diff --git a/libc/src/__support/RPC/rpc.h b/libc/src/__support/RPC/rpc.h
--- a/libc/src/__support/RPC/rpc.h
+++ b/libc/src/__support/RPC/rpc.h
@@ -248,6 +248,9 @@
   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);
 
@@ -285,6 +288,9 @@
   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);