[PATCH bpf] libbpf: fix race when pinning maps in parallel

[Martynas Pumputis <m@lambda.lt>]

When loading in parallel multiple programs which use the same to-be
pinned map, it is possible that two instances of the loader will call
bpf_object__create_maps() at the same time. If the map doesn't exist
when both instances call bpf_object__reuse_map(), then one of the
instances will fail with EEXIST when calling bpf_map__pin().

Fix the race by retrying creating a map if bpf_map__pin() returns
EEXIST. The fix is similar to the one in iproute2: e4c4685fd6e4 ("bpf:
Fix race condition with map pinning").

Cc: Joe Stringer <joe@wand.net.nz>
Signed-off-by: Martynas Pumputis <m@lambda.lt>
---
 tools/lib/bpf/libbpf.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 1e04ce724240..7a31c7c3cd21 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -4616,10 +4616,12 @@ bpf_object__create_maps(struct bpf_object *obj)
 	char *cp, errmsg[STRERR_BUFSIZE];
 	unsigned int i, j;
 	int err;
+	bool retried = false;
 
 	for (i = 0; i < obj->nr_maps; i++) {
 		map = &obj->maps[i];
 
+retry:
 		if (map->pin_path) {
 			err = bpf_object__reuse_map(map);
 			if (err) {
@@ -4660,9 +4662,13 @@ bpf_object__create_maps(struct bpf_object *obj)
 		if (map->pin_path && !map->pinned) {
 			err = bpf_map__pin(map, NULL);
 			if (err) {
+				zclose(map->fd);
+				if (!retried && err == EEXIST) {
+					retried = true;
+					goto retry;
+				}
 				pr_warn("map '%s': failed to auto-pin at '%s': %d\n",
 					map->name, map->pin_path, err);
-				zclose(map->fd);
 				goto err_out;
 			}
 		}
-- 
2.32.0