Fix 'atomic-box-compare-and-swap!'.

Fixes <https://bugs.gnu.org/32786>.

'scm_atomic_compare_and_swap_scm' is a thin wrapper around
'atomic_compare_exchange_weak' (where available), and therefore it may
spuriously fail on some platforms, leaving the atomic object unchanged
even when the observed value is equal to the expected value.  Since
'scm_atomic_compare_and_swap_scm' returns both a boolean result and the
observed value, the caller is able to detect spurious failures when
using that API.

'atomic-box-compare-and-swap!' presents a simpler API, returning only
the observed value.  The documentation advises callers to assume that
the exchange succeeded if the observed value is 'eq?' to the expected
value.  It's therefore not possible to report spurious failures with
this API.

'atomic-box-compare-and-swap!' uses 'scm_atomic_compare_and_swap_scm',
and prior to this commit would simply ignore the boolean result and
return the observed value.  In case of spurious failures, the caller
would legitimately conclude that the exchange had succeeded.

With this commit, 'atomic-box-compare-and-swap!' now retries in case of
spurious failures.

* libguile/atomic.c (scm_atomic_box_compare_and_swap_x): If
'scm_atomic_compare_and_swap_scm' returns false and the observed value
is equal to 'expected', then try again.
* libguile/intrinsics.c (atomic_compare_and_swap_scm): Ditto.

1 files changed, 15 insertions, 3 deletions

diff --git a/libguile/intrinsics.c b/libguile/intrinsics.c
index a619fda90..437441032 100644
--- a/libguile/intrinsics.c
+++ b/libguile/intrinsics.c
@@ -1,4 +1,4 @@
-/* Copyright 2018
+/* Copyright 2018-2019
      Free Software Foundation, Inc.
 
    This file is part of Guile.
@@ -459,8 +459,20 @@ atomic_swap_scm (SCM *loc, SCM val)
 static SCM
 atomic_compare_and_swap_scm (SCM *loc, SCM expected, SCM desired)
 {
-  scm_atomic_compare_and_swap_scm (loc, &expected, desired);
-  return expected;
+  SCM result = expected;
+
+  while (!scm_atomic_compare_and_swap_scm (loc, &result, desired)
+         && scm_is_eq (result, expected))
+    {
+      /* 'scm_atomic_compare_and_swap_scm' has spuriously failed,
+         i.e. it has returned 0 to indicate failure, although the
+         observed value is 'eq?' to EXPECTED.  In this case, we *must*
+         try again, because the API of 'atomic-box-compare-and-swap!'
+         provides no way to indicate to the caller that the exchange
+         failed when the observed value is 'eq?' to EXPECTED.  */
+    }
+
+  return result;
 }
 
 void