Kernel: Fix some Lock problems and VERIFY statements

When a Lock blocks (e.g. due to a mode mismatch or because someone
else holds it) the lock mode will be updated to what was requested.

There were also some cases where restoring locks may have not worked
as intended as it may have been held already by the same thread.

Fixes #8787

2 files changed, 110 insertions, 35 deletions

diff --git a/Kernel/Lock.cpp b/Kernel/Lock.cpp
index 45cb54208f..6afc672ea5 100644
--- a/Kernel/Lock.cpp
+++ b/Kernel/Lock.cpp
@@ -55,9 +55,19 @@ void Lock::lock(Mode mode)
         if (m_holder != 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
+            VERIFY(m_mode == mode);
         }
 
-        VERIFY(m_shared_holders.is_empty());
+        if (m_mode == Mode::Exclusive) {
+            VERIFY(m_holder == current_thread);
+            VERIFY(m_shared_holders.is_empty());
+        } 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());
+        }
 
         if constexpr (LOCK_TRACE_DEBUG) {
             if (mode == Mode::Exclusive)
@@ -66,10 +76,12 @@ void Lock::lock(Mode mode)
                 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)
+        if (!did_block) {
+            // if we didn't block we must still be an exclusive lock
+            VERIFY(m_mode == Mode::Exclusive);
             m_times_locked++;
+        }
 
 #if LOCK_DEBUG
         current_thread->holding_lock(*this, 1, location);
@@ -78,29 +90,40 @@ void Lock::lock(Mode mode)
     }
     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::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, "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;
+            VERIFY(m_mode == mode);
         }
 
         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 {
+        if (m_mode == Mode::Shared) {
+            VERIFY(!m_holder);
+            VERIFY(!did_block || m_shared_holders.contains(current_thread));
+        } else if (did_block) {
+            VERIFY(mode == Mode::Exclusive);
+            VERIFY(m_holder == current_thread);
+            VERIFY(m_shared_holders.is_empty());
+        }
+
+        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);
@@ -307,24 +330,47 @@ void Lock::restore_lock(Mode mode, u32 lock_count)
     ScopedSpinLock lock(m_lock);
     switch (mode) {
     case Mode::Exclusive: {
-        if (m_mode != Mode::Unlocked) {
+        auto previous_mode = m_mode;
+        bool need_to_block = false;
+        if (m_mode == Mode::Exclusive && m_holder != current_thread)
+            need_to_block = true;
+        else if (m_mode == Mode::Shared && (m_shared_holders.size() != 1 || !m_shared_holders.contains(current_thread)))
+            need_to_block = true;
+        if (need_to_block) {
             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, "Lock::restore_lock @ {}: restoring {} with lock count {}, was unlocked", this, mode_to_string(mode), lock_count);
+        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_mode != Mode::Shared);
         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 (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
@@ -334,9 +380,11 @@ void Lock::restore_lock(Mode mode, u32 lock_count)
     }
     case Mode::Shared: {
         auto previous_mode = m_mode;
-        if (m_mode != Mode::Unlocked && m_mode != Mode::Shared) {
+        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, "Lock::restore_lock @ {}: restoring {} with lock count {}, was {}",
@@ -344,15 +392,29 @@ void Lock::restore_lock(Mode mode, u32 lock_count)
 
         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 (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;
+            }
         }
 
 #if LOCK_DEBUG
diff --git a/Kernel/Lock.h b/Kernel/Lock.h
index cfd69eb7f9..c0908dc136 100644
--- a/Kernel/Lock.h
+++ b/Kernel/Lock.h
@@ -41,7 +41,20 @@ public:
 
     void unlock();
     [[nodiscard]] Mode force_unlock_if_locked(u32&);
-    [[nodiscard]] bool is_locked() const { return m_mode != Mode::Unlocked; }
+    [[nodiscard]] bool is_locked() const
+    {
+        ScopedSpinLock lock(m_lock);
+        return m_mode != Mode::Unlocked;
+    }
+    [[nodiscard]] bool own_lock() const
+    {
+        ScopedSpinLock 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;
+    }
 
     [[nodiscard]] const char* name() const { return m_name; }
 
@@ -89,7 +102,7 @@ private:
     BlockedThreadList m_blocked_threads_list_exclusive;
     BlockedThreadList m_blocked_threads_list_shared;
 
-    SpinLock<u8> m_lock;
+    mutable SpinLock<u8> m_lock;
 };
 
 class Locker {