Kernel: Ignore subsequent calls to Process::die

It's possible that another thread might try to exit the process just
about the same time another thread does the same, or a crash happens.
Also, we may not be able to kill all other threads instantly as they
may be blocked in the kernel (though in this case they would get killed
before ever returning back to user mode. So keep track of whether
Process::die was already called and ignore it on subsequent calls.

Fixes #8485

Kernel/Process.cpp

@@ -581,7 +581,7 @@ void Process::finalize()
     // allow us to take references anymore.
     ProcFSComponentRegistry::the().unregister_process(*this);
 
-    m_dead = true;
+    m_state.store(State::Dead, AK::MemoryOrder::memory_order_release);
 
     {
         // FIXME: PID/TID BUG
@@ -626,6 +626,15 @@ void Process::unblock_waiters(Thread::WaitBlocker::UnblockFlags flags, u8 signal
 
 void Process::die()
 {
+    auto expected = State::Running;
+    if (!m_state.compare_exchange_strong(expected, State::Dying, AK::memory_order_acquire)) {
+        // It's possible that another thread calls this at almost the same time
+        // as we can't always instantly kill other threads (they may be blocked)
+        // So if we already were called then other threads should stop running
+        // momentarily and we only really need to service the first thread
+        return;
+    }
+
     // Let go of the TTY, otherwise a slave PTY may keep the master PTY from
     // getting an EOF when the last process using the slave PTY dies.
     // If the master PTY owner relies on an EOF to know when to wait() on a