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bpf: Remove __rcu tagging in st_link->map

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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>
Signed-off-by: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Reviewed-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://patch.msgid.link/20260917200542.3689605-2-ameryhung@gmail.com
This commit is contained in:
Martin KaFai Lau 2026-09-17 13:05:27 -07:00 committed by Alexei Starovoitov
parent 72a930db88
commit 0120895121
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1 changed files with 14 additions and 15 deletions
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29
kernel/bpf/bpf_struct_ops.c
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@ -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;
};
@ -1297,8 +1297,7 @@ 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);
@ -1313,11 +1312,11 @@ 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,
@ -1327,11 +1326,11 @@ 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;
}
@ -1354,7 +1353,7 @@ 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;
@ -1376,7 +1375,7 @@ 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:
@ -1393,7 +1392,7 @@ 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;
@ -1402,7 +1401,7 @@ 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().
*/
@ -1422,7 +1421,7 @@ 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 = {
@ -1478,7 +1477,7 @@ 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);