[RFC PATCH bpf-next 01/12] bpf: Remove __rcu tagging in st_link->map

[Martin KaFai Lau <martin.lau@linux.dev>]

From: Martin KaFai Lau <martin.lau@kernel.org>

st_link->map is always written under update_mutex. The paths that read
st_link->map with rcu_read_lock() are not in the fast path, so they can
simply take update_mutex instead. Remove the __rcu annotation and replace
all RCU accessors with direct pointer reads under update_mutex. Use
READ_ONCE() in bpf_struct_ops_map_link_poll() which reads the pointer
without holding update_mutex.

It is a simplification change.

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
---
 kernel/bpf/bpf_struct_ops.c | 29 ++++++++++++++---------------
 1 file changed, 14 insertions(+), 15 deletions(-)

diff --git a/kernel/bpf/bpf_struct_ops.c b/kernel/bpf/bpf_struct_ops.c
index 521cb9d7e8c7..08791180d71d 100644
--- a/kernel/bpf/bpf_struct_ops.c
+++ b/kernel/bpf/bpf_struct_ops.c
@@ -57,7 +57,7 @@ struct bpf_struct_ops_map {
 
 struct bpf_struct_ops_link {
 	struct bpf_link link;
-	struct bpf_map __rcu *map;
+	struct bpf_map *map;
 	wait_queue_head_t wait_hup;
 };
 
@@ -1220,8 +1220,7 @@ static void bpf_struct_ops_map_link_dealloc(struct bpf_link *link)
 	struct bpf_struct_ops_map *st_map;
 
 	st_link = container_of(link, struct bpf_struct_ops_link, link);
-	st_map = (struct bpf_struct_ops_map *)
-		rcu_dereference_protected(st_link->map, true);
+	st_map = (struct bpf_struct_ops_map *)st_link->map;
 	if (st_map) {
 		st_map->st_ops_desc->st_ops->unreg(&st_map->kvalue.data, link);
 		bpf_map_put(&st_map->map);
@@ -1236,11 +1235,11 @@ static void bpf_struct_ops_map_link_show_fdinfo(const struct bpf_link *link,
 	struct bpf_map *map;
 
 	st_link = container_of(link, struct bpf_struct_ops_link, link);
-	rcu_read_lock();
-	map = rcu_dereference(st_link->map);
+	mutex_lock(&update_mutex);
+	map = st_link->map;
 	if (map)
 		seq_printf(seq, "map_id:\t%d\n", map->id);
-	rcu_read_unlock();
+	mutex_unlock(&update_mutex);
 }
 
 static int bpf_struct_ops_map_link_fill_link_info(const struct bpf_link *link,
@@ -1250,11 +1249,11 @@ static int bpf_struct_ops_map_link_fill_link_info(const struct bpf_link *link,
 	struct bpf_map *map;
 
 	st_link = container_of(link, struct bpf_struct_ops_link, link);
-	rcu_read_lock();
-	map = rcu_dereference(st_link->map);
+	mutex_lock(&update_mutex);
+	map = st_link->map;
 	if (map)
 		info->struct_ops.map_id = map->id;
-	rcu_read_unlock();
+	mutex_unlock(&update_mutex);
 	return 0;
 }
 
@@ -1277,7 +1276,7 @@ static int bpf_struct_ops_map_link_update(struct bpf_link *link, struct bpf_map
 
 	mutex_lock(&update_mutex);
 
-	old_map = rcu_dereference_protected(st_link->map, lockdep_is_held(&update_mutex));
+	old_map = st_link->map;
 	if (!old_map) {
 		err = -ENOLINK;
 		goto err_out;
@@ -1299,7 +1298,7 @@ static int bpf_struct_ops_map_link_update(struct bpf_link *link, struct bpf_map
 		goto err_out;
 
 	bpf_map_inc(new_map);
-	rcu_assign_pointer(st_link->map, new_map);
+	WRITE_ONCE(st_link->map, new_map);
 	bpf_map_put(old_map);
 
 err_out:
@@ -1316,7 +1315,7 @@ static int bpf_struct_ops_map_link_detach(struct bpf_link *link)
 
 	mutex_lock(&update_mutex);
 
-	map = rcu_dereference_protected(st_link->map, lockdep_is_held(&update_mutex));
+	map = st_link->map;
 	if (!map) {
 		mutex_unlock(&update_mutex);
 		return 0;
@@ -1325,7 +1324,7 @@ static int bpf_struct_ops_map_link_detach(struct bpf_link *link)
 
 	st_map->st_ops_desc->st_ops->unreg(&st_map->kvalue.data, link);
 
-	RCU_INIT_POINTER(st_link->map, NULL);
+	WRITE_ONCE(st_link->map, NULL);
 	/* Pair with bpf_map_get() in bpf_struct_ops_link_create() or
 	 * bpf_map_inc() in bpf_struct_ops_map_link_update().
 	 */
@@ -1345,7 +1344,7 @@ static __poll_t bpf_struct_ops_map_link_poll(struct file *file,
 
 	poll_wait(file, &st_link->wait_hup, pts);
 
-	return rcu_access_pointer(st_link->map) ? 0 : EPOLLHUP;
+	return READ_ONCE(st_link->map) ? 0 : EPOLLHUP;
 }
 
 static const struct bpf_link_ops bpf_struct_ops_map_lops = {
@@ -1401,7 +1400,7 @@ int bpf_struct_ops_link_create(union bpf_attr *attr)
 		link = NULL;
 		goto err_out;
 	}
-	RCU_INIT_POINTER(link->map, map);
+	link->map = map;
 	mutex_unlock(&update_mutex);
 
 	return bpf_link_settle(&link_primer);
-- 
2.53.0-Meta