diff --git a/spike/x86/cost.sh b/spike/x86/cost.sh
index 5b9b105..8bc0f35 100755
--- a/spike/x86/cost.sh
+++ b/spike/x86/cost.sh
@@ -54,8 +54,15 @@ test -x "$flan" || { echo "build failed" >&2; exit 1; }
 # Not mktemp under /tmp by default: this writes a few hundred executables of
 # a megabyte or two, and a full /tmp on this machine has already frozen one
 # session. The guard is cheap and a wedged run is not.
-out=${COST_OUT:-${TMPDIR:-/tmp}/flan-cost.$$}
-mkdir -p "$out" || exit 1
+# A directory of this run's own, made with a plain mkdir so that a second
+# copy of this script cannot land in the first one's: two runs sharing a
+# scratch directory overwrite each other's `l` and `x` between the build and
+# the timing, and the result is a row of numbers that belong to two different
+# programs. That happened once here and the numbers looked entirely ordinary.
+work=${COST_OUT:-${TMPDIR:-/tmp}/flan-cost}
+mkdir -p "$work" || exit 1
+out=$work/run.$$
+mkdir "$out" || exit 1
 trap 'rm -rf "$out"' EXIT
 free=$(df -Pk "$out" | awk 'NR==2 {print $4}')
 [ "$free" -gt 2000000 ] || { echo "less than 2GB free at $out" >&2; exit 1; }