Only set new thread once an unblocked thread is found.

Author: Patrick-6

src/concurrency/thread.rs

@@ -724,12 +724,12 @@ trait EvalContextPrivExt<'tcx>: MiriInterpCxExt<'tcx> {
                     );
                     this.unblock_thread(next_thread_id, BlockReason::Genmc)?;
                 }
-                // Set the new active thread.
-                let thread_manager = &mut this.machine.threads;
-                thread_manager.active_thread = next_thread_id;
-                // A thread blocked by GenMC may get blocked again during the unblocking callback.
+                // The thread we just unblocked may have been blocked again during the unblocking callback.
                 // In that case, we need to ask for a different thread to run next.
-                if thread_manager.threads[thread_manager.active_thread].state.is_enabled() {
+                let thread_manager = &mut this.machine.threads;
+                if thread_manager.threads[next_thread_id].state.is_enabled() {
+                    // Set the new active thread.
+                    thread_manager.active_thread = next_thread_id;
                     return interp_ok(SchedulingAction::ExecuteStep);
                 }
             }