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-rw-r--r--libguile/atomic.c19
-rw-r--r--libguile/vm-engine.c19
2 files changed, 29 insertions, 9 deletions
diff --git a/libguile/atomic.c b/libguile/atomic.c
index 950874030..180622b47 100644
--- a/libguile/atomic.c
+++ b/libguile/atomic.c
@@ -1,4 +1,4 @@
-/* Copyright (C) 2016 Free Software Foundation, Inc.
+/* Copyright (C) 2016, 2018 Free Software Foundation, Inc.
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public License
@@ -95,10 +95,21 @@ SCM_DEFINE (scm_atomic_box_compare_and_swap_x,
"if the return value is @code{eq?} to @var{expected}.")
#define FUNC_NAME s_scm_atomic_box_compare_and_swap_x
{
+ SCM result = expected;
+
SCM_VALIDATE_ATOMIC_BOX (1, box);
- scm_atomic_compare_and_swap_scm (scm_atomic_box_loc (box),
- &expected, desired);
- return expected;
+ while (!scm_atomic_compare_and_swap_scm (scm_atomic_box_loc (box),
+ &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;
}
#undef FUNC_NAME
diff --git a/libguile/vm-engine.c b/libguile/vm-engine.c
index 19ff3e498..53d7f3522 100644
--- a/libguile/vm-engine.c
+++ b/libguile/vm-engine.c
@@ -3868,16 +3868,25 @@ VM_NAME (scm_i_thread *thread, struct scm_vm *vp,
{
scm_t_uint16 dst, box;
scm_t_uint32 expected, desired;
- SCM scm_box, scm_expected;
+ SCM scm_box, scm_expected, scm_result;
UNPACK_12_12 (op, dst, box);
UNPACK_24 (ip[1], expected);
UNPACK_24 (ip[2], desired);
scm_box = SP_REF (box);
VM_VALIDATE_ATOMIC_BOX (scm_box, "atomic-box-compare-and-swap!");
- scm_expected = SP_REF (expected);
- scm_atomic_compare_and_swap_scm (scm_atomic_box_loc (scm_box),
- &scm_expected, SP_REF (desired));
- SP_SET (dst, scm_expected);
+ scm_result = scm_expected = SP_REF (expected);
+ while (!scm_atomic_compare_and_swap_scm (scm_atomic_box_loc (scm_box),
+ &scm_result, SP_REF (desired))
+ && scm_is_eq (scm_result, scm_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. */
+ }
+ SP_SET (dst, scm_result);
NEXT (3);
}