[libc][rpc] Fix the memory ordering in lock.

[JonChesterfield]

Summary

Prevent operation reordering with fence instead of a comment.

The mailboxes are in shared memory and the locks structure in device memory.
If the mailboxes are read and then the lock taken, the lock says nothing
about the current or future state of those mail boxes. The relaxed atomic
fetch_or can be reordered before the relaxed atomic loads of unrelated
variables unless there is a fence preventing this.

Patches both Client::try_open and Server::try_open, one of which is missing
an optimisation and the other is missing the comment, but which otherwise
could be Process::try_open followed by buffer->opcode = opcode in Client.

[jhuber6]

Thanks, I forgot about the potential ordering hazard here.

libc/src/__support/RPC/rpc.h

 /// port if we find an index that is in a valid sending state. That is, there
 /// are send operations pending that haven't been serviced on this port. Each
 /// port instance uses an associated \p opcode to tell the server what to do.
 LIBC_INLINE cpp::optional<Port> Client::try_open(uint16_t opcode) {
   // Attempt to acquire the lock on this index.
   if (lock->fetch_or(1, cpp::MemoryOrder::RELAXED))
     return cpp::nullopt;
 
+  // The mailbox state must be read with the lock held.
+  atomic_thread_fence(cpp::MemoryOrder::ACQUIRE);
+
   uint32_t in = inbox->load(cpp::MemoryOrder::RELAXED);
   uint32_t out = outbox->load(cpp::MemoryOrder::RELAXED);
 
   // Once we acquire the index we need to check if we are in a valid sending
   // state.
   if (!can_send_data(in, out)) {
     lock->store(0, cpp::MemoryOrder::RELAXED);
     return cpp::nullopt;
LIBC_INLINE cpp::optional<Port> Server::try_open() {
   // opening a port.
   if (!can_recv_data(in, out))
     return cpp::nullopt;
 
   // Attempt to acquire the lock on this index.
   if (lock->fetch_or(1, cpp::MemoryOrder::RELAXED))
     return cpp::nullopt;
 
+  // The mailbox state must be read with the lock held.
+  atomic_thread_fence(cpp::MemoryOrder::ACQUIRE);
+
   in = inbox->load(cpp::MemoryOrder::RELAXED);
   out = outbox->load(cpp::MemoryOrder::RELAXED);
 
   if (!can_recv_data(in, out)) {
     lock->store(0, cpp::MemoryOrder::RELAXED);
     return cpp::nullopt;
   }