Kernel+LibPthread+LibC: Add a naive futex and use it for pthread_cond_t

This patch implements a simple version of the futex (fast userspace
mutex) API in the kernel and uses it to make the pthread_cond_t API's
block instead of busily sched_yield().

An arbitrary userspace address is passed to the kernel as a "token"
that identifies the futex and you can then FUTEX_WAIT and FUTEX_WAKE
that specific userspace address.

FUTEX_WAIT corresponds to pthread_cond_wait() and FUTEX_WAKE is used
for pthread_cond_signal() and pthread_cond_broadcast().

I'm pretty sure I'm missing something in this implementation, but it's
hopefully okay for a start. :^)

Kernel/Process.cpp

@@ -3807,3 +3807,47 @@ Thread& Process::any_thread()
     ASSERT(found_thread);
     return *found_thread;
 }
+
+WaitQueue& Process::futex_queue(i32* userspace_address)
+{
+    auto& queue = m_futex_queues.ensure((u32)userspace_address);
+    if (!queue)
+        queue = make<WaitQueue>();
+    return *queue;
+}
+
+int Process::sys$futex(const Syscall::SC_futex_params* params)
+{
+    if (!validate_read_typed(params))
+        return -EFAULT;
+
+    auto& [userspace_address, futex_op, value, timeout] = *params;
+
+    if (!validate_read_typed(userspace_address))
+        return -EFAULT;
+
+    if (timeout && !validate_read_typed(timeout))
+        return -EFAULT;
+
+    switch (futex_op) {
+    case FUTEX_WAIT:
+        if (*userspace_address != value)
+            return -EAGAIN;
+        // FIXME: This is supposed to be interruptible by a signal, but right now WaitQueue cannot be interrupted.
+        // FIXME: Support timeout!
+        current->wait_on(futex_queue(userspace_address));
+        break;
+    case FUTEX_WAKE:
+        if (value == 0)
+            return 0;
+        if (value == 1) {
+            futex_queue(userspace_address).wake_one();
+        } else {
+            // FIXME: Wake exactly (value) waiters.
+            futex_queue(userspace_address).wake_all();
+        }
+        break;
+    }
+
+    return 0;
+}

Kernel/Process.h

@@ -233,6 +233,7 @@ public:
     int sys$profiling_enable(pid_t);
     int sys$profiling_disable(pid_t);
     void* sys$get_kernel_info_page();
+    int sys$futex(const Syscall::SC_futex_params*);
 
     static void initialize();
 
@@ -391,6 +392,9 @@ private:
     u64 m_alarm_deadline { 0 };
 
     int m_icon_id { -1 };
+
+    WaitQueue& futex_queue(i32*);
+    HashMap<u32, OwnPtr<WaitQueue>> m_futex_queues;
 };
 
 class ProcessInspectionHandle {

Kernel/Syscall.h

@@ -150,7 +150,8 @@ typedef u32 socklen_t;
     __ENUMERATE_SYSCALL(set_shared_buffer_volatile) \
     __ENUMERATE_SYSCALL(profiling_enable)           \
     __ENUMERATE_SYSCALL(profiling_disable)          \
-    __ENUMERATE_SYSCALL(get_kernel_info_page)
+    __ENUMERATE_SYSCALL(get_kernel_info_page)       \
+    __ENUMERATE_SYSCALL(futex)
 
 namespace Syscall {
 
@@ -260,6 +261,13 @@ struct SC_setsockopt_params {
     socklen_t value_size;
 };
 
+struct SC_futex_params {
+    i32* userspace_address;
+    int futex_op;
+    i32 val;
+    const timespec* timeout;
+};
+
 struct SC_create_thread_params {
     unsigned int m_detach_state = 0; // JOINABLE or DETACHED
     int m_schedule_priority = 2;     // ThreadPriority::Normal

Kernel/UnixTypes.h

@@ -46,6 +46,9 @@
 
 #define FD_CLOEXEC 1
 
+#define FUTEX_WAIT 1
+#define FUTEX_WAKE 2
+
 /* c_cc characters */
 #define VINTR 0
 #define VQUIT 1