perf maps: Locking tidy up of nr_maps

After this change maps__nr_maps is only used by tests, existing users
are migrated to maps__empty. Compute maps__empty under the read lock.
Signed-off-by: Ian Rogers <irogers@google.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: K Prateek Nayak <kprateek.nayak@amd.com>
Cc: James Clark <james.clark@arm.com>
Cc: Vincent Whitchurch <vincent.whitchurch@axis.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Colin Ian King <colin.i.king@gmail.com>
Cc: Changbin Du <changbin.du@huawei.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Song Liu <song@kernel.org>
Cc: Leo Yan <leo.yan@linux.dev>
Cc: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Artem Savkov <asavkov@redhat.com>
Cc: bpf@vger.kernel.org
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20240210031746.4057262-7-irogers@google.com

tools/perf/util/machine.c

@@ -440,7 +440,7 @@ static struct thread *findnew_guest_code(struct machine *machine,
 		return NULL;
 
 	/* Assume maps are set up if there are any */
-	if (maps__nr_maps(thread__maps(thread)))
+	if (!maps__empty(thread__maps(thread)))
 		return thread;
 
 	host_thread = machine__find_thread(host_machine, -1, pid);

tools/perf/util/maps.c

@@ -541,7 +541,13 @@ void maps__remove(struct maps *maps, struct map *map)
 
 bool maps__empty(struct maps *maps)
 {
-	return maps__nr_maps(maps) == 0;
+	bool res;
+
+	down_read(maps__lock(maps));
+	res = maps__nr_maps(maps) == 0;
+	up_read(maps__lock(maps));
+
+	return res;
 }
 
 bool maps__equal(struct maps *a, struct maps *b)
@@ -871,7 +877,7 @@ int maps__copy_from(struct maps *dest, struct maps *parent)
 
 	parent_maps_by_address = maps__maps_by_address(parent);
 	n = maps__nr_maps(parent);
-	if (maps__empty(dest)) {
+	if (maps__nr_maps(dest) == 0) {
 		/* No existing mappings so just copy from parent to avoid reallocs in insert. */
 		unsigned int nr_maps_allocated = RC_CHK_ACCESS(parent)->nr_maps_allocated;
 		struct map **dest_maps_by_address =

tools/perf/util/maps.h

@@ -43,8 +43,8 @@ int maps__for_each_map(struct maps *maps, int (*cb)(struct map *map, void *data)
 void maps__remove_maps(struct maps *maps, bool (*cb)(struct map *map, void *data), void *data);
 
 struct machine *maps__machine(const struct maps *maps);
-unsigned int maps__nr_maps(const struct maps *maps);
-refcount_t *maps__refcnt(struct maps *maps);
+unsigned int maps__nr_maps(const struct maps *maps); /* Test only. */
+refcount_t *maps__refcnt(struct maps *maps); /* Test only. */
 
 #ifdef HAVE_LIBUNWIND_SUPPORT
 void *maps__addr_space(const struct maps *maps);