) != LockMode::Unlocked)
+        dbgln("Process {} big lock had {} locks", *this, dropped_lock_count);
 }
 
 void Process::kill_all_threads()

Kernel/Thread.cpp

@@ -175,6 +175,100 @@ Thread::~Thread()
     }
 }
 
+void Thread::block(Kernel::Lock& lock, ScopedSpinLock<SpinLock<u8>>& lock_lock, u32 lock_count)
+{
+    VERIFY(!Processor::current().in_irq());
+    VERIFY(this == Thread::current());
+    ScopedCritical critical;
+    VERIFY(!s_mm_lock.own_lock());
+
+    ScopedSpinLock block_lock(m_block_lock);
+    VERIFY(!m_in_block);
+    m_in_block = true;
+
+    ScopedSpinLock scheduler_lock(g_scheduler_lock);
+
+    switch (state()) {
+    case Thread::Stopped:
+        // It's possible that we were requested to be stopped!
+        break;
+    case Thread::Running:
+        VERIFY(m_blocker == nullptr);
+        break;
+    default:
+        VERIFY_NOT_REACHED();
+    }
+    VERIFY(!m_blocking_lock);
+    m_blocking_lock = &lock;
+    m_lock_requested_count = lock_count;
+
+    set_state(Thread::Blocked);
+
+    scheduler_lock.unlock();
+    block_lock.unlock();
+
+    lock_lock.unlock();
+
+    dbgln_if(THREAD_DEBUG, "Thread {} blocking on Lock {}", *this, &lock);
+
+    for (;;) {
+        // Yield to the scheduler, and wait for us to resume unblocked.
+        VERIFY(!g_scheduler_lock.own_lock());
+        VERIFY(Processor::current().in_critical());
+        yield_while_not_holding_big_lock(); // We might hold the big lock though!
+        VERIFY(Processor::current().in_critical());
+
+        ScopedSpinLock block_lock2(m_block_lock);
+        if (should_be_stopped() || state() == Stopped) {
+            dbgln("Thread should be stopped, current state: {}", state_string());
+            set_state(Thread::Blocked);
+            continue;
+        }
+
+        VERIFY(!m_blocking_lock);
+        VERIFY(m_in_block);
+        m_in_block = false;
+        break;
+    }
+
+    lock_lock.lock();
+}
+
+u32 Thread::unblock_from_lock(Kernel::Lock& lock)
+{
+    ScopedSpinLock block_lock(m_block_lock);
+    VERIFY(m_blocking_lock == &lock);
+    auto requested_count = m_lock_requested_count;
+    block_lock.unlock();
+
+    auto do_unblock = [&]() {
+        ScopedSpinLock scheduler_lock(g_scheduler_lock);
+        ScopedSpinLock block_lock(m_block_lock);
+        VERIFY(m_blocking_lock == &lock);
+        VERIFY(!Processor::current().in_irq());
+        VERIFY(g_scheduler_lock.own_lock());
+        VERIFY(m_block_lock.own_lock());
+        VERIFY(m_blocking_lock == &lock);
+        dbgln_if(THREAD_DEBUG, "Thread {} unblocked from Lock {}", *this, &lock);
+        m_blocking_lock = nullptr;
+        if (Thread::current() == this) {
+            set_state(Thread::Running);
+            return;
+        }
+        VERIFY(m_state != Thread::Runnable && m_state != Thread::Running);
+        set_state(Thread::Runnable);
+    };
+    if (Processor::current().in_irq()) {
+        Processor::current().deferred_call_queue([do_unblock = move(do_unblock), self = make_weak_ptr()]() {
+            if (auto this_thread = self.strong_ref())
+                do_unblock();
+        });
+    } else {
+        do_unblock();
+    }
+    return requested_count;
+}
+
 void Thread::unblock_from_blocker(Blocker& blocker)
 {
     auto do_unblock = [&]() {
@@ -202,6 +296,8 @@ void Thread::unblock(u8 signal)
     VERIFY(m_block_lock.own_lock());
     if (m_state != Thread::Blocked)
         return;
+    if (m_blocking_lock)
+        return;
     VERIFY(m_blocker);
     if (signal != 0) {
         if (is_handling_page_fault()) {
@@ -314,9 +410,11 @@ void Thread::exit(void* exit_value)
 void Thread::yield_while_not_holding_big_lock()
 {
     VERIFY(!g_scheduler_lock.own_lock());
+    // Disable interrupts here. This ensures we don't accidentally switch contexts twice
+    InterruptDisabler disable;
+    Scheduler::yield(); // flag a switch
     u32 prev_flags;
     u32 prev_crit = Processor::current().clear_critical(prev_flags, true);
-    Scheduler::yield();
     // NOTE: We may be on a different CPU now!
     Processor::current().restore_critical(prev_crit, prev_flags);
 }
@@ -324,12 +422,14 @@ void Thread::yield_while_not_holding_big_lock()
 void Thread::yield_without_holding_big_lock()
 {
     VERIFY(!g_scheduler_lock.own_lock());
+    // Disable interrupts here. This ensures we don't accidentally switch contexts twice
+    InterruptDisabler disable;
+    Scheduler::yield(); // flag a switch
     u32 lock_count_to_restore = 0;
     auto previous_locked = unlock_process_if_locked(lock_count_to_restore);
     // NOTE: Even though we call Scheduler::yield here, unless we happen
     // to be outside of a critical section, the yield will be postponed
     // until leaving it in relock_process.
-    Scheduler::yield();
     relock_process(previous_locked, lock_count_to_restore);
 }
 
@@ -387,8 +487,11 @@ const char* Thread::state_string() const
         return "Stopped";
     case Thread::Blocked: {
         ScopedSpinLock block_lock(m_block_lock);
-        VERIFY(m_blocker != nullptr);
-        return m_blocker->state_string();
+        if (m_blocking_lock)
+            return "Lock";
+        if (m_blocker)
+            return m_blocker->state_string();
+        VERIFY_NOT_REACHED();
     }
     }
     PANIC("Thread::state_string(): Invalid state: {}", (int)state());
@@ -705,7 +808,7 @@ void Thread::resume_from_stopped()
     VERIFY(g_scheduler_lock.own_lock());
     if (m_stop_state == Blocked) {
         ScopedSpinLock block_lock(m_block_lock);
-        if (m_blocker) {
+        if (m_blocker || m_blocking_lock) {
             // Hasn't been unblocked yet
             set_state(Blocked, 0);
         } else {

Kernel/Thread.h

@@ -116,6 +116,7 @@ class Thread
     AK_MAKE_NONCOPYABLE(Thread);
     AK_MAKE_NONMOVABLE(Thread);
 
+    friend class Lock;
     friend class Process;
     friend class ProtectedProcessBase;
     friend class Scheduler;
@@ -823,6 +824,8 @@ public:
         }
     }
 
+    void block(Kernel::Lock&, ScopedSpinLock<SpinLock<u8>>&, u32);
+
     template<typename BlockerType, class... Args>
     [[nodiscard]] BlockResult block(const BlockTimeout& timeout, Args&&... args)
     {
@@ -954,6 +957,7 @@ public:
         return result;
     }
 
+    u32 unblock_from_lock(Kernel::Lock&);
     void unblock_from_blocker(Blocker&);
     void unblock(u8 signal = 0);
 
@@ -1280,6 +1284,9 @@ private:
     Optional<Range> m_thread_specific_range;
     Array<SignalActionData, NSIG> m_signal_action_data;
     Blocker* m_blocker { nullptr };
+    Kernel::Lock* m_blocking_lock { nullptr };
+    u32 m_lock_requested_count { 0 };
+    IntrusiveListNode<Thread> m_blocked_threads_list_node;
     bool m_may_die_immediately { true };
 
 #if LOCK_DEBUG

Kernel/VM/Region.cpp

@@ -436,7 +436,7 @@ PageFaultResponse Region::handle_fault(const PageFault& fault, ScopedSpinLock<Re
             remap_vmobject_page(translate_to_vmobject_page(page_index_in_region));
             return PageFaultResponse::Continue;
         }
-        return handle_zero_fault(page_index_in_region);
+        return handle_zero_fault(page_index_in_region, mm_lock);
 #else
         dbgln("BUG! Unexpected NP fault at {}", fault.vaddr());
         return PageFaultResponse::ShouldCrash;
@@ -448,7 +448,7 @@ PageFaultResponse Region::handle_fault(const PageFault& fault, ScopedSpinLock<Re
         auto* phys_page = physical_page(page_index_in_region);
         if (phys_page->is_shared_zero_page() || phys_page->is_lazy_committed_page()) {
             dbgln_if(PAGE_FAULT_DEBUG, "NP(zero) fault in Region({})[{}] at {}", this, page_index_in_region, fault.vaddr());
-            return handle_zero_fault(page_index_in_region);
+            return handle_zero_fault(page_index_in_region, mm_lock);
         }
         return handle_cow_fault(page_index_in_region);
     }
@@ -456,12 +456,29 @@ PageFaultResponse Region::handle_fault(const PageFault& fault, ScopedSpinLock<Re
     return PageFaultResponse::ShouldCrash;
 }
 
-PageFaultResponse Region::handle_zero_fault(size_t page_index_in_region)
+PageFaultResponse Region::handle_zero_fault(size_t page_index_in_region, ScopedSpinLock<RecursiveSpinLock>& mm_lock)
 {
     VERIFY_INTERRUPTS_DISABLED();
     VERIFY(vmobject().is_anonymous());
 
-    Locker locker(vmobject().m_paging_lock);
+    bool can_lock = Thread::current() && !g_scheduler_lock.own_lock();
+    if (can_lock) {
+        // TODO: This seems rather weird. If we don't have a current thread
+        // then we're in the Kernel in early initialization still. So we
+        // can't actually wait on the paging lock. And if we currently
+        // own the scheduler lock and we trigger zero faults, we also
+        // can't really wait. But do we actually need to wait here?
+        mm_lock.unlock();
+        VERIFY(!s_mm_lock.own_lock());
+
+        vmobject().m_paging_lock.lock();
+
+        mm_lock.lock();
+    }
+    ScopeGuard guard([&]() {
+        if (can_lock)
+            vmobject().m_paging_lock.unlock();
+    });
 
     auto& page_slot = physical_page_slot(page_index_in_region);
     auto page_index_in_vmobject = translate_to_vmobject_page(page_index_in_region);

Kernel/VM/Region.h

@@ -243,7 +243,7 @@ private:
 
     PageFaultResponse handle_cow_fault(size_t page_index);
     PageFaultResponse handle_inode_fault(size_t page_index, ScopedSpinLock<RecursiveSpinLock>&);
-    PageFaultResponse handle_zero_fault(size_t page_index);
+    PageFaultResponse handle_zero_fault(size_t page_index, ScopedSpinLock<RecursiveSpinLock>&);
 
     bool map_individual_page_impl(size_t page_index);