bpf: Update the comments in maybe_wait_bpf_programs()

Since commit 638e4b825d52 ("bpf: Allows per-cpu maps and map-in-map in
sleepable programs"), sleepable BPF program can also use map-in-map, but
maybe_wait_bpf_programs() doesn't handle it accordingly. The main reason
is that using synchronize_rcu_tasks_trace() to wait for the completions
of these sleepable BPF programs may incur a very long delay and
userspace may think it is hung, so the wait for sleepable BPF programs
is skipped. Update the comments in maybe_wait_bpf_programs() to reflect
the reason.

Signed-off-by: Hou Tao <houtao1@huawei.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/r/20231211083447.1921178-1-houtao@huaweicloud.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 06320d9..d63c1ed 100644 (file)
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -142,9 +142,13 @@ static u32 bpf_map_value_size(const struct bpf_map *map)
 
 static void maybe_wait_bpf_programs(struct bpf_map *map)
 {
-       /* Wait for any running BPF programs to complete so that
-        * userspace, when we return to it, knows that all programs
-        * that could be running use the new map value.
+       /* Wait for any running non-sleepable BPF programs to complete so that
+        * userspace, when we return to it, knows that all non-sleepable
+        * programs that could be running use the new map value. For sleepable
+        * BPF programs, synchronize_rcu_tasks_trace() should be used to wait
+        * for the completions of these programs, but considering the waiting
+        * time can be very long and userspace may think it will hang forever,
+        * so don't handle sleepable BPF programs now.
         */
        if (map->map_type == BPF_MAP_TYPE_HASH_OF_MAPS ||
            map->map_type == BPF_MAP_TYPE_ARRAY_OF_MAPS)