Kernel: Fix Lock race causing infinite spinning between two threads

We need to account for how many shared lock instances the current
thread owns, so that we can properly release such references when
yielding execution.

We also need to release the process lock when donating.

CMakeLists.txt

@@ -62,7 +62,7 @@ if (ALL_THE_DEBUG_MACROS)
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DICMP_DEBUG -DICO_DEBUG -DImage_DEBUG -DIMAGE_DECODER_CLIENT_DEBUG -DIMAGE_DECODER_DEBUG -DIMAGE_LOADER_DEBUG -DINTERRUPT_DEBUG -DIOAPIC_DEBUG -DIPC_DEBUG -DIPV4_DEBUG -DIPV4_SOCKET_DEBUG -DIRC_DEBUG -DIRQ_DEBUG")
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DJOB_DEBUG -DJPG_DEBUG")
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DKEYBOARD_DEBUG -DKEYBOARD_SHORTCUTS_DEBUG -DKMALLOC_DEBUG_LARGE_ALLOCATIONS")
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DLEXER_DEBUG -DLoader_DEBUG -DLOCK_DEBUG -DLOOKUPSERVER_DEBUG")
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DLEXER_DEBUG -DLoader_DEBUG -DLOCK_DEBUG -DLOCK_TRACE_DEBUG -DLOCK_RESTORE_DEBUG -DLOOKUPSERVER_DEBUG")
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DMALLOC_DEBUG -DMASTERPTY_DEBUG -DMBR_DEBUG -DMEMORY_DEBUG -DMENU_DEBUG -DMINIMIZE_ANIMATION_DEBUG -DMM_DEBUG -DMOVE_DEBUG -DMULTIPROCESSOR_DEBUG")
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DNETWORK_TASK_DEBUG -DNT_DEBUG")
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DOBJECT_DEBUG -DOCCLUSIONS_DEBUG -DOFFD_DEBUG")

Kernel/Lock.cpp

@@ -27,6 +27,10 @@
 #include <AK/TemporaryChange.h>
 #include <Kernel/KSyms.h>
 #include <Kernel/Lock.h>
+#include <Kernel/Thread.h>
+
+//#define LOCK_TRACE_DEBUG
+//#define LOCK_RESTORE_DEBUG
 
 namespace Kernel {
 
@@ -46,38 +50,78 @@ void Lock::lock(Mode mode)
     ASSERT(!Processor::current().in_irq());
     ASSERT(mode != Mode::Unlocked);
     auto current_thread = Thread::current();
-    ScopedCritical critical;
+    ScopedCritical critical; // in case we're not in a critical section already
     for (;;) {
         if (m_lock.exchange(true, AK::memory_order_acq_rel) == false) {
             do {
                 // FIXME: Do not add new readers if writers are queued.
-                bool can_lock;
-                switch (m_mode) {
-                case Lock::Mode::Unlocked:
-                    can_lock = true;
-                    break;
-                case Lock::Mode::Shared:
-                    can_lock = (mode == Lock::Mode::Shared);
-                    break;
-                case Lock::Mode::Exclusive:
-                    can_lock = (m_holder == current_thread);
-                    break;
-                }
-                if (can_lock) {
-                    // We got the lock!
-                    if (m_mode == Lock::Mode::Unlocked) {
-                        m_mode = mode;
-                        ASSERT(m_times_locked == 0);
-                        if (mode == Mode::Exclusive)
-                            m_holder = current_thread;
-                    }
-#ifdef LOCK_DEBUG
-                    current_thread->holding_lock(*this, true, file, line);
+                auto current_mode = m_mode.load(AK::MemoryOrder::memory_order_relaxed);
+                switch (current_mode) {
+                case Mode::Unlocked: {
+#ifdef LOCK_TRACE_DEBUG
+                    dbg() << "Lock::lock @ " << this << ": acquire " << mode_to_string(mode) << ", currently unlocked";
 #endif
+                    m_mode.store(mode, AK::MemoryOrder::memory_order_relaxed);
+                    ASSERT(!m_holder);
+                    ASSERT(m_shared_holders.is_empty());
+                    if (mode == Mode::Exclusive) {
+                        m_holder = current_thread;
+                    } else {
+                        ASSERT(mode == Mode::Shared);
+                        m_shared_holders.set(current_thread, 1);
+                    }
+                    ASSERT(m_times_locked == 0);
                     m_times_locked++;
+#ifdef LOCK_DEBUG
+                    current_thread->holding_lock(*this, 1, file, line);
+#endif
                     m_lock.store(false, AK::memory_order_release);
                     return;
                 }
+                case Mode::Exclusive: {
+                    ASSERT(m_holder);
+                    if (m_holder != current_thread)
+                        break;
+                    ASSERT(m_shared_holders.is_empty());
+#ifdef LOCK_TRACE_DEBUG
+                    if (mode == Mode::Exclusive)
+                        dbg() << "Lock::lock @ " << this << ": acquire " << mode_to_string(mode) << ", currently exclusive, holding: " << m_times_locked;
+                    else
+                        dbg() << "Lock::lock @ " << this << ": acquire exclusive (requested " << mode_to_string(mode) << "), currently exclusive, holding " << m_times_locked;
+#endif
+                    ASSERT(mode == Mode::Exclusive || mode == Mode::Shared);
+                    ASSERT(m_times_locked > 0);
+                    m_times_locked++;
+#ifdef LOCK_DEBUG
+                    current_thread->holding_lock(*this, 1, file, line);
+#endif
+                    m_lock.store(false, AK::memory_order_release);
+                    return;
+                }
+                case Mode::Shared: {
+                    ASSERT(!m_holder);
+                    if (mode != Mode::Shared)
+                        break;
+#ifdef LOCK_TRACE_DEBUG
+                    dbg() << "Lock::lock @ " << this << ": acquire " << mode_to_string(mode) << ", currently shared, locks held: " << m_times_locked;
+#endif
+                    ASSERT(m_times_locked > 0);
+                    m_times_locked++;
+                    ASSERT(!m_shared_holders.is_empty());
+                    auto it = m_shared_holders.find(current_thread);
+                    if (it != m_shared_holders.end())
+                        it->value++;
+                    else
+                        m_shared_holders.set(current_thread, 1);
+#ifdef LOCK_DEBUG
+                    current_thread->holding_lock(*this, 1, file, line);
+#endif
+                    m_lock.store(false, AK::memory_order_release);
+                    return;
+                }
+                default:
+                    ASSERT_NOT_REACHED();
+                }
                 m_lock.store(false, AK::memory_order_release);
             } while (m_queue.wait_on(nullptr, m_name) == Thread::BlockResult::NotBlocked);
         } else {
@@ -93,28 +137,53 @@ void Lock::unlock()
     // and also from within critical sections!
     ASSERT(!Processor::current().in_irq());
     auto current_thread = Thread::current();
-    ScopedCritical critical;
+    ScopedCritical critical; // in case we're not in a critical section already
     for (;;) {
         if (m_lock.exchange(true, AK::memory_order_acq_rel) == false) {
-            ASSERT(m_times_locked);
-            --m_times_locked;
+            auto current_mode = m_mode.load(AK::MemoryOrder::memory_order_relaxed);
+#ifdef LOCK_TRACE_DEBUG
+            if (current_mode == Mode::Shared)
+                dbg() << "Lock::unlock @ " << this << ": release " << mode_to_string(current_mode) << ", locks held: " << m_times_locked;
+            else
+                dbg() << "Lock::unlock @ " << this << ": release " << mode_to_string(current_mode) << ", holding: " << m_times_locked;
+#endif
+            ASSERT(current_mode != Mode::Unlocked);
 
-            ASSERT(m_mode != Mode::Unlocked);
+            ASSERT(m_times_locked > 0);
+            m_times_locked--;
 
-            if (m_mode == Mode::Exclusive) {
+            switch (current_mode) {
+            case Mode::Exclusive:
                 ASSERT(m_holder == current_thread);
+                ASSERT(m_shared_holders.is_empty());
                 if (m_times_locked == 0)
                     m_holder = nullptr;
+                break;
+            case Mode::Shared: {
+                ASSERT(!m_holder);
+                auto it = m_shared_holders.find(current_thread);
+                ASSERT(it != m_shared_holders.end());
+                if (it->value > 1) {
+                    it->value--;
+                } else {
+                    ASSERT(it->value > 0);
+                    m_shared_holders.remove(it);
                 }
+                break;
+            }
+            default:
+                ASSERT_NOT_REACHED();
+            }
+
+            if (m_times_locked == 0) {
+                ASSERT(current_mode == Mode::Exclusive ? !m_holder : m_shared_holders.is_empty());
+                m_mode.store(Mode::Unlocked, AK::MemoryOrder::memory_order_relaxed);
+            }
+
 #ifdef LOCK_DEBUG
-            current_thread->holding_lock(*this, false);
+            current_thread->holding_lock(*this, -1);
 #endif
 
-            if (m_times_locked > 0) {
-                m_lock.store(false, AK::memory_order_release);
-                return;
-            }
-            m_mode = Mode::Unlocked;
             m_lock.store(false, AK::memory_order_release);
             m_queue.wake_one();
             return;
@@ -124,39 +193,153 @@ void Lock::unlock()
     }
 }
 
-bool Lock::force_unlock_if_locked()
+auto Lock::force_unlock_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!
     ASSERT(!Processor::current().in_irq());
-    ScopedCritical critical;
+    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) {
-            if (m_holder != Thread::current()) {
+            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);
-                return false;
+                    lock_count_to_restore = 0;
+                    return Mode::Unlocked;
                 }
-            ASSERT(m_mode != Mode::Shared);
-            ASSERT(m_times_locked == 1);
-#ifdef LOCK_DEBUG
-            m_holder->holding_lock(*this, false);
+#ifdef LOCK_RESTORE_DEBUG
+                dbg() << "Lock::force_unlock_if_locked @ " << this << ": unlocking exclusive with lock count: " << m_times_locked;
 #endif
                 m_holder = nullptr;
-            m_mode = Mode::Unlocked;
+                ASSERT(m_times_locked > 0);
+                lock_count_to_restore = m_times_locked;
                 m_times_locked = 0;
+                m_mode.store(Mode::Unlocked, AK::MemoryOrder::memory_order_relaxed);
                 m_lock.store(false, AK::memory_order_release);
-            m_queue.wake_one();
+#ifdef LOCK_DEBUG
+                m_holder->holding_lock(*this, -(int)lock_count_to_restore);
+#endif
+                previous_mode = Mode::Exclusive;
                 break;
             }
+            case Mode::Shared: {
+                ASSERT(!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;
+                }
+#ifdef LOCK_RESTORE_DEBUG
+                dbg() << "Lock::force_unlock_if_locked @ " << this << ": unlocking exclusive with lock count: " << it->value << ", total locks: " << m_times_locked;
+#endif
+                ASSERT(it->value > 0);
+                lock_count_to_restore = it->value;
+                ASSERT(lock_count_to_restore > 0);
+#ifdef LOCK_DEBUG
+                m_holder->holding_lock(*this, -(int)lock_count_to_restore);
+#endif
+                m_shared_holders.remove(it);
+                ASSERT(m_times_locked >= lock_count_to_restore);
+                m_times_locked -= lock_count_to_restore;
+                if (m_times_locked == 0)
+                    m_mode.store(Mode::Unlocked, AK::MemoryOrder::memory_order_relaxed);
+                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:
+                ASSERT_NOT_REACHED();
+            }
+            m_queue.wake_one();
+            return previous_mode;
+        }
+        // I don't know *who* is using "m_lock", so just yield.
+        Scheduler::yield_from_critical();
+    }
+}
+
+#ifdef LOCK_DEBUG
+void Lock::restore_lock(Mode mode, u32 lock_count)
+{
+    return restore_lock("unknown", 0, mode, lock_count);
+}
+
+void Lock::restore_lock(const char* file, int line, Mode mode, u32 lock_count)
+#else
+void Lock::restore_lock(Mode mode, u32 lock_count)
+#endif
+{
+    ASSERT(mode != Mode::Unlocked);
+    ASSERT(lock_count > 0);
+    ASSERT(!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, AK::MemoryOrder::memory_order_relaxed))
+                    break;
+#ifdef LOCK_RESTORE_DEBUG
+                dbg() << "Lock::restore_lock @ " << this << ": restoring " << mode_to_string(mode) << " with lock count " << lock_count << ", was unlocked";
+#endif
+                ASSERT(m_times_locked == 0);
+                m_times_locked = lock_count;
+                ASSERT(!m_holder);
+                ASSERT(m_shared_holders.is_empty());
+                m_holder = current_thread;
+                m_lock.store(false, AK::memory_order_release);
+#ifdef LOCK_DEBUG
+                m_holder->holding_lock(*this, (int)lock_count, file, line);
+#endif
+                return;
+            }
+            case Mode::Shared: {
+                auto expected_mode = Mode::Unlocked;
+                if (!m_mode.compare_exchange_strong(expected_mode, Mode::Shared, AK::MemoryOrder::memory_order_relaxed) && expected_mode != Mode::Shared)
+                    break;
+#ifdef LOCK_RESTORE_DEBUG
+                dbg() << "Lock::restore_lock @ " << this << ": restoring " << mode_to_string(mode) << " with lock count " << lock_count << ", was " << mode_to_string(expected_mode);
+#endif
+                ASSERT(expected_mode == Mode::Shared || m_times_locked == 0);
+                m_times_locked += lock_count;
+                ASSERT(!m_holder);
+                ASSERT((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
+                ASSERT(set_result == AK::HashSetResult::InsertedNewEntry);
+                m_lock.store(false, AK::memory_order_release);
+#ifdef LOCK_DEBUG
+                m_holder->holding_lock(*this, (int)lock_count, file, line);
+#endif
+                return;
+            }
+            default:
+                ASSERT_NOT_REACHED();
+            }
+
+            m_lock.store(false, AK::memory_order_relaxed);
+        }
         // I don't know *who* is using "m_lock", so just yield.
         Scheduler::yield_from_critical();
     }
-    return true;
 }
 
 void Lock::clear_waiters()
 {
-    ASSERT(m_mode != Mode::Shared);
+    ASSERT(m_mode.load(AK::MemoryOrder::memory_order_relaxed) != Mode::Shared);
     m_queue.wake_all();
 }
 

Kernel/Lock.h

@@ -28,44 +28,60 @@
 
 #include <AK/Assertions.h>
 #include <AK/Atomic.h>
+#include <AK/HashMap.h>
 #include <AK/Types.h>
 #include <Kernel/Arch/i386/CPU.h>
 #include <Kernel/Forward.h>
-#include <Kernel/Thread.h>
+#include <Kernel/LockMode.h>
 #include <Kernel/WaitQueue.h>
 
 namespace Kernel {
 
 class Lock {
+    AK_MAKE_NONCOPYABLE(Lock);
+    AK_MAKE_NONMOVABLE(Lock);
+
 public:
+    using Mode = LockMode;
+
     Lock(const char* name = nullptr)
         : m_name(name)
     {
     }
     ~Lock() { }
 
-    enum class Mode {
-        Unlocked,
-        Shared,
-        Exclusive
-    };
-
     void lock(Mode = Mode::Exclusive);
 #ifdef LOCK_DEBUG
     void lock(const char* file, int line, Mode mode = Mode::Exclusive);
+    void restore_lock(const char* file, int line, Mode, u32);
 #endif
     void unlock();
-    bool force_unlock_if_locked();
-    bool is_locked() const { return m_holder; }
+    [[nodiscard]] Mode force_unlock_if_locked(u32&);
+    void restore_lock(Mode, u32);
+    bool is_locked() const { return m_mode.load(AK::MemoryOrder::memory_order_relaxed) != Mode::Unlocked; }
     void clear_waiters();
 
     const char* name() const { return m_name; }
 
+    static const char* mode_to_string(Mode mode)
+    {
+        switch (mode) {
+        case Mode::Unlocked:
+            return "unlocked";
+        case Mode::Exclusive:
+            return "exclusive";
+        case Mode::Shared:
+            return "shared";
+        default:
+            return "invalid";
+        }
+    }
+
 private:
     Atomic<bool> m_lock { false };
     const char* m_name { nullptr };
     WaitQueue m_queue;
-    Mode m_mode { Mode::Unlocked };
+    Atomic<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.
@@ -77,6 +93,7 @@ private:
     // When locked exclusively, this is always the one thread that holds the
     // lock.
     RefPtr<Thread> m_holder;
+    HashMap<Thread*, u32> m_shared_holders;
 };
 
 class Locker {
@@ -103,8 +120,10 @@ private:
 
 #ifdef LOCK_DEBUG
 #    define LOCKER(...) Locker locker(__FILE__, __LINE__, __VA_ARGS__)
+#    define RESTORE_LOCK(lock, ...) (lock).restore_lock(__FILE__, __LINE__, __VA_ARGS__)
 #else
 #    define LOCKER(...) Locker locker(__VA_ARGS__)
+#    define RESTORE_LOCK(lock, ...) (lock).restore_lock(__VA_ARGS__)
 #endif
 
 template<typename T>

Kernel/LockMode.h

@@ -0,0 +1,37 @@
+/*
+ * Copyright (c) 2020, The SerenityOS developers.
+ * 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.
+ */
+
+#pragma once
+
+namespace Kernel {
+
+enum class LockMode {
+    Unlocked,
+    Shared,
+    Exclusive
+};
+
+}

Kernel/Syscalls/execve.cpp

@@ -348,7 +348,8 @@ int Process::do_exec(NonnullRefPtr<FileDescription> main_program_description, Ve
         ScopedSpinLock lock(g_scheduler_lock);
         new_main_thread->set_state(Thread::State::Runnable);
     }
-    big_lock().force_unlock_if_locked();
+    u32 lock_count_to_restore;
+    (void)big_lock().force_unlock_if_locked(lock_count_to_restore);
     ASSERT_INTERRUPTS_DISABLED();
     ASSERT(Processor::current().in_critical());
     return 0;

Kernel/Syscalls/sched.cpp

@@ -41,14 +41,11 @@ int Process::sys$donate(pid_t tid)
     if (tid < 0)
         return -EINVAL;
 
-    // We don't strictly need to grab the scheduler lock here, but it
-    // will close a race where we can find the thread but it disappears
-    // before we call Scheduler::donate_to.
-    ScopedSpinLock lock(g_scheduler_lock);
+    ScopedCritical critical;
     auto thread = Thread::from_tid(tid);
     if (!thread || thread->pid() != pid())
         return -ESRCH;
-    Scheduler::donate_to(thread, "sys$donate");
+    Thread::current()->donate_without_holding_big_lock(thread, "sys$donate");
     return 0;
 }
 

Kernel/Thread.cpp

@@ -212,7 +212,8 @@ void Thread::die_if_needed()
     if (!m_should_die)
         return;
 
-    unlock_process_if_locked();
+    u32 unlock_count;
+    (void)unlock_process_if_locked(unlock_count);
 
     ScopedCritical critical;
     set_should_die();
@@ -238,7 +239,8 @@ void Thread::exit(void* exit_value)
     ASSERT(Thread::current() == this);
     m_join_condition.thread_did_exit(exit_value);
     set_should_die();
-    unlock_process_if_locked();
+    u32 unlock_count;
+    (void)unlock_process_if_locked(unlock_count);
     die_if_needed();
 }
 
@@ -255,25 +257,33 @@ void Thread::yield_while_not_holding_big_lock()
 void Thread::yield_without_holding_big_lock()
 {
     ASSERT(!g_scheduler_lock.own_lock());
-    bool did_unlock = unlock_process_if_locked();
+    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(did_unlock);
+    relock_process(previous_locked, lock_count_to_restore);
 }
 
-bool Thread::unlock_process_if_locked()
+void Thread::donate_without_holding_big_lock(RefPtr<Thread>& thread, const char* reason)
 {
-    return process().big_lock().force_unlock_if_locked();
+    ASSERT(!g_scheduler_lock.own_lock());
+    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::donate_to(thread, reason);
+    relock_process(previous_locked, lock_count_to_restore);
 }
 
-void Thread::lock_process()
+LockMode Thread::unlock_process_if_locked(u32& lock_count_to_restore)
 {
-    process().big_lock().lock();
+    return process().big_lock().force_unlock_if_locked(lock_count_to_restore);
 }
 
-void Thread::relock_process(bool did_unlock)
+void Thread::relock_process(LockMode previous_locked, u32 lock_count_to_restore)
 {
     // Clearing the critical section may trigger the context switch
     // flagged by calling Scheduler::donate_to or Scheduler::yield
@@ -282,13 +292,15 @@ void Thread::relock_process(bool did_unlock)
     u32 prev_flags;
     u32 prev_crit = Processor::current().clear_critical(prev_flags, true);
 
-    if (did_unlock) {
-        // We've unblocked, relock the process if needed and carry on.
-        process().big_lock().lock();
-    }
+    // CONTEXT SWITCH HAPPENS HERE!
 
     // NOTE: We may be on a different CPU now!
     Processor::current().restore_critical(prev_crit, prev_flags);
+
+    if (previous_locked != LockMode::Unlocked) {
+        // We've unblocked, relock the process if needed and carry on.
+        RESTORE_LOCK(process().big_lock(), previous_locked, lock_count_to_restore);
+    }
 }
 
 auto Thread::sleep(clockid_t clock_id, const timespec& duration, timespec* remaining_time) -> BlockResult

Kernel/Thread.h

@@ -38,6 +38,7 @@
 #include <Kernel/Arch/i386/CPU.h>
 #include <Kernel/Forward.h>
 #include <Kernel/KResult.h>
+#include <Kernel/LockMode.h>
 #include <Kernel/Scheduler.h>
 #include <Kernel/ThreadTracer.h>
 #include <Kernel/TimerQueue.h>
@@ -781,6 +782,7 @@ public:
     [[nodiscard]] BlockResult block(const BlockTimeout& timeout, Args&&... args)
     {
         ASSERT(!Processor::current().in_irq());
+        ASSERT(this == Thread::current());
         ScopedCritical critical;
         ScopedSpinLock scheduler_lock(g_scheduler_lock);
         ScopedSpinLock block_lock(m_block_lock);
@@ -844,12 +846,14 @@ public:
         block_lock.unlock();
 
         bool did_timeout = false;
-        bool did_unlock = false;
+        auto previous_locked = LockMode::Unlocked;
+        u32 lock_count_to_restore = 0;
         for (;;) {
             scheduler_lock.unlock();
 
             // Yield to the scheduler, and wait for us to resume unblocked.
-            did_unlock |= unlock_process_if_locked();
+            if (previous_locked == LockMode::Unlocked)
+                previous_locked = unlock_process_if_locked(lock_count_to_restore);
 
             ASSERT(!g_scheduler_lock.own_lock());
             ASSERT(Processor::current().in_critical());
@@ -896,10 +900,10 @@ public:
             // (e.g. if it's on another processor)
             TimerQueue::the().cancel_timer(timer.release_nonnull());
         }
-        if (did_unlock) {
+        if (previous_locked != LockMode::Unlocked) {
             // NOTE: this may trigger another call to Thread::block(), so
             // we need to do this after we're all done and restored m_in_block!
-            lock_process();
+            relock_process(previous_locked, lock_count_to_restore);
         }
         return result;
     }
@@ -907,6 +911,13 @@ public:
     void unblock_from_blocker(Blocker&);
     void unblock(u8 signal = 0);
 
+    template<class... Args>
+    Thread::BlockResult wait_on(WaitQueue& wait_queue, const Thread::BlockTimeout& timeout, Args&&... args)
+    {
+        ASSERT(this == Thread::current());
+        return block<Thread::QueueBlocker>(timeout, wait_queue, forward<Args>(args)...);
+    }
+
     BlockResult sleep(clockid_t, const timespec&, timespec* = nullptr);
     BlockResult sleep(const timespec& duration, timespec* remaining_time = nullptr)
     {
@@ -1062,29 +1073,32 @@ public:
     RecursiveSpinLock& get_lock() const { return m_lock; }
 
 #ifdef LOCK_DEBUG
-    void holding_lock(Lock& lock, bool holding, const char* file = nullptr, int line = 0)
+    void holding_lock(Lock& lock, int refs_delta, const char* file = nullptr, int line = 0)
     {
-        m_holding_locks.fetch_add(holding ? 1 : -1, AK::MemoryOrder::memory_order_relaxed);
+        ASSERT(refs_delta != 0);
+        m_holding_locks.fetch_add(refs_delta, AK::MemoryOrder::memory_order_relaxed);
         ScopedSpinLock list_lock(m_holding_locks_lock);
-        if (holding) {
+        if (refs_delta > 0) {
             bool have_existing = false;
             for (size_t i = 0; i < m_holding_locks_list.size(); i++) {
                 auto& info = m_holding_locks_list[i];
                 if (info.lock == &lock) {
                     have_existing = true;
-                    info.count++;
+                    info.count += refs_delta;
                     break;
                 }
             }
             if (!have_existing)
                 m_holding_locks_list.append({ &lock, file ? file : "unknown", line, 1 });
         } else {
+            ASSERT(refs_delta < 0);
             bool found = false;
             for (size_t i = 0; i < m_holding_locks_list.size(); i++) {
                 auto& info = m_holding_locks_list[i];
                 if (info.lock == &lock) {
-                    ASSERT(info.count > 0);
-                    if (--info.count == 0)
+                    ASSERT(info.count >= (unsigned)-refs_delta);
+                    info.count -= (unsigned)-refs_delta;
+                    if (info.count == 0)
                         m_holding_locks_list.remove(i);
                     found = true;
                     break;
@@ -1162,9 +1176,8 @@ private:
         bool m_thread_did_exit { false };
     };
 
-    bool unlock_process_if_locked();
-    void lock_process();
-    void relock_process(bool did_unlock);
+    LockMode unlock_process_if_locked(u32&);
+    void relock_process(LockMode, u32);
     String backtrace_impl();
     void reset_fpu_state();
 
@@ -1234,6 +1247,7 @@ private:
     Atomic<bool> m_have_any_unmasked_pending_signals { false };
 
     void yield_without_holding_big_lock();
+    void donate_without_holding_big_lock(RefPtr<Thread>&, const char*);
     void yield_while_not_holding_big_lock();
     void update_state_for_thread(Thread::State previous_state);
 };

Kernel/WaitQueue.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2020, The SerenityOS developers.
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without

Kernel/WaitQueue.h

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2020, The SerenityOS developers.
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without