Kernel: Add simplistic work queues

We can't use deferred functions for anything that may require preemption,
such as copying from/to user or accessing the disk. For those purposes
we should use a work queue, which is essentially a kernel thread that
may be preempted or blocked.

Kernel/CMakeLists.txt

@@ -228,6 +228,7 @@ set(KERNEL_SOURCES
     VM/Space.cpp
     VM/VMObject.cpp
     WaitQueue.cpp
+    WorkQueue.cpp
     init.cpp
     kprintf.cpp
 )

Kernel/Devices/AsyncDeviceRequest.h

@@ -37,6 +37,8 @@ namespace Kernel {
 
 class Device;
 
+extern WorkQueue* g_io_work;
+
 class AsyncDeviceRequest : public RefCounted<AsyncDeviceRequest> {
     AK_MAKE_NONCOPYABLE(AsyncDeviceRequest);
     AK_MAKE_NONMOVABLE(AsyncDeviceRequest);

Kernel/Forward.h

@@ -76,6 +76,7 @@ class UserOrKernelBuffer;
 class VFS;
 class VMObject;
 class WaitQueue;
+class WorkQueue;
 
 template<typename T>
 class KResultOr;

Kernel/Storage/IDEChannel.cpp

@@ -35,6 +35,7 @@
 #include <Kernel/Storage/IDEController.h>
 #include <Kernel/Storage/PATADiskDevice.h>
 #include <Kernel/VM/MemoryManager.h>
+#include <Kernel/WorkQueue.h>
 
 namespace Kernel {
 
@@ -122,7 +123,7 @@ void IDEChannel::complete_current_request(AsyncDeviceRequest::RequestResult resu
     // This is important so that we can safely write the buffer back,
     // which could cause page faults. Note that this may be called immediately
     // before Processor::deferred_call_queue returns!
-    Processor::deferred_call_queue([this, result]() {
+    g_io_work->queue([this, result]() {
         dbgln_if(PATA_DEBUG, "IDEChannel::complete_current_request result: {}", (int)result);
         ScopedSpinLock lock(m_request_lock);
         VERIFY(m_current_request);

Kernel/WorkQueue.cpp

@@ -0,0 +1,79 @@
+/*
+ * Copyright (c) 2021, 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.
+ */
+
+#include <Kernel/Process.h>
+#include <Kernel/SpinLock.h>
+#include <Kernel/WaitQueue.h>
+#include <Kernel/WorkQueue.h>
+
+namespace Kernel {
+
+WorkQueue* g_io_work;
+
+void WorkQueue::initialize()
+{
+    g_io_work = new WorkQueue("IO WorkQueue");
+}
+
+WorkQueue::WorkQueue(const char* name)
+    : m_name(name)
+{
+    RefPtr<Thread> thread;
+    Process::create_kernel_process(thread, name, [this] {
+        for (;;) {
+            WorkItem* item;
+            bool have_more;
+            {
+                ScopedSpinLock lock(m_lock);
+                item = m_items.take_first();
+                have_more = !m_items.is_empty();
+            }
+            if (item) {
+                item->function(item->data);
+                if (item->free_data)
+                    item->free_data(item->data);
+                delete item;
+
+                if (have_more)
+                    continue;
+            }
+            [[maybe_unused]] auto result = m_wait_queue.wait_on({});
+        }
+    });
+    // If we can't create the thread we're in trouble...
+    m_thread = thread.release_nonnull();
+}
+
+void WorkQueue::do_queue(WorkItem* item)
+{
+    {
+        ScopedSpinLock lock(m_lock);
+        m_items.append(*item);
+    }
+    m_wait_queue.wake_one();
+}
+
+}

Kernel/WorkQueue.h

@@ -0,0 +1,94 @@
+/*
+ * Copyright (c) 2021, 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
+
+#include <AK/IntrusiveList.h>
+#include <Kernel/Forward.h>
+
+namespace Kernel {
+
+extern WorkQueue* g_io_work;
+
+class WorkQueue {
+    AK_MAKE_NONCOPYABLE(WorkQueue);
+    AK_MAKE_NONMOVABLE(WorkQueue);
+
+public:
+    static void initialize();
+
+    WorkQueue(const char*);
+
+    void queue(void (*function)(void*), void* data = nullptr, void (*free_data)(void*) = nullptr)
+    {
+        auto* item = new WorkItem; // TODO: use a pool
+        item->function = function;
+        item->data = data;
+        item->free_data = free_data;
+        do_queue(item);
+    }
+
+    template<typename Function>
+    void queue(Function function)
+    {
+        auto* item = new WorkItem; // TODO: use a pool
+        item->function = [](void* f) {
+            (*reinterpret_cast<Function*>(f))();
+        };
+        if constexpr (sizeof(Function) <= sizeof(item->inline_data)) {
+            item->data = new (item->inline_data) Function(move(function));
+            item->free_data = [](void* f) {
+                reinterpret_cast<Function*>(f)->~Function();
+            };
+
+        } else {
+            item->data = new Function(move(function));
+            item->free_data = [](void* f) {
+                delete reinterpret_cast<Function*>(f);
+            };
+        }
+        do_queue(item);
+    }
+
+private:
+    struct WorkItem {
+        IntrusiveListNode m_node;
+        void (*function)(void*);
+        void* data;
+        void (*free_data)(void*);
+        u8 inline_data[4 * sizeof(void*)];
+    };
+
+    void do_queue(WorkItem*);
+
+    const char* const m_name;
+    RefPtr<Thread> m_thread;
+    WaitQueue m_wait_queue;
+    IntrusiveList<WorkItem, &WorkItem::m_node> m_items;
+    SpinLock<u8> m_lock;
+};
+
+}

Kernel/init.cpp

@@ -72,6 +72,7 @@
 #include <Kernel/Tasks/SyncTask.h>
 #include <Kernel/Time/TimeManagement.h>
 #include <Kernel/VM/MemoryManager.h>
+#include <Kernel/WorkQueue.h>
 #include <Kernel/kstdio.h>
 
 // Defined in the linker script
@@ -189,6 +190,8 @@ extern "C" UNMAP_AFTER_INIT [[noreturn]] void init()
     Process::initialize();
     Scheduler::initialize();
 
+    WorkQueue::initialize();
+
     {
         RefPtr<Thread> init_stage2_thread;
         Process::create_kernel_process(init_stage2_thread, "init_stage2", init_stage2, nullptr);