Apply suggestions from code review

Co-authored-by: Ralf Jung <post@ralfj.de>

Author: nia-e

src/bin/miri.rs

@@ -756,7 +756,8 @@ fn main() {
         #[cfg(target_os = "linux")]
         if unsafe { miri::native_lib::init_sv() }.is_err() {
             eprintln!(
-                "warning: The native-lib tracer could not be started. Is this a Linux system, and does Miri have permissions to ptrace?"
+                "warning: The native-lib tracer could not be started. Is this a Linux system, and does Miri have permissions to ptrace?\n\
+                Falling back to non-tracing native-lib mode."
             );
         }
     }

src/shims/native_lib/trace/stub.rs

@@ -14,6 +14,7 @@ impl Supervisor {
         _: T,
         f: impl FnOnce() -> InterpResult<'tcx, crate::ImmTy<'tcx>>,
     ) -> InterpResult<'tcx, (crate::ImmTy<'tcx>, Option<super::MemEvents>)> {
+        // We acquire the lock to ensure that no two FFI calls run concurrently.
         let _g = SUPERVISOR.lock().unwrap();
         f().map(|v| (v, None))
     }