On Fri, Mar 05, 2021 at 06:07:23PM +0100, Ævar Arnfjörð Bjarmason wrote:

+# Setting SPATCH_XARGS overrides SPATCH_BATCH_SIZE. To get concurrency
+# when targeting a single contrib/coccinelle/%.patch use e.g. "-P" if
+# your xargs(1) supports it:
+#
+#    make contrib/coccinelle/strbuf.cocci.patch SPATCH_XARGS="xargs -P 8 -n 8"
+#
+# Or a combination -jN and "xargs -P":
+#
+#    make -j4 coccicheck SPATCH_XARGS="xargs -P 2 -n 8"

I don't think this is actually safe to do. At least not if you care
about the output patch. All of the parallel processes will go to the
same output file:

-	if test $(SPATCH_BATCH_SIZE) = 0; then \
-		limit=; \
-	else \
-		limit='-n $(SPATCH_BATCH_SIZE)'; \
-	fi; \
-	if ! echo $(COCCI_SOURCES) | xargs $$limit \
+	if ! echo $(COCCI_SOURCES) | $(SPATCH_XARGS) \
 		$(SPATCH) --sp-file $< $(SPATCH_FLAGS) \
 		>$@+ 2>$@.log; \
 	then \

which means they run the risk of getting jumbled racily. It might work
OK in practice if the patches are smaller than spatch puts into a single
write() call, and what the OS will treat as an atomic file-write (which
is really under-specified in POSIX for regular files).

-Peff