add comments explaining the parameter mismatch check in the Apple futex code

Author: joboet

src/shims/unix/macos/sync.rs

@@ -154,7 +154,9 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
             .unwrap();
 
         // Detect mismatches between the flags and sizes used on this address
-        // by comparing with the parameters stored with the first waiter.
+        // by comparing it with the parameters used by the other waiters in
+        // the current list. If the list is currently empty, update those
+        // parameters.
         if futex.futex.waiters() == 0 {
             futex.size.set(size);
             futex.shared.set(is_shared);
@@ -246,7 +248,10 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
         if futex.futex.waiters() == 0 {
             this.set_last_error_and_return(LibcError("ENOENT"), dest)?;
             return interp_ok(());
-        // Detect mismatches between the flags and sizes used on this address.
+        // If there are waiters in the queue, they have all used the parameters
+        // stored in `futex` (we check this in `os_sync_wait_on_address` above).
+        // Detect mismatches between "our" parameters and the parameters used by
+        // the waiters and return an error in that case.
         } else if futex.size.get() != size || futex.shared.get() != is_shared {
             this.set_last_error_and_return(LibcError("EINVAL"), dest)?;
             return interp_ok(());