Kernel: Clean up around Scheduler::yield() a bit.

Also add assertion in Lock that the scheduler isn't currently active.
I've been seeing occasional fuckups that I suspect might be someone called
by the scheduler trying to take a busy lock.

AK/Lock.h

@@ -3,7 +3,7 @@
 #include "Assertions.h"
 #include "Types.h"
 #include "i386.h"
-int sched_yield();
+#include <Kernel/Scheduler.h>
 
 class Process;
 extern Process* current;
@@ -52,6 +52,7 @@ private:
 
 inline void Lock::lock()
 {
+    ASSERT(!Scheduler::is_active());
     for (;;) {
         if (CAS(&m_lock, 1, 0) == 0) {
             if (!m_holder || m_holder == current) {
@@ -63,7 +64,7 @@ inline void Lock::lock()
             }
             m_lock = 0;
         }
-        sched_yield();
+        Scheduler::yield();
     }
 }
 
@@ -84,7 +85,7 @@ inline void Lock::unlock()
             m_lock = 0;
             return;
         }
-        sched_yield();
+        Scheduler::yield();
     }
 }
 

Kernel/Process.cpp

@@ -1109,7 +1109,7 @@ ssize_t Process::sys$read(int fd, void* outbuf, size_t nread)
         if (!descriptor->can_read(*this)) {
             m_blocked_fd = fd;
             block(BlockedRead);
-            sched_yield();
+            Scheduler::yield();
             if (m_was_interrupted_while_blocked)
                 return -EINTR;
         }
@@ -1572,7 +1572,7 @@ pid_t Process::sys$waitpid(pid_t waitee, int* wstatus, int options)
 
     m_waitee_pid = waitee;
     block(BlockedWait);
-    sched_yield();
+    Scheduler::yield();
     if (m_was_interrupted_while_blocked)
         return -EINTR;
     Process* waitee_process;
@@ -1612,7 +1612,7 @@ void Process::block(Process::State new_state)
 void block(Process::State state)
 {
     current->block(state);
-    sched_yield();
+    Scheduler::yield();
 }
 
 void sleep(dword ticks)
@@ -1620,7 +1620,7 @@ void sleep(dword ticks)
     ASSERT(current->state() == Process::Running);
     current->set_wakeup_time(system.uptime + ticks);
     current->block(Process::BlockedSleep);
-    sched_yield();
+    Scheduler::yield();
 }
 
 static bool is_inside_kernel_code(LinearAddress laddr)

Kernel/Scheduler.cpp

@@ -3,6 +3,7 @@
 #include "system.h"
 #include "RTC.h"
 #include "i8253.h"
+#include <AK/TemporaryChange.h>
 
 //#define LOG_EVERY_CONTEXT_SWITCH
 //#define SCHEDULER_DEBUG
@@ -12,17 +13,27 @@ static const dword time_slice = 5; // *10 = 50ms
 Process* current;
 Process* g_last_fpu_process;
 static Process* s_colonel_process;
-static volatile bool s_in_yield;
 
 struct TaskRedirectionData {
     word selector;
     TSS32 tss;
 };
 static TaskRedirectionData s_redirection;
+static bool s_active;
+
+bool Scheduler::is_active()
+{
+    return s_active;
+}
 
 bool Scheduler::pick_next()
 {
     ASSERT_INTERRUPTS_DISABLED();
+    ASSERT(!s_active);
+
+    TemporaryChange<bool> change(s_active, true);
+
+    ASSERT(s_active);
 
     if (!current) {
         // XXX: The first ever context_switch() goes to the idle process.
@@ -176,22 +187,12 @@ bool Scheduler::pick_next()
 bool Scheduler::yield()
 {
     InterruptDisabler disabler;
-    ASSERT(!s_in_yield);
-    s_in_yield = true;
-
-    if (!current) {
-        kprintf("PANIC: sched_yield() with !current");
-        HANG;
-    }
-
+    ASSERT(current);
     //dbgprintf("%s<%u> yield()\n", current->name().characters(), current->pid());
 
-    if (!pick_next()) {
-        s_in_yield = false;
+    if (!pick_next())
         return 1;
-    }
 
-    s_in_yield = false;
     //dbgprintf("yield() jumping to new process: %x (%s)\n", current->far_ptr().selector, current->name().characters());
     switch_now();
     return 0;
@@ -269,11 +270,6 @@ bool Scheduler::context_switch(Process& process)
     return true;
 }
 
-int sched_yield()
-{
-    return Scheduler::yield();
-}
-
 static void initialize_redirection()
 {
     auto& descriptor = get_gdt_entry(s_redirection.selector);
@@ -319,7 +315,6 @@ void Scheduler::initialize()
     s_colonel_process = Process::create_kernel_process("colonel", nullptr);
     current = nullptr;
     g_last_fpu_process = nullptr;
-    s_in_yield = false;
     load_task_register(s_redirection.selector);
 }
 

Kernel/Scheduler.h

@@ -19,8 +19,7 @@ public:
     static bool context_switch(Process&);
     static void prepare_to_modify_tss(Process&);
     static Process* colonel();
+    static bool is_active();
 private:
     static void prepare_for_iret_to_new_process();
 };
-
-int sched_yield();